peerj_json_files/PeerJ_Json_117.json

{
    "v1_Abstract": "The Laotian rock rat, Laonastes aenigmamus, is one of the most recently discovered species of rodent, and displays a cranial morphology that is highly specialised. The rostrum of L. aenigmamus is exceptionally elongate and bears a large attachment site for the infraorbital portion of the zygomaticomandibularis muscle (IOZM), which is particularly well-developed in this species. In this study, we used finite element analysis to investigate the biomechanical performance of the Laotian rock rat cranium and to elucidate the function of the IOZM. A finite element model of the skull of L. aenigmamus was constructed and solved for biting on each of the teeth (incisors, premolar and molars). Further load cases were created and solved in which the origin of the IOZM had been moved anteriorly and posteriorly along the rostrum. Finally, a set of load cases were produced in which the IOZM was removed entirely, and its force was redistributed between the remaining masticatory muscles. The analysis showed that, during biting, the most stressed areas of the skull were the zygomatic and orbital regions. Compared to other rodents, L. aenigmamus is highly efficient at incisor gnawing, but less efficient at molar chewing. However, a relatively constant bite force across the molar tooth row may be an adaptation to folivory. Movement of the origin of the IOZM had little on the patterns of von Mises stresses, or the overall stress experienced by the cranium. However, removal of the IOZM had a substantial effect on the total deformation experienced by the skull. In addition, the positioning and presence of the IOZM had large impact on bite force. Moving the IOZM origin to the anterior tip of the rostrum led to a substantially reduced bite force at all teeth. This was hypothesised to be a result of the increasing horizontal component to the pull of this muscle as it is moved anteriorly along the rostrum. Removal of the IOZM also resulted in a reduced bite force, even when the total input muscle force was maintained at the same level. It was thus concluded that the function of the IOZM in L. aenigmamus is to increase bite force whilst reducing cranial deformation. If the IOZM can be",
    "v1_col_introduction": "introduction  : The Rodentia is the most speciose of all mammalian orders, with over 2,200 extant species (Wilson & Reeder, 2005) in addition to a large number of fossil forms. Despite such taxonomic diversity, morphological variation within the order is relatively limited, particularly with regard to the skull and mandible (Wood, 1965; Hautier et al., 2011). Hence, determining relationships between rodent species based on morphology is difficult, and rodent taxonomy has been historically controversial, with two competing classifications arising in the second half of the 19 th century. First, Brandt (1855) split the rodents into three suborders (Sciuromorpha, Hystricomorpha and Myomorpha) based on the morphology of the masseter muscle and its attachment to the rostrum (see Cox & Jeffery, 2011 for details). Later, Tullberg (1899) divided the rodents into two groups (Sciurognathi and Hystricognathi) based on the position of the angular process of the mandible relative to the incisor. Recent molecular phylogenies (Blanga-Kanfi et al., 2009; Churakov et al., 2010; Fabre et al., 2012) have shown that neither of these classifications accurately resolves the evolutionary relationships between the rodents. The three suborders of Brandt (1855), also used by Simpson (1945), have now been discarded, as they are thought to represent polyphyletic groupings of rodents, although the names have been retained in their adjectival form (sciuromorphous, etc) to describe the three morphotypes of the skull and\n1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n12\n13\n14\n15\n16\n17\n18\nPeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013)\nR ev ie w in g M an\nus cr ip t\nmasseter (Wood, 1965; Cox & Jeffery, 2011). The classification of Tullberg (1899) has fared slightly better, with the Hystricognathi still recognised as a monophyletic clade (the Sciurognathi is paraphyletic with respect to the Hystricognathi). However, recent work (Hautier et al., 2011) has questioned the usefulness of sciurognathy and hystricognathy as morphological terms, noting that there exists a continuum of mandibular forms rather than two discrete morphologies, and that some members of the Hystricognathi actually have mandibles that appear to be almost sciurognathous in form.\nThe Laotian rock rat, Laonastes aenigmamus (Jenkins et al., 2005), is a recently discovered species of rodent from south-east Asia. It shows an unusual mixture of cranial, mandibular and muscular morphologies, combining a large part of the zygomaticomandibularis muscle that extends through the enlarged infraorbital foramen to attach to the rostrum (Hautier & Saksiri, 2009) with a weak lateral displacement of the angular process of the mandible (Hautier et al., 2011). Thus, the Laotian rock rat brings together a hystricomorphous skull and masseter with a lower jaw that is intermediate between sciurognathous and hystricognathous. This combination of characters has made its phylogenetic relationships difficult to ascertain. When first described, a new family, the Laonastidae, was created to house L. aenigmamus (Jenkins et al., 2005). This family was placed within the Hystricognathi as the sister-group to African mole-rats (Bathyergidae) or the dassie rat (Petromuridae). A subsequent analysis (Dawson et al., 2006) showed that L. aenigmamus was in fact a member of the Diatomyidae, a family of rodents previously thought to have gone extinct in the Miocene. Further work then showed the Laotian rock rat to be the sister-taxon to the Ctenodactylidae (Huchon et al., 2007), a family of rodents that also display the combination of a hystricomorphous skull with a sciurognathous mandible (Hautier, 2010). The Ctenodactylidae and Diatomyidae together form the sister-group the Hystricognathi, within the more inclusive clade Ctenohystrica (Fabre et al., 2012).\nOne particularly notable characteristic of the Laotian rock rat is the morphology of the zygomaticomandibularis muscle. This muscle is the innermost layer of the masseter, and in the hystricomorph condition extends anteriorly through the orbit and the grossly enlarged infraorbital foramen to take an attachment on the rostrum (Wood, 1965; Cox & Jeffery, 2011). In L. aenigmamus, the infraorbital portion of the zygomaticomandibularis (IOZM) is especially welldeveloped, extending over halfway along the rostrum (Hautier & Saksiri, 2009), which is itself exceptionally elongated compared to other rodents (Herrel et al., 2012). Various functions have\n19\n20\n21\n22\n23\n24\n25\n26\n27\n28\n29\n30\n31\n32\n33\n34\n35\n36\n37\n38\n39\n40\n41\n42\n43\n44\n45\n46\n47\n48\n49\nPeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013)\nR ev ie w in g M an\nus cr ip t\nbeen proposed for the IOZM. Becht (1953) noted that its origin on the rostrum between the incisors and cheek teeth would enable it to function as a second class lever during chewing at the molars, but as a third class lever during gnawing at the incisors. Thus, it can produce both fine control at the incisors and strong pressure at the molars. Using electromyography, Weijs & Dantuma (1975) found that the IOZM was firing at low intensities during jaw opening in rats, and suggested that it may have a role in the fine control of the opening phase of mastication. With regard to L. aenigmamus specifically, Herrel et al. (2012) proposed that the strong development of the IOZM and its anterior origin on the rostrum would produce a strong horizontal force component to the bite. This would result in optimal functioning at low gape angles and the generation of uniform force along the tooth row, both of which would be beneficial for a folivorous diet, which has been suggested for the Laotian rock rat (Scopin et al., 2011).\nThe aim of this study is to investigate the biomechanics of feeding in Laonastes aenigmamus. Finite element analysis (FEA) will be used to examine the response of the cranium during gnawing at the incisors and chewing at the molars. FEA is a computer-based engineering technique that enables the prediction of stress, strain and deformation in a complex geometric object subjected to a load (Rayfield, 2007). It has been successfully used to study the biomechanics of feeding in a number of mammalian groups (e.g. Dumont, Piccirillo & Grosse, 2005; Kupczik et al., 2007; Wroe, 2010; Bright & Rayfield, 2011) including rodents (Cox et al., 2011, 2012). The major advantage of FEA in the study of biological systems is that elements such as muscle or bone can be modified or removed at will, without the practical and ethical concerns that would arise with in vivo work. In this study, the contribution of the IOZM to feeding will be investigated by changing its attachment site and removing it altogether. In this way, we aim to understand the function of the IOZM, which is so highly developed in the Laotian rock rat, and to elucidate why such an unusual morphology has evolved in this species.",
    "v2_Abstract": "The Laotian rock rat, Laonastes aenigmamus, is one of the most recently discovered species of rodent, and displays a cranial morphology that is highly specialised. The rostrum of L. aenigmamus is exceptionally elongate and bears a large attachment site for the infraorbital portion of the zygomaticomandibularis (IOZM), which is particularly well-developed in this species. In this study, we used finite element analysis to investigate the biomechanical performance of the Laotian rock rat cranium and to elucidate the function of the IOZM muscle. A finite element model of the skull of L. aenigmamus was constructed and solved for biting on each of the teeth (incisors, premolar and molars). Further load cases were created and solved in which the origin of the IOZM had been moved anteriorly and posteriorly along the rostrum. Finally, a set of load cases were produced in which the IOZM was removed entirely, and its force was redistributed between the remaining masticatory muscles. The analysis showed that, during biting, the most stressed areas of the skull were the zygomatic and orbital regions. Compared to other rodents, L. aenigmamus is highly efficient at incisor gnawing, but less efficient at molar chewing. However, a relatively constant bite force across the molar tooth row may be an adaptation to folivory. Neither movement nor removal of the IOZM had much effect on the patterns of von Mises stresses, or the overall stress experienced by the cranium. However, the positioning and presence of the IOZM did have a large effect on bite force. Moving the IOZM origin to the anterior tip of the rostrum led to a substantially reduced bite force at all teeth. This was hypothesised to be a result of the increasing horizontal component to the pull of this muscle as it moves anteriorly along the rostrum. Removal of the IOZM also resulted in a reduced bite force, even when the total input muscle force is maintained at the same level, and in addition increased the total deformation across the skull. It was thus concluded that the function of the IOZM in L. aenigmamus is to increase bite force whilst reducing cranial deformation. If the IOZM can be shown to have this function in other rodent groups, this may help explain the evolution of this muscle, and may also provide an understanding of why it has evolved independently several times within rodents.",
    "v2_col_introduction": "introduction  : The Rodentia is the most speciose of all mammalian orders, with over 2,200 extant species (Wilson & Reeder, 2005) in addition to a large number of fossil forms. Despite such specific diversity, morphological variation within the order is relatively limited, particularly with regard to the skull and mandible (Wood, 1965; Hautier et al., 2011). Hence, determining relationships between rodent species based on morphology is difficult, and rodent taxonomy has been historically controversial, with two competing classifications arising in the second half of the 19 th century. First, Brandt (1855) split the rodents into three suborders (Sciuromorpha, Hystricomorpha and Myomorpha) based on the morphology of the masseter muscle and its attachment to the rostrum (see Cox & Jeffery, 2011 for details). Later, Tullberg (1899) divided the rodents into two groups (Sciurognathi and Hystricognathi) based on the position of the angular process of the mandible relative to the incisor. Recent molecular phylogenies (Blanga-Kanfi et al., 2009; Churakov et al., 2010; Fabre et al., 2012) have shown that neither of these classifications accurately resolves the evolutionary relationships between the rodents. The three suborders of Brandt (1855), also used by Simpson (1945), have now been discarded, as they are thought to represent polyphyletic groupings of rodents, although the names have been retained in their adjectival form (sciuromorphous, etc) to describe the three morphotypes of the skull and\n1\n2 3 4 5 6 7 8\n9 10 11 12 13\n14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30\nPeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013)\nR ev ie w in g M an\nus cr ip t\nmasseter (Wood, 1965; Cox & Jeffery, 2011). The classification of Tullberg (1899) has fared slightly better, with the Hystricognathi still recognised as a monophyletic clade (the Sciurognathi is paraphyletic with respect to the Hystricognathi). However, recent work (Hautier et al., 2011) has questioned the usefulness of sciurognathy and hystricognathy as morphological terms, noting that there exists a continuum of mandibular forms rather than two discrete morphologies, and that some members of the Hystricognathi actually have mandibles that appear to be almost sciurognathous in form.\nThe Laotian rock rat, Laonastes aenigmamus (Jenkins et al., 2005), is a recently discovered species of rodent from south-east Asia. It shows an unusual mixture of cranial, mandibular and muscular morphologies, combining a large part of the zygomaticomandibularis muscle that extends through the enlarged infraorbital foramen to attach to the rostrum (Hautier & Saksiri, 2009) with a weak lateral displacement of the angular process of the mandible (Hautier et al., 2011). Thus, the Laotian rock rat brings together a hystricomorphous skull and masseter with a lower jaw that is intermediate between sciurognathous and hystricognathous. This combination of characters has made its phylogenetic relationships difficult to ascertain. When first described, a new family, the Laonastidae, was created to house L. aenigmamus (Jenkins et al., 2005). This family was placed within the Hystricognathi as the sister-group to African mole-rats (Bathyergidae) or the dassie rat (Petromuridae). A subsequent analysis (Dawson et al., 2006) showed that L. aenigmamus was in fact a member of the Diatomyidae, a family of rodents previously thought to have gone extinct in the Miocene. Further work then showed the Laotian rock rat to be the sister-taxon to the Ctenodactylidae (Huchon et al., 2007), a family of rodents that also display the combination of a hystricomorphous skull with a sciurognathous mandible (Hautier, 2010). The Ctenodactylidae and Diatomyidae together form the sister-group the Hystricognathi, within the more inclusive clade Ctenohystrica (Fabre et al., 2012).\nOne particularly notable characteristic of the Laotian rock rat is the morphology of the zygomaticomandibularis muscle. This muscle is the innermost layer of the masseter, and in the hystricomorph condition extends anteriorly through the orbit and the grossly enlarged infraorbital foramen to take an attachment on the rostrum (Wood, 1965; Cox & Jeffery, 2011). In L. aenigmamus, the infraorbital portion of the zygomaticomandibularis (IOZM) is especially well-developed, extending over halfway along the rostrum (Hautier & Saksiri, 2009), which is itself exceptionally elongated compared to other rodents (Herrel et al., 2012). Various functions\n31 32 33 34 35 36 37\n38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54\n55 56 57 58 59 60 61\nPeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013)\nR ev ie w in g M an\nus cr ip t\nhave been proposed for the IOZM. Becht (1953) noted that its origin on the rostrum between the incisors and cheek teeth would enable it to function as a second class lever during chewing at the molars, but as a third class lever during gnawing at the incisors. Thus, it can produce both fine control at the incisors and strong pressure at the molars. Using electromyography, Weijs & Dantuma (1975) found that the IOZM was firing at low intensities during jaw opening in rats, and suggested that it may have a role in the fine control of the opening phase of mastication. With regard to L. aenigmamus specifically, Herrel et al. (2012) proposed that the strong development of the IOZM and its anterior origin on the rostrum would produce a strong horizontal force component to the bite. This would result in optimal functioning at low gape angles and the generation of uniform force along the tooth row, both of which would be beneficial for a folivorous diet, which has been suggested for the Laotian rock rat (Scopin et al., 2011).\nThe aim of this study is to investigate the biomechanics of feeding in Laonastes aenigmamus. Finite element analysis (FEA) will be used to examine the response of the cranium during gnawing at the incisors and chewing at the molars. FEA is a computer-based engineering technique that enables the prediction of stress, strain and deformation in a complex geometric object subjected to a load (Rayfield, 2007). It has been successfully used to study the biomechanics of feeding in a number of mammalian groups (Dumont, Piccirillo & Grosse, 2005; Kupczik et al., 2007; Wroe, 2010; Bright & Rayfield, 2011) including rodents (Cox et al., 2011, 2012). The major advantage of FEA in the study of biological systems is that elements such as muscle or bone can be modified or removed at will, without the practical and ethical concerns that would arise with in vivo work. In this study, the contribution of the IOZM to feeding will be investigated by changing its attachment site and removing it altogether. In this way, we aim to understand the function of the IOZM, which is so highly developed in the Laotian rock rat, and to elucidate why such an unusual morphology has evolved in this species.",
    "v1_text": "materials & methods : results : Figure 3 shows the von Mises stress patterns generated across the skull of L. aenigmamus during biting at the incisor, first molar and third molar. It can be seen that, aside from the biting tooth, the zygomatic arch is the most stressed region of the skull, followed by the orbital region. The rostrum experiences a moderate degree of stress during incisor gnawing, but is unstressed during molar chewing, and the occipital region is unstressed during all bites. From visual inspection of the stress distribution figures it is difficult to determine a great deal of variation between bites at different points along the tooth row, even between incisors and molars. However, by studying the mean von Mises stresses across the skull (Table 3) it can be seen that there are indeed subtle differences between bites on different teeth. In general, overall stress increases as the bite point moves closer to the jaw articulation from the premolar to the third molar. However, the incisor bite does not fit this trend, and shows the greatest mean stress across the skull of all bites, presumably because the rostrum is stressed in incisor bites but not in molar bites. The maximum bite force predicted at each tooth is given in Table 4. As would be expected from simple lever mechanics, bite force increases the closer the bite point (the biting tooth) is to the fulcrum (the TMJ). By dividing the bite forces by total input muscle force, the mechanical efficiency of biting has been calculated and can be compared with previous analyses on other rodents (Cox et al., 2012). It can be seen (Figure 4) that the Laotian rock rat performs well at the incisors and premolars, outcompeting the squirrel and guinea pig, and even matching the rat for efficiency at the incisors. However, compared to the other rodents, the mechanical efficiency tails off towards the distal molars and at the third molar, L. aenigmamus has the least efficient bite. The impact of the IOZM muscle was investigated by altering the position of its origin on the rostrum in VOX-FE. Figure 3 shows the von Mises stress patterns generated across the skull in models with the origin of the IOZM moved anteriorly and posteriorly, as well as in its original position. Despite quite large changes in muscle origins, very few differences can be seen between the models. This situation is confirmed by examining the mean von Mises stresses across the skull (Table 3). It can be seen that moving the IOZM origin anteriorly or posteriorly results in very little change (less than 6%) in mean von Mises stress during both incisor gnawing and molar 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t biting. Although not presented here, the same lack of difference was found on examination of the principal strains across the skull. Predicted bite force was also affected very little by moving the IOZM origin caudally to the most posterior part of the rostrum. However, Table 4 shows that an anterior shift of the IOZM origin resulted in quite a substantial reduction in bite force (between 10 and 13%). The effect of removing the IOZM muscle (and redistributing its force between the remaining masticatory muscles) can be seen in Figure 3. As when the IOZM origin was moved, few differences could be detected between the von Mises stress patterns by visual inspection alone, and again, this assertion is supported by examination of the mean von Mises stresses across the skull (Table 3). However, it can be seen from Table 4 that the action of the IOZM has a noticeable effect on bite force. It is clear that the presence of the IOZM enables L. aenigmamus to generate a greater bite force than if it were absent, even when the total input muscle force is the same. The reduction in bite force resulting from removal of the IOZM is approximately 10% for bites on both incisors and molars, which is similar to the effect of shifting the origin of this muscle to the anterior part of the rostrum. In order to analyse the subtle differences between the deformation patterns generated by the various load cases described above, a geometric morphometric analysis was performed. Landmark data from the original solved models, the models with the IOZM moved anteriorly and posteriorly, and models with the IOZM removed and its force redistributed, as well as landmarks from the unsolved model, were all subjected to Procrustes superimposition and PCA. The plot of the first two principal components (together comprising over 98% of the variation; PC1, 63.2%; PC2, 35.1%) is shown in Figure 5. The first principal component largely separates the unloaded model from the solved load cases. The point representing the unloaded model (star symbol) is on the far left of the plot whilst the points representing the loaded models are spread out down the right hand side of the diagram. The warped reconstructions indicate that the difference in deformation between the loaded and unloaded models is largely concentrated in the zygomatic region (also shown in Fig. 3). The incisor bites (squares) are clearly separated from bites on the other teeth on PC2. The cheek teeth bites are more closely grouped together, but separable into bites on each of the different teeth and positioned in order along the second principal component from the premolar (diamonds) to the third molar (lines). It can be seen from the reconstructions at the extremes of PC2 that incisor bites tend to deflect the rostrum dorsally (relative to the orbito- 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t temporal region), but that molar bites lead to a dorsal movement of the orbital region (relative to the rostrum). Within bites on each tooth, the models with the IOZM origin in its original position (blue symbols), moved anteriorly (green symbols) and moved posteriorly (red symbols) all group together closely at a similar distance from the unloaded model. This demonstrates that a similar amount of deformation is occurring in each of these load cases. However, the symbols representing the models with the IOZM muscle force redistributed between the other masticatory muscles (orange symbols), are positioned further from the unloaded model than the other load cases, indicating that even more deformation is occurring in these models. As the displacement of these symbols is along PC1, it can be seen that the redistribution of the IOZM is leading to greater deformation mainly in the zygomatic region. acknowledgements : We thank Jean-Pierre Hugot for providing access to the specimens and Dominique Adriaens, Loes Brabant and Luc Van Hoorebeke for scanning the specimens at the University of Ghent CT 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t facility (UGCT). We are grateful to Paul O\u2019Higgins and Michael Fagan for access to VOX-FE finite element software. Thanks are due to Laura Fitton for help with model construction, Peter Bazira for technical support with high-performance computing, and Andrew McIntosh and Thomas P\u00fcschel for assistance with GMM software. discussion : A finite element model of the skull of the Laotian rock rat, Laonastes aenigmamus, was created, loaded, constrained and solved. It was shown that the area of the skull experiencing the highest levels of stress was the zygomatic arch, including the zygomatic processes of the maxillary and frontal bones, which is likely to be a result of the large amount of masticatory musculature that attaches directly to this area. Similarly high zygomatic stresses have been noted in other rodents (Cox et al., 2012) as well as in other mammalian groups (Dumont, Piccirillo & Grosse, 2005; Bright & Rayfield, 2011; Dumont et al., 2011). It has been suggested that, in primates, the downward pull of the masseter muscle on the zygomatic arch may be counterbalanced to some degree by the upward pull of a soft tissue structure, namely the temporal fascia (Curtis et al., 2011). Despite many careful dissections (e.g. Baverstock, Jeffery & Cobb, 2013), no temporal fascia has been found in rodents, and this is also true of L. aenigmamus (Hautier & Saksiri, 2009). Thus, for the time being, it must be assumed that, although the zygomatic stresses are high in L. aegnimamus, they are not so high as to pose a danger of bone fracture. The bite forces predicted in this study demonstrate that the skull of L. aenigmamus can generate bites of 29 N during gnawing and between 53 and 74 N during chewing. By dividing these values by the total input muscle force, the mechanical efficiency of biting was calculated. It was shown that the Laotian rock rat is particularly efficient at incisor biting, having a mechanical efficiency greater than squirrels or guinea pigs, and similar to that of rats (Cox et al., 2012), but is less efficient at molar bites compared to these three rodent species. This would seem to indicate that L. aenigmamus is well-adapted for gnawing and less so for chewing, a conclusion that at first 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t glance appears somewhat at odds with the suggested diet of this species, which is thought to be largely folivorous (Scopin et al., 2011). However, Onoda et al. (2011) have proposed that the generation of a uniform bite force across the tooth row may be beneficial in the processing of fibrous plant material such as leaves. Viewed in this light, L. aenigmamus is well-adapted to folivory with a difference of just 20 N between its premolar and M3 bites, compared to 30-35 N in squirrels and guinea pigs. One of the more intriguing results of this study is that the position of the IOZM muscle has little effect on the overall stress experienced by the skull of L. aenigmamus. It can be seen from both Figure 3 and Table 3 that neither the pattern of von Mises stresses nor the mean stress across the skull are greatly affected by moving the origin of the IOZM forwards or backwards. This is may be because the IOZM is contributing a relatively small proportion of the total muscle force (around 8%). Although the IOZM appears quite a large muscle in lateral view (see Hautier & Saksiri, 2009), it is relatively small compared to the superficial and deep masseters. Moreover, it also possesses the longest muscle fibres of all masticatory muscles, and so its PCSA (and hence muscle force) is comparatively reduced (Table 1). Thus, its ability to impact the overall stress patterns across the skull may be fairly limited and, furthermore, these patterns may be more strongly influenced by skull morphology than by muscle geometry. In contrast to the position of its attachment, the presence or absence of the IOZM has a much stronger effect on the cranial biomechanics of L. aenigmamus. Figure 5 shows that the symbols representing the models without the IOZM are positioned further from the unloaded model that the other load cases, indicating that the removal of the IOZM leads to greater deformation (and therefore strain) across the skull. Thus, one of the major functions of the IOZM, at least in L. aenigmamus, appears to be to minimise strain during feeding. This conclusion holds true at all bites, both gnawing and chewing. In addition to its effect on cranial deformation, it was also found that the IOZM has a strong impact on the bite force produced by L. aenigmamus (Table 4). Specifically, removing the IOZM altogether (and redistributing its force between the remaining masticatory muscles) reduces the bite force generated at all teeth. Similarly, moving the origin of the IOZM to the anterior tip of the rostrum reduces bite force to a similar degree. It is likely that this effect is a result of the wrapping of the IOZM around the zygomatic process of the maxilla, which means that as the 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t IOZM is moved forward, its vector of pull becomes more horizontal (Fig.1), and so less able to generate bite force. On the other hand, moving the origin of the IOZM posteriorly appears to have little effect on bite force. Interestingly, the reduction in bite force resulting from the anterior movement or removal of the IOZM is largely constant along the tooth row \u2013 around 10%. Therefore, it would seem that the extension of the zygomaticomandibularis on to the rostrum evolved in order to increase bite force (as proposed by Becht, 1953). Given that there is minimal difference in bite force between the models with the IOZM in its original position and those with the origin moved posteriorly, the position of the IOZM origin halfway along the rostrum may simply have coevolved with the lengthening of the rostrum in this species. Alternatively, Herrel et al. (2012) have suggested that the anterior insertion of the IOZM in L. aenigmamus increases the horizontal component of biting, which leads to anterior displacement of the mandible during jaw closing, which may be advantageous for the processing of leaves. Whatever the driving force behind the anterior extension of the IOZM, the results here indicate that the reason that the IOZM does not extend any further along the rostrum, as is seen in some other hystricomorph rodents such as the springhare (Offermans & De Vree, 1989) and capybara (M\u00fcller, 1933), is because this would lead to a reduction in bite force at both the incisors and cheek teeth (Table 4). The results of this study have provided some important insights into the role of the IOZM muscle in the feeding behaviour of L. aenigmamus. Further investigations into other rodents, particularly other hystricomorphs, will enable us to understand whether the ability of the IOZM to increase bite force is unique to L. aenigmamus or common to all rodents that possess it. If the latter scenario is true, this could provide a selective advantage that may have driven the evolution of the IOZM, and could explain why it has evolved independently in several rodent groups (Ctenohystrica, Dipodidae, Anomaluroidea and Gliridae). However, finite element models can only shed light on static loading, and therefore cannot inform about the dynamic processes to which the IOZM may contribute, such as propalineal (antero-posterior) movements of the lower jaw, jaw opening or the fine control of gnawing. These activities can be addressed with dynamic modelling techniques, such as multibody dynamics analysis (e.g. Jones et al., 2012), and may well provide a fruitful avenue of research in the future. corresponding author: philip g. cox : Address: Centre for Anatomical and Human Sciences, Hull York Medical School, University of Hull, Cottingham Road, Hull, HU6 7RX, UK Telephone: 01904 321744 Email: philip.cox@hyms.ac.uk model solution and analysis : The finite element model of Laonastes aenigmamus was solved for biting at each tooth along the dental arcade, using VOX-FE. Gnawing was assumed always to be bilateral, as a result of the close apposition of the incisors, whereas chewing at the premolars and molars was modelled unilaterally on the left side. Von Mises stress patterns across the skull and bite forces at the teeth were recorded from the solved models. Following Cox et al. (2011) and O\u2019Higgins et al. (2011), geometric morphometrics was used to study the deformation patterns across the cranium. Thus, a set of landmarks (3D co-ordinate data) was recorded from each loaded skull, as well as from the original unsolved model. The landmark set was partially based on that in Cox et al. (2011) and is shown in Figure 2. Landmarks were concentrated in the rostral, orbital and zygomatic regions, as these were the areas experiencing the highest strains. The landmarks were then subjected to a geometric morphometric analysis using MorphoJ software (Klingenberg, 2011). This consisted of co-registering the landmarks via Procrustes superimposition followed by a principal components analysis (PCA). Cranial deformations along PC axes were visualised using the EVAN toolbox (http://www.evan-society.org). The results generated by the Laotian rock rat model were compared to those found in other rodents, such as squirrels, guinea pigs and rats (Cox et al., 2012). As it difficult to compare absolute bite force between models of different sizes, the mechanical efficiency of biting was calculated. Mechanical efficiency of biting is the ratio of output bite force to input muscle force, and represents the proportion of muscle force that is translated into bite force i.e. is not lost to deformation of the mandible or generation of joint reaction force at the condyles (Dumont et al., 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t 2011). As it is a proportion, mechanical efficiency is size-independent and facilitates clearer comparisons between skulls of varying sizes. authors: philip g cox1, joanna kirkham2,3 and anthony herrel4 : 1Centre for Anatomical and Human Sciences, Hull York Medical School, University of Hull, Hull, UK 2Centre for Anatomical and Human Sciences, Hull York Medical School, University of York, York, UK 3College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK 4UMR CNRS/MNHN 7179, M\u00e9canismes adaptatifs: des organismes aux communaut\u00e9s, Paris, France sample and model creation : MicroCT scans of an adult female Laonastes aenigmamus (specimen number KY213), previously obtained for an earlier research project (Herrel et al., 2012), were provided by AH. Voxels were isometric and the voxel resolution was 0.137 mm. Further details of the scanning protocol can be found in Herrel et al. (2012). 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Avizo 7.0 (Visualization Sciences Group, Burlington, MA, USA) was used to create a 3D volume reconstruction of the cranium of the Laotian rock rat. The bone and teeth were segmented separately so that different elastic properties could be assigned to them. Within the incisors, the enamel, dentine and pulp were not differentiated, as the scan resolution was not sufficient to distinguish these materials from one another. In addition, varying the material properties of these components has been shown to make little difference to the overall deformation of the skull (Cox et al., 2011). For similar reasons, the periodontal ligament was not included (Wood et al., 2011). The model was converted to an eight-noded FE mesh by direct voxel conversion, using VOX-FE, custom-built FE software (Liu et al., 2012), resulting in a model of 1734787 elements. Material properties, constraints and loads Bone was assigned a Young\u2019s modulus of 17 GPa, and the teeth were given a Young\u2019s modulus of 30 GPa. Both materials were modelled to be linearly elastic and isotropic with a Poisson\u2019s ratio of 0.3. These values were based on previous nano-indentation work on rodents (Cox et al., 2012) and FE studies on other mammals (Kupczik et al., 2012). The model was constrained at three locations: the left and right temporo-mandibular joints (TMJ) and the biting tooth. The TMJs were constrained on the ventral surface of the zygomatic process of the squamosal in all three dimensions. However, the biting tooth was only constrained in the dorsoventral axis (i.e. perpendicular to the occlusal plane). The number of nodes constrained at each location varied between 191 and 221. Loads were added to the model to represent the forces generated by the following muscles: superficial masseter; deep masseter; zygomaticomandibularis (anterior, posterior and infraorbital parts); temporalis (main, orbital and posterior parts); internal pterygoid; and external pterygoid. Muscle attachment sites and directions of pull were assigned based on the detailed dissections presented in Hautier & Saksiri (2009) and are shown in Figure 1. Table 1 gives the muscle mass and mean fibre length of each masticatory muscle, measured from the dissection of specimen KY213. Muscle masses were converted to volumes, assuming a muscle density of 1.0564 gcm-3 (Murphy & Beardsley, 1974), and the physiological cross-sectional area (PCSA) of each muscle (given in Table 1) was calculated by dividing the muscle volume by mean fibre length. Muscle forces (Table 2) were calculated by multiplying PCSAs by an intrinsic muscle stress value of 0.3 Nmm-2 (van Spronsen et al., 1989). In order to investigate the function of the IOZM, alternate 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t versions of the models were created with the origin of this muscle moved anteriorly and posteriorly, and also with the IOZM omitted completely on each side. In the models with a posteriorly shifted IOZM, the insertion point of the IOZM was kept the same, resulting in a more vertically directed muscle vector. However, in the models with an anteriorly shifted IOZM, the insertion point was moved to reflect the wrapping of the muscle around the zygomatic process of the maxilla, resulting in a highly horizontally directed muscle vector (see Fig. 1). To facilitate comparisons between models with and without the IOZM whilst retaining the same overall input muscle load, where it was omitted, the force of the IOZM was redistributed across the remaining masticatory muscles. This was done in such a way as to preserve their relative proportions. The muscle forces applied in the absence of the IOZM are given in Table 2. masticatory biomechanics of the laotian rock rat, : Laonastes aenigmamus, and the Function of the Zygomaticomandibularis Muscle The Laotian rock rat, Laonastes aenigmamus, is one of the most recently discovered species of rodent, and displays a cranial morphology that is highly specialised. The rostrum of L. aenigmamus is exceptionally elongate and bears a large attachment site for the infraorbital portion of the zygomaticomandibularis muscle (IOZM), which is particularly well-developed in this species. In this study, we used finite element analysis to investigate the biomechanical performance of the Laotian rock rat cranium and to elucidate the function of the IOZM. A finite element model of the skull of L. aenigmamus was constructed and solved for biting on each of the teeth (incisors, premolar and molars). Further load cases were created and solved in which the origin of the IOZM had been moved anteriorly and posteriorly along the rostrum. Finally, a set of load cases were produced in which the IOZM was removed entirely, and its force was redistributed between the remaining masticatory muscles. The analysis showed that, during biting, the most stressed areas of the skull were the zygomatic and orbital regions. Compared to other rodents, L. aenigmamus is highly efficient at incisor gnawing, but less efficient at molar chewing. However, a relatively constant bite force across the molar tooth row may be an adaptation to folivory. Movement of the origin of the IOZM had little on the patterns of von Mises stresses, or the overall stress experienced by the cranium. However, removal of the IOZM had a substantial effect on the total deformation experienced by the skull. In addition, the positioning and presence of the IOZM had large impact on bite force. Moving the IOZM origin to the anterior tip of the rostrum led to a substantially reduced bite force at all teeth. This was hypothesised to be a result of the increasing horizontal component to the pull of this muscle as it is moved anteriorly along the rostrum. Removal of the IOZM also resulted in a reduced bite force, even when the total input muscle force was maintained at the same level. It was thus concluded that the function of the IOZM in L. aenigmamus is to increase bite force whilst reducing cranial deformation. If the IOZM can be PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t shown to have this function in other rodent groups, this may help explain the evolution of this muscle, and may also provide an understanding of why it has evolved independently several times within rodents. PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t 16, midpoint between 15 and 17; 17, anteriormost attachment of zygomatic arch to rostrum; : 18; posteriormost point of infraorbital margin; 19, point on ventrolateral margin of zygomatic : arch in same coronal plane as midpoint of M1; 20, apex of tubercle on anterior orbital margin; 21, dorsalmost point on orbital margin; 22, ventralmost point on orbital margin; 23, midpoint : between 21 and 24; 24, posteriormost point on orbital margin. Landmarks 11-24recorded on both sides. PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Figure 3 Predicted distribution of von Mises stresses across the skull of Laonastes aenigmamus. Arrows indicate the biting tooth. First column, incisor bites; second column, M1 bites; third column, M3 bites. First line, original models; second line, origin of IOZM moved anteriorly to front of rostrum; third line, origin of IOZM moved posteriorly to back of rostrum; fourth line, IOZM muscle force removed and redistributed proportionally between the remaining masticatory muscles. PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Figure 4 Mechanical efficiency of biting at each tooth Predicted from FE models of squirrel, guinea pig, rat and Laotian rock rat skulls. Data for squirrel, guinea pig and rat from Cox et al. (2012). Abbreviations: I, incisor; PM, premolar (absent in rats); M, molar. PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Figure 5 GMM analysis of cranial deformations in Laonastes aengimamus. Plot of the first two principal components from a GMM analysis of 24 cranial landmarks. Cranial reconstructions and thin-plate splines indicate shape changes (x 200) along the first and second principal components. PC1 and PC2 not to same scale. Key: star, unloaded model; squares, incisor bites; diamonds, premolar bites; triangles, M1 bites; circles, M2 bites; lines, M3 bites; blue, IOZM in original position; green, IOZM moved anteriorly; red, IOZM moved posteriorly; orange, IOZM force redistributed between other masticatory muscles. PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Table 1(on next page) Muscle mass (g), mean fibre length (mm) and PCSA (cm2) of masticatory muscles of Laonastes aenigmamus, specimen KY213. abbreviations: zm, zygomaticomandibularis : PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Mass (g) Fibre length (mm) PCSA (cm2) Superficial masseter 0.348 8.73 0.376 Deep masseter 0.235 7.71 0.287 Anterior ZM 0.285 9.16 0.293 Posterior ZM 0.033 3.70 0.084 Infraorbital ZM 0.152 9.71 0.148 Main temporalis 0.047 8.58 0.052 Orbital temporalis 0.019 4.67 0.037 Posterior temporalis 0.017 3.70 0.042 Internal pterygoid 0.188 6.34 0.280 External pterygoid 0.118 4.67 0.239 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Table 2(on next page) Muscle loads (N) applied to each side of each model in the presence and absence of the IOZM. PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t With IOZM Without IOZM Superficial masseter 11.31 12.30 Deep masseter 8.62 9.37 Anterior ZM 8.80 9.57 Posterior ZM 2.50 2.72 Infraorbital ZM 4.44 - Main temporalis 1.56 1.70 Orbital temporalis 1.12 1.21 Posterior temporalis 1.27 1.38 Internal pterygoid 8.40 9.13 External pterygoid 7.15 7.78 Total 55.17 55.17 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Table 3(on next page) Mean von Mises stress (MPa) across the skull. Abbreviations: I, incisor; PM, premolar; M, molar. PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Original model IOZM origin moved anteriorly IOZM origin moved posteriorly IOZM force redistributed I 2.31 2.27 2.32 2.34 PM 2.14 2.15 2.13 2.15 M1 2.29 2.28 2.23 2.28 M2 2.66 2.59 2.60 2.57 M3 3.04 2.86 2.99 2.95 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Table 4(on next page) Predicted bite forces (N) across the skull. Abbreviations: I, incisor; PM, premolar; M, molar. PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t Original model IOZM origin moved anteriorly IOZM origin moved posteriorly IOZM force redistributed I 29.27 25.74 29.05 26.61 PM 53.74 46.93 53.72 48.80 M1 61.00 54.93 61.70 55.78 M2 68.97 61.92 70.32 62.05 M3 73.93 65.35 74.38 66.61 PeerJ reviewing PDF | (v2013:07:700:1:0:CHECK 23 Aug 2013) R ev ie w in g M an us cr ip t",
    "v2_text": "materials & methods : results : Figure 2 shows the von Mises stress patterns generated across the skull of L. aenigmamus during biting at the incisor, first molar and third molar. It can be seen that, aside from the biting tooth, the zygomatic arch is the most stressed region of the skull, followed by the orbital region. The rostrum experiences a moderate degree of stress during incisor gnawing, but is unstressed during molar chewing, and the occipital region is unstressed during all bites. From visual inspection of the stress distribution figures it is difficult to determine a great deal of variation between bites at different points along the tooth row, even between incisors and molars. However, by studying the mean von Mises stresses across the skull (Table 3) it can be seen that there are indeed subtle differences between bites on different teeth. In general, overall stress increases as the bite point moves closer to the jaw articulation from the premolar to the third molar. However, the incisor bite does not fit this trend, and shows the greatest mean stress across the skull of all bites, presumably because the rostrum is stressed in incisor bites but not in molar bites. The maximum bite force predicted at each tooth is given in Table 4. As would be expected from simple lever mechanics, bite force increases the closer the bite point (the biting tooth) is to the fulcrum (the TMJ). By dividing the bite forces by total input muscle force, the mechanical efficiency of biting has been calculated and can be compared with previous analyses on other rodents (Cox et al., 2012). It can be seen (Figure 3) that the Laotian rock rat performs well at the incisors and premolars, outcompeting the squirrel and guinea pig, and even matching the rat for efficiency at the incisors. However, compared to the other rodents, the mechanical efficiency tails off towards the distal molars and at the third molar, L. aenigmamus has the least efficient bite. The impact of the IOZM muscle was investigated by altering the position of its origin on the rostrum in VOX-FE. Figure 2 shows the von Mises stress patterns generated across the skull in models with the origin of the IOZM moved anteriorly and posteriorly, as well as in its original position. Despite quite large changes in muscle origins, very few differences can be seen between the models. This situation is confirmed by examining the mean von Mises stresses across the skull (Table 3). It can be seen that moving the IOZM origin anteriorly or posteriorly results in very little change (less than 6%) in mean von Mises stress during both incisor gnawing and molar 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t biting. Although not presented here, the same lack of difference was found on examination of the principal strains across the skull. Predicted bite force was also affected very little by moving the IOZM origin caudally to the most posterior part of the rostrum. However, Table 4 demonstrates that an anterior shift of the IOZM origin resulted in quite a substantial reduction in bite force (between 10 and 13%). The effect of removing the IOZM muscle (and redistributing its force between the remaining masticatory muscles) can be seen in Figure 2. As when the IOZM origin was moved, few differences could be detected between the von Mises stress patterns by visual inspection alone, and again, this assertion is supported by examination of the mean von Mises stresses across the skull (Table 3). However, it can be seen from Table 4 that the action of the IOZM has a noticeable effect on bite force. It is clear that the presence of the IOZM enables L. aenigmamus to generate a greater bite force than if it were absent, even when the total input muscle force is the same. The reduction in bite force resulting from removal of the IOZM is approximately 10% for bites on both incisors and molars, which is similar to the effect of shifting the origin of this muscle to the anterior part of the rostrum. In order to analyse the subtle differences between the deformation patterns generated by the various load cases described above, a geometric morphometric analysis was performed. Landmark data from the original solved models, the models with the IOZM moved anteriorly and posteriorly, and models with the IOZM removed and its force redistributed, as well as landmarks from the unsolved model, were all subjected to Procrustes superimposition and PCA. The plot of the first two principal components (together comprising over 98% of the variation; PC1, 63.2%; PC2, 35.1%) is shown in Figure 4. The first principal component largely separates the unloaded model from the solved load cases. The point representing the unloaded model (star symbol) is on the far left of the plot whilst the points representing the loaded models are spread out down the right hand side of the diagram. The warped reconstructions indicate that the difference in deformation between the loaded and unloaded models is largely concentrated in the zygomatic region. The incisor bites (squares) are clearly separated from bites on the other teeth. The cheek teeth bites are more closely grouped together, but separable into bites on each of the different teeth and positioned in order along the second principal component from the premolar (diamonds) to the third molar (lines). It can be seen from the reconstructions at the extremes of PC2 that incisor bites tend to deflect the rostrum dorsally (relative to the orbito-temporal region), 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t but that molar bites lead to a ventral deflection of the rostrum. Within bites on each tooth, the models with the IOZM origin in its original position (blue symbols), moved anteriorly (green symbols) and moved posteriorly (red symbols) all group together closely at a similar distance from the unloaded model. This demonstrates that a similar amount of deformation is occurring in each of these load cases. However, the symbols representing the models with the IOZM muscle force redistributed between the other masticatory muscles (orange symbols), are positioned further from the unloaded model than the other load cases, indicating that even more deformation is occurring in these models. As the displacement of these symbols is along PC1, it can be inferred that the redistribution of the IOZM is leading to greater deformation in the zygomatic region. acknowledgements : We thank Jean-Pierre Hugot for providing access to the specimens and Dominique Adriaens, Loes Brabant and Luc Van Hoorebeke for scanning the specimens at the University of Ghent CT facility (UGCT). We are grateful to Paul O\u2019Higgins and Michael Fagan for access to VOX-FE finite element software. Thanks are due to Laura Fitton for help with model construction, Peter 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t Bazira for technical support with high-performance computing, and Andrew McIntosh and Thomas P\u00fcschel for assistance with GMM software. discussion : A finite element model of the skull of the Laotian rock rat, Laonastes aenigmamus, was created, loaded and solved. It was shown that the area of the skull experiencing the highest levels of stress was the zygomatic arch, including the zygomatic processes of the maxillary and frontal bones, which is likely to be a result of the large amount of masticatory musculature that attaches directly to this area. Similarly high zygomatic stresses have been noted in other rodents (Cox et al., 2012) as well as in other mammalian groups (Dumont, Piccirillo & Grosse, 2005; Bright & Rayfield, 2011; Dumont et al., 2011). It has been suggested that, in primates, the downward pull of the masseter muscle on the zygomatic arch may be counterbalanced to some degree by the upward pull of a soft tissue structure, namely the temporal fascia (Curtis et al., 2011). Despite many careful dissections (e.g. Baverstock, Jeffery & Cobb, 2013), no temporal fascia has been found in rodents, and this is also true of L. aenigmamus (Hautier & Saksiri, 2009). However, there is a possibility that one, or part of one, of the masticatory muscles may be acting as a counterbalance. Specifically, if part of the temporalis were to run from the temporal fossa to the zygomatic arch it could have this role. This function of the temporalis is at the moment hypothetical and will require more detailed dissections of L. aenigmamus for confirmation. The bite forces predicted in this study demonstrate that the skull of L. aenigmamus can generate bites of 29 N during gnawing and between 53 and 74 N during chewing. By dividing these values by the total input muscle force, the mechanical efficiency of biting was calculated. It was shown that the Laotian rock rat is particularly efficient at incisor biting, having a mechanical efficiency greater than squirrels or guinea pigs, and similar to that of rats (Cox et al., 2012), but are less 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t efficient at molar bites compared to these three rodent species. This would seem to indicate that L. aenigmamus is well-adapted for gnawing and less so for chewing, a conclusion that at first glance appears somewhat at odds with the suggested diet of this species, which is thought to be largely folivorous (Scopin et al., 2011). However, Onoda et al. (2011) have proposed that the generation of a uniform bite force across the tooth row may be beneficial in the processing of fibrous plant material such as leaves. Viewed in this light, L. aenigmamus is well-adapted to folivory with a difference of just 20 N between its premolar and M3 bites, compared to 30-35 N in squirrels and guinea pigs. One of the more intriguing results of this study is that the presence and position of the IOZM muscle has little effect on the overall stress experienced by the skull of L. aenigmamus. It can be seen from both Figure 2 and Table 3 that neither the pattern of von Mises stresses nor the mean stress across the skull are affected by moving the origin of the IOZM forwards or backwards, or by removing the IOZM altogether (and redistributing its force between the remaining muscles). This is probably because the IOZM is contributing a relatively small proportion of the total muscle force (around 8%). Although the IOZM appears quite a large muscle in lateral view (see Hautier & Saksiri, 2009), it is relatively small compared to the superficial and deep masseters. Moreover, it also possesses the longest muscle fibres of all masticatory muscles, and so its PCSA (and hence muscle force) is comparatively reduced (Table 1). Thus, its ability to impact the overall stress patterns across the skull may be fairly limited. In contrast to the lack of influence on stress distributions, it was found that the IOZM has a strong impact on the bite force produced by L. aenigmamus (Table 4). Specifically, removing the IOZM altogether (and redistributing its force between the remaining masticatory muscles) reduces the bite force generated at all teeth. Similarly, moving the origin of the IOZM to the anterior tip of the rostrum reduces bite force to a similar degree. It is likely that this effect is a result of the wrapping of the IOZM around the zygomatic process of the maxilla, which means that as the IOZM is moved forward, its vector of pull becomes more horizontal, and so less able to generate bite force. On the other hand, moving the origin of the IOZM posteriorly appears to have little effect on bite force. Interestingly, the reduction in bite force resulting from the anterior movement or removal of the IOZM is largely constant along the tooth row \u2013 around 10%. Furthermore, the GMM analysis of deformations (Figure 4) has shown that the absence of the IOZM leads to increased deformations across the skull, again at bites on all teeth. Therefore, it would seem that 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t the extension of the zygomaticomandibularis on to the rostrum evolved in order to increase bite force (as proposed by Becht, 1953) and to decrease cranial deformation. Given that there is minimal difference in bite force between the models with the IOZM in its original position and those with the origin moved posteriorly, the position of the IOZM origin halfway along the rostrum may simply have coevolved with the lengthening of the rostrum in this species. Alternatively, Herrel et al. (2012) have suggested that the anterior insertion of the IOZM in L. aenigmamus increases the horizontal component of biting, which leads to anterior displacement of the mandible during jaw closing, which may be advantageous for the processing of leaves. Whatever the driving force behind the anterior extension of the IOZM, the results here indicate that the reason that the IOZM does not extend any further along the rostrum, as is seen in some other hystricomorph rodents such as the springhare (Offermans & De Vree, 1989) and capybara (M\u00fcller, 1933), is because this would lead to a reduction in bite force at both the incisors and cheek teeth. The results of this study have provided some important insights into the role of the IOZM muscle in the feeding behaviour of L. aenigmamus. Further investigations into other rodents, particularly other hystricomorphs, will enable us to understand whether the ability of the IOZM to increase bite force is unique to L. aenigmamus or common to all rodents that possess it. If the latter scenario is true, this could provide a selective advantage that may have driven the evolution of the IOZM, and could explain why it has evolved independently in several rodent groups (Ctenohystrica, Dipodidae, Anomaluroidea and Gliridae). However, finite element models can only shed light on static loading, and therefore cannot inform about the dynamic processes to which the IOZM may contribute, such as propalineal (antero-posterior) movements of the lower jaw, jaw opening or the fine control of gnawing. These activities can be addressed with dynamic modelling techniques, such as multibody dynamics analysis (e.g. Jones et al., 2012), and may well provide a fruitful avenue of research in the future. corresponding author: philip g. cox : Address: Centre for Anatomical and Human Sciences, Hull York Medical School, University of Hull, Cottingham Road, Hull, HU6 7RX, UK Telephone: 01904 321744 Email: philip.cox@hyms.ac.uk tables : Table 1. Muscle mass (g), mean fibre length (mm) and PCSA (cm2) of masticatory muscles in Laonastes aenigmamus, specimen KY213. Mass (g) Fibre length (mm) PCSA (cm2) Superficial masseter 0.348 8.73 0.376 Deep masseter 0.235 7.71 0.287 Anterior ZM 0.285 9.16 0.293 Posterior ZM 0.033 3.70 0.084 Infraorbital ZM 0.152 9.71 0.148 Main temporalis 0.047 8.58 0.052 Orbital temporalis 0.019 4.67 0.037 Posterior temporalis 0.017 3.70 0.042 Internal pterygoid 0.188 6.34 0.280 External pterygoid 0.118 4.67 0.239 Abbreviations: ZM, zygomaticomandibularis. Table 2. Muscle loads (N) applied to each side of each model in the presence and absence of the IOZM. 407 408 409 410 411 412 414 415 416 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t Original model IOZM origin moved anteriorly IOZM origin moved posteriorly IOZM force redistribut ed I 29.27 25.74 29.05 26.61 PM 53.74 46.93 53.72 48.80 M1 61.00 54.93 61.70 55.78 M2 68.97 61.92 70.32 62.05 M3 73.93 65.35 74.38 66.61 418 419 420 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t Figure 1 Landmarks used in GMM analysis of skull deformations. Reconstruction of skull of Laonastes aenigmamus in (A) dorsal, (B) ventral and (C) left lateral view. 1, anteriormost point on internasal suture; 2, midpoint on cranium between anterior roots of zygomatic arch; 3, midpoint between medialmost points on orbital margins; 4, midpoint on skull roof between zygomatic processes of the squamosal; 5, posteriormost point in dorsal midline; 6, midpoint between ventral margins of incisal alveoli; 7, midpoint between anteriormost points of premolars; 8, posteriormost midline point on palate; 9, midpoint between posterior margins of pterygoid flanges; 10, ventralmost point on margin of foramen magnum; 11, anteriormost point on naso-frontal suture; 12, dorsalmost point on incisal alveolar margin; 13, rostralmost point of infraorbital fossa; 14, midpoint between incisor and premolar on ventrolateral rostral margin; 15, midpoint of dorsal margin of infraorbital fossa; 16, midpoint between 15 and 17; 17, anteriormost attachment of zygomatic arch to rostrum; 18; posteriormost point of infraorbital margin; 19, point on ventrolateral margin of zygomatic arch in same coronal plane as midpoint of M1; 20, apex of tubercle on anterior orbital margin; 21, dorsalmost point on orbital margin; 22, ventralmost point on orbital margin; 23, midpoint between 21 and 24; 24, posteriormost point on orbital margin. Landmarks 11-24 recorded on both sides. PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t Figure 2 Predicted distribution of von Mises stresses across the skull of Laonastes aenigmamus. Arrows indicate the biting tooth. Top row, incisor bites; middle row, M1 bites; bottom row, M3 bites. First column, original models; second column, origin of IOZM moved anteriorly to front of rostrum; third column, origin of IOZM moved posteriorly to back of rostrum; fourth column, IOZM muscle force removed and redistributed proportionally between the remaining masticatory muscles. PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t Figure 3 Mechanical efficiency of biting at each tooth. Predicted from FE models of squirrel, guinea pig, rat and Laotian rock rat skulls. Data for squirrel, guinea pig and rat from Cox et al. (2012). Abbreviations: I, incisor; PM, premolar (absent in rats); M, molar. PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t Figure 4 GMM analysis of cranial deformations in Laonastes aenigmamus. Plot of the first two principal components from a GMM analysis of 24 cranial landmarks. Cranial reconstructions indicate shape changes (x 100) along the first and second principal components. Key: star, unloaded model; squares, incisor bites; diamonds, premolar bites; triangles, M1 bites; circles, M2 bites; lines, M3 bites; blue, IOZM in original position; green, IOZM moved anteriorly; red, IOZM moved posteriorly; orange, IOZM force redistributed between other masticatory muscles. PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t model solution and analysis : The finite element model of Laonastes aenigmamus was solved for biting at each tooth along the dental arcade, using VOX-FE. Gnawing was assumed always to be bilateral, as a result of the close apposition of the incisors, whereas chewing at the premolars and molars was modelled unilaterally on the left side. Von Mises stress patterns across the skull and bite forces at the teeth were recorded from the solved models. Following, Cox et al. (2011) and O\u2019Higgins et al. (2011), geometric morphometrics was used to study the deformation patterns across the cranium. Thus, a set of landmarks (3D co-ordinate data) was recorded from each loaded skull, as well as from the original unsolved model. The landmark set was partially based on that in Cox et al. (2011) and is shown in Figure 1. Landmarks were concentrated in the rostral, orbital and zygomatic regions, as these were the areas experiencing the highest strains. The landmarks were then subjected to a geometric morphometric analysis using MorphoJ software (Klingenberg, 2011). This consisted of co-registering the landmarks via Procrustes superimposition followed by a principal components analysis (PCA). Cranial deformations along PC axes were visualised using the EVAN toolbox (http://www.evan-society.org). The results generated by the Laotian rock rat model were compared to those found in other rodents, such as squirrels, guinea pigs and rats (Cox et al., 2012). As it difficult to compare absolute bite force between models of different sizes, the mechanical efficiency of biting was calculated. Mechanical efficiency of biting is the ratio of output bite force to input muscle force, and represents the proportion of muscle force that is translated into bite force i.e. is not lost to deformation of the mandible or generation of joint reaction force at the condyles (Dumont et al., 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t 2011). As it is a ratio, mechanical efficiency is size-independent and facilitates clearer comparisons between skulls of varying sizes. authors: philip g cox1, joanna kirkham2,3 and anthony herrel4 : 1Centre for Anatomical and Human Sciences, Hull York Medical School, University of Hull, Hull, UK 2Centre for Anatomical and Human Sciences, Hull York Medical School, University of York, York, UK 3College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK 4UMR CNRS/MNHN 7179, M\u00e9canismes adaptatifs: des organismes aux communaut\u00e9s, Paris, France sample and model creation : MicroCT scans of an adult female Laonastes aenigmamus (specimen number KY213), previously obtained for an earlier research project (Herrel et al., 2012), were provided by AH. Voxels were isometric and the voxel resolution was 0.137 mm. Further details of the scanning protocol can be found in Herrel et al. (2012). 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t Avizo 7.0 (Visualization Sciences Group, Burlington, MA, USA) was used to create a 3D volume reconstruction of the cranium of the Laotian rock rat. The bone and teeth were segmented separately so that different elastic properties could be assigned to them. Within the incisors, the enamel, dentine and pulp were not differentiated, as the scan resolution was not sufficient to distinguish these materials from one another. In addition, varying the material properties of these components has been shown to make little difference to the overall deformation of the skull (Cox et al., 2011). For similar reasons, the periodontal ligament was not included (Wood et al., 2011). The model was converted to an eight-noded FE mesh by direct voxel conversion, using VOX-FE, custom-built FE software (Liu et al., 2012), resulting in a model of 1734787 elements. Material properties, constraints and loads Bone was assigned a Young\u2019s modulus of 17 GPa, and the teeth were given a Young\u2019s modulus of 30 GPa. Both materials were modelled to be linearly elastic and isotropic with a Poisson\u2019s ratio of 0.3. These values were based on previous nano-indentation work on rodents (Cox et al.., 2012) and FE studies on other mammals (Kupczik et al.., 2012). The model was constrained at three locations: the left and right temporo-mandibular joints (TMJ) and the biting tooth. The TMJs were constrained on the ventral surface of the zygomatic process of the squamosal in all three dimensions. However, the biting tooth was only constrained in the dorsoventral axis (i.e. perpendicular to the occlusal plane). The number of nodes constrained at each location varied between 191 and 221. Loads were added to the model to represent the forces generated by the following muscles: superficial masseter; deep masseter; zygomaticomandibularis (anterior, posterior and infraorbital parts); temporalis (main, orbital and posterior parts); internal pterygoid; and external pterygoid. Muscle attachment sites and directions of pull were assigned based on the detailed dissections presented in Hautier & Saksiri (2009). Table 1 gives the muscle mass and mean fibre length of each masticatory muscle, measured from the dissection of specimen KY213. Muscle masses were converted to volumes, assuming a muscle density of 1.0564 gcm-3 (Murphy & Beardsley, 1974), and the physiological cross-sectional area (PCSA) of each muscle (given in Table 1) was calculated by dividing the muscle volume by mean fibre length. Muscle forces (Table 2) were calculated by multiplying PCSAs by an intrinsic muscle stress value of 0.3 Nmm-2 (van Spronsen et al., 1989). In order to investigate the function of the IOZM, alternate versions of the models 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 PeerJ reviewing PDF | (v2013:07:700:0:1:NEW 1 Aug 2013) R ev ie w in g M an us cr ip t were created with the origin of this muscle moved anteriorly and posteriorly, and also with the IOZM omitted completely on each side. In the models with a posteriorly shifted IOZM, the insertion point of the IOZM was kept the same, resulting in a more vertically directed muscle vector. However, in the models with an anteriorly shifted IOZM, the insertion point was moved to reflect the wrapping of the muscle around the zygomatic process of the maxilla, resulting in a highly horizontally directed muscle vector. To facilitate comparisons between models with and without the IOZM whilst retaining the same overall input muscle load, where it was omitted, the force of the IOZM was redistributed across the remaining masticatory muscles. This was done in such a way as to preserve their relative proportions. The muscle forces applied in the absence of the IOZM are given in Table 2.",
    "url": "https://peerj.com/articles/161/reviews/",
    "review_1": "Marco Iacoboni \u00b7 Aug 27, 2013 \u00b7 Academic Editor\nACCEPT\nThe addition of Experiment 2 not only clarifies the nature of the results in Experiment 1 but also provides strong evidence of how temporal dynamics are powerful modulators of these effects. An excellent study that will undoubtedly inspire more research on this important topic.",
    "review_2": "Marco Iacoboni \u00b7 Aug 22, 2013 \u00b7 Academic Editor\nMAJOR REVISIONS\nThe reviewers made a number of important comments that you may want to address in your revision. I also read the paper and have some comments, which partially overlap the comments of the reviewers.\n\nYour paper reports a clever new experimental variation of the automatic imitation task (AIT). You propose that this new task, the controlled imitation task (CIT), can be used in studies \u201cof self-other control processes and more generally, internally generated versus externally triggered actions.\u201d\n\nI think that with regard to the self-other control processes the task lacks an important control condition, that is, non biological cues requiring subjects to override their initially planned action. In this control condition, rather than seeing an action 100 ms after the cue had given subjects instructions on which action to perform, subjects would see either another symbolic or a spatial cue. The RT costs in the incongruent trials of this control task, subtracted to the RT costs in the CIT, would give us a true estimate of self-other control process. Without this control task, the only thing we can conclude is that the whopping RT costs (120 ms!) shown in this study are costs in reprogramming an action. Indeed, I am not even sure one can conclude that the task in its current form can give us an index of \u201cinternally generated versus externally triggered actions\u201d, since the first planned action is also externally cued.\n\nTwo other minor comments:\n\nWhy 100 ms? What happens if you manipulate that time interval? Does the RT cost get bigger, smaller, it disappear completely? These are definitely interesting follow up experimental manipulations.\n\nThere are documented orthogonal spatial s-r effects. I think you should look into that literature and make sure that those effects do no play a role here.",
    "review_3": "Jonas Kaplan \u00b7 Aug 15, 2013\nBasic reporting\nI find the manuscript to be well-written, clear, and concise. I enjoyed reading it. The reporting is simple and straight-forward.\nExperimental design\nThe experiment as it stands appears to be well-executed, but I do feel that the experiment is lacking an important control which would allow the authors to draw conclusions that they draw about the usefulness of this task for studying self-other issues. In order to say this result has anything to do with imitation, it would be important to use a cue that was not another person's movement. The current results, which show that people were slower to respond when their instruction was changed mid-trial by a hand moving, may be explained rather simply without invoking any explanation relating to imitation: people make slower responses when their instruction is changed mid-movement compared to when they are not. In fact, it\u2019s hard to see how any other result would be obtained regardless of what the interrupting cue was. It is clearly going to take more time to change a response than it is to not change a response.\n\nTherefore, to make the case that this task has any relevance to imitation, the authors would need to show that the imitative cue itself has some special power to interfere with self-inhibition, over and above the simple cost of changing action mid-stream. This concept is briefly discussed in the discussion, but it seems to me that it is crucial to the interpretation that the task involves a conflict between self and other representations, which is the way the manuscript frames the usefulness of this task. It would be a fairly simple control condition to add to the experiment.\n\n(\n-The N is fairly small for a simple behavioral study, although the results are significant nevertheless. I wonder about the impact of the handedness of the participants, given that 3 of the 12 were left handed. It appears that everyone used the right hand to respond and the cues from the figure appear to be a left hand such that participants were doing \"mirror imitation\".\n\n-How is reaction time measured? Is it the time when the key on the keyboard is released?\n)\nValidity of the findings\nAs discussed above, it seems to me that the results don't directly speak to the issue of imitation or self/other representation, and instead may be explained in simple motor terms: changing a target response midstream leads to slower responses compared with not changing it. I don't think the manuscript succeeds in making the case that this novel task is measuring anything about representations of self-other control as promised in the title.\n\nThat said, I like the idea of this task, and if it turns out that the interference effect is related to representations of observed movement it has the potential to be very useful as a complement to the AIT.\nAdditional comments\nNo comments.\nCite this review as\nKaplan J (2013) Peer Review #1 of \"The controlled imitation task: a new paradigm for studying self-other control (v0.1)\". PeerJ https://doi.org/10.7287/peerj.161v0.1/reviews/1",
    "review_4": "Reviewer 2 \u00b7 Aug 6, 2013\nBasic reporting\nThe authors introduce a novel task, based on the automatic imitation task, which aims at testing for prioritization of imitative responses. The basic rationale of the task is a useful addition, and if constructed well is likely to spawn a large number of studies using this addition, or extensions thereof.\n- It would be helpful to elaborate in more detail why the effects in the AIT are not explained by other processes, such as preparation and consequent competition resolution. In other words, what is so special about this specific process, or is there a unifying account \u2013 intuitively, having dedicated but separated systems for imitation and selection seems odd. For example, to which degree can prior expectation explain the reviewed results \u2013 participants have to respond against an expectation? It is hardly surprising that action representations with higher prior probability will be faster to express than unlikely ones, irrespective of the type of information on which such an expectation is build? See also my point below regarding the CIT. I also note that I am not an expert on action observation or imitation, so the difference may be obvious and thus easy to point out.\n- As far as I understand, the CIT is essentially a countermanding task, with matched probabilities. Other cousins include reprogramming tasks (cf work by Rushworth) and stopping tasks. I can see how the AIT is incomplete, and the CIT is therefore a useful addition, but embedding the task into the vast literature on related inhibition tasks would seem useful. This point is essential as it concerns the construct validity of the task, and this reviewer is not entirely sure the task divorces itself from the various paradigms in this literature.\n- It seems to me that the authors basically aim to extend the AIT to become a fully factorial task. Instead of being complementary, this author wonders if they are indeed both necessary, i.e. accurate inference requires both to be tested. One may be tempted to attribute any outcome to \u2018other\u2019 vs \u2018self\u2019 generated movements, but this essentially requires comparison of both, and neither task achieves that alone. Viewed in this way, the authors basically add another factor to the AIT, which distinguishes reference frame. That may or may not be seen as a novel task, but in any case advocating the use of the CIT as standalone seems to suffer from the same limitations as using the AIT alone, i.e. poor control for \u201creference frame\u201d.\n- Then again, having said the above, comparability between the AIT and CIT is somewhat limited because the temporal structure is different. One may deem this unlikely to explain any observed differences but in terms of experimental design and control this seems to be a problem. It would be useful to device a novel task in which the temporal structure in both tasks is matched\n- Please report measures of effect size\n- tDCS is a terrible method for anatomical inference. Such suggestions should be avoided as they permeate the field in unhealthy and incorrect ways. More generally, anatomical predictions would seem only the second step. First, what about the novel hypotheses concerning processing of observed actions that can now be addressed? As per my comment above, it would seem also that any anatomical prediction would have to include the extensive work on conflict, inhibition etc, and this reviewer wonders whether the current task would reveal anything surprising here.\n- Related, if the authors want to advocate the use of this task, it may be useful to anticipate modifications. One could vary the likelihood of incongruent/congruent trials, for example, which may help to address the previous issue about preparation/prior expectation. The authors use a fixed interval of 100ms between cue and target stimulus, which may not be optimal (nb. How was this interval chosen?). In any case, it may be useful to explore the temporal nature of the process, by varying this interval similar to SSRT tasks. Note also that there is debate about the nature of these processes, e.g. whether the selection and stop process occur in parallel or not, and what the dynamics of this process are.\nExperimental design\nsee above\nValidity of the findings\nsee above\nAdditional comments\nsee above\nCite this review as\nAnonymous Reviewer (2013) Peer Review #2 of \"The controlled imitation task: a new paradigm for studying self-other control (v0.1)\". PeerJ https://doi.org/10.7287/peerj.161v0.1/reviews/2",
    "pdf_1": "https://peerj.com/articles/161v0.2/submission",
    "pdf_2": "https://peerj.com/articles/161v0.1/submission",
    "review_5": "Hugo Th\u00e9oret \u00b7 Aug 1, 2013\nBasic reporting\nNo comments\nExperimental design\nNo comments\nValidity of the findings\nNo comments\nAdditional comments\nIn this paper, the authors present a novel task that requires participants to suppress self-activated motor reponses following an initial cue. The authors argue that this task, which complement the well known \u00ab automatic imitation task \u00bb, may be used to study self-other representations.\n\nThe paper presents a good argument for the development of such a task, and the protocol and task appear well thought. I do not have concerns regarding the manuscript as it stands. I wonder, however, how performance on the CIT correlates with performance on the classical AIT\u2026 It would have been quite interesting to have participants perform both tasks\u2026\nCite this review as\nTh\u00e9oret H (2013) Peer Review #3 of \"The controlled imitation task: a new paradigm for studying self-other control (v0.1)\". PeerJ https://doi.org/10.7287/peerj.161v0.1/reviews/3",
    "all_reviews": "Review 1: Marco Iacoboni \u00b7 Aug 27, 2013 \u00b7 Academic Editor\nACCEPT\nThe addition of Experiment 2 not only clarifies the nature of the results in Experiment 1 but also provides strong evidence of how temporal dynamics are powerful modulators of these effects. An excellent study that will undoubtedly inspire more research on this important topic.\nReview 2: Marco Iacoboni \u00b7 Aug 22, 2013 \u00b7 Academic Editor\nMAJOR REVISIONS\nThe reviewers made a number of important comments that you may want to address in your revision. I also read the paper and have some comments, which partially overlap the comments of the reviewers.\n\nYour paper reports a clever new experimental variation of the automatic imitation task (AIT). You propose that this new task, the controlled imitation task (CIT), can be used in studies \u201cof self-other control processes and more generally, internally generated versus externally triggered actions.\u201d\n\nI think that with regard to the self-other control processes the task lacks an important control condition, that is, non biological cues requiring subjects to override their initially planned action. In this control condition, rather than seeing an action 100 ms after the cue had given subjects instructions on which action to perform, subjects would see either another symbolic or a spatial cue. The RT costs in the incongruent trials of this control task, subtracted to the RT costs in the CIT, would give us a true estimate of self-other control process. Without this control task, the only thing we can conclude is that the whopping RT costs (120 ms!) shown in this study are costs in reprogramming an action. Indeed, I am not even sure one can conclude that the task in its current form can give us an index of \u201cinternally generated versus externally triggered actions\u201d, since the first planned action is also externally cued.\n\nTwo other minor comments:\n\nWhy 100 ms? What happens if you manipulate that time interval? Does the RT cost get bigger, smaller, it disappear completely? These are definitely interesting follow up experimental manipulations.\n\nThere are documented orthogonal spatial s-r effects. I think you should look into that literature and make sure that those effects do no play a role here.\nReview 3: Jonas Kaplan \u00b7 Aug 15, 2013\nBasic reporting\nI find the manuscript to be well-written, clear, and concise. I enjoyed reading it. The reporting is simple and straight-forward.\nExperimental design\nThe experiment as it stands appears to be well-executed, but I do feel that the experiment is lacking an important control which would allow the authors to draw conclusions that they draw about the usefulness of this task for studying self-other issues. In order to say this result has anything to do with imitation, it would be important to use a cue that was not another person's movement. The current results, which show that people were slower to respond when their instruction was changed mid-trial by a hand moving, may be explained rather simply without invoking any explanation relating to imitation: people make slower responses when their instruction is changed mid-movement compared to when they are not. In fact, it\u2019s hard to see how any other result would be obtained regardless of what the interrupting cue was. It is clearly going to take more time to change a response than it is to not change a response.\n\nTherefore, to make the case that this task has any relevance to imitation, the authors would need to show that the imitative cue itself has some special power to interfere with self-inhibition, over and above the simple cost of changing action mid-stream. This concept is briefly discussed in the discussion, but it seems to me that it is crucial to the interpretation that the task involves a conflict between self and other representations, which is the way the manuscript frames the usefulness of this task. It would be a fairly simple control condition to add to the experiment.\n\n(\n-The N is fairly small for a simple behavioral study, although the results are significant nevertheless. I wonder about the impact of the handedness of the participants, given that 3 of the 12 were left handed. It appears that everyone used the right hand to respond and the cues from the figure appear to be a left hand such that participants were doing \"mirror imitation\".\n\n-How is reaction time measured? Is it the time when the key on the keyboard is released?\n)\nValidity of the findings\nAs discussed above, it seems to me that the results don't directly speak to the issue of imitation or self/other representation, and instead may be explained in simple motor terms: changing a target response midstream leads to slower responses compared with not changing it. I don't think the manuscript succeeds in making the case that this novel task is measuring anything about representations of self-other control as promised in the title.\n\nThat said, I like the idea of this task, and if it turns out that the interference effect is related to representations of observed movement it has the potential to be very useful as a complement to the AIT.\nAdditional comments\nNo comments.\nCite this review as\nKaplan J (2013) Peer Review #1 of \"The controlled imitation task: a new paradigm for studying self-other control (v0.1)\". PeerJ https://doi.org/10.7287/peerj.161v0.1/reviews/1\nReview 4: Reviewer 2 \u00b7 Aug 6, 2013\nBasic reporting\nThe authors introduce a novel task, based on the automatic imitation task, which aims at testing for prioritization of imitative responses. The basic rationale of the task is a useful addition, and if constructed well is likely to spawn a large number of studies using this addition, or extensions thereof.\n- It would be helpful to elaborate in more detail why the effects in the AIT are not explained by other processes, such as preparation and consequent competition resolution. In other words, what is so special about this specific process, or is there a unifying account \u2013 intuitively, having dedicated but separated systems for imitation and selection seems odd. For example, to which degree can prior expectation explain the reviewed results \u2013 participants have to respond against an expectation? It is hardly surprising that action representations with higher prior probability will be faster to express than unlikely ones, irrespective of the type of information on which such an expectation is build? See also my point below regarding the CIT. I also note that I am not an expert on action observation or imitation, so the difference may be obvious and thus easy to point out.\n- As far as I understand, the CIT is essentially a countermanding task, with matched probabilities. Other cousins include reprogramming tasks (cf work by Rushworth) and stopping tasks. I can see how the AIT is incomplete, and the CIT is therefore a useful addition, but embedding the task into the vast literature on related inhibition tasks would seem useful. This point is essential as it concerns the construct validity of the task, and this reviewer is not entirely sure the task divorces itself from the various paradigms in this literature.\n- It seems to me that the authors basically aim to extend the AIT to become a fully factorial task. Instead of being complementary, this author wonders if they are indeed both necessary, i.e. accurate inference requires both to be tested. One may be tempted to attribute any outcome to \u2018other\u2019 vs \u2018self\u2019 generated movements, but this essentially requires comparison of both, and neither task achieves that alone. Viewed in this way, the authors basically add another factor to the AIT, which distinguishes reference frame. That may or may not be seen as a novel task, but in any case advocating the use of the CIT as standalone seems to suffer from the same limitations as using the AIT alone, i.e. poor control for \u201creference frame\u201d.\n- Then again, having said the above, comparability between the AIT and CIT is somewhat limited because the temporal structure is different. One may deem this unlikely to explain any observed differences but in terms of experimental design and control this seems to be a problem. It would be useful to device a novel task in which the temporal structure in both tasks is matched\n- Please report measures of effect size\n- tDCS is a terrible method for anatomical inference. Such suggestions should be avoided as they permeate the field in unhealthy and incorrect ways. More generally, anatomical predictions would seem only the second step. First, what about the novel hypotheses concerning processing of observed actions that can now be addressed? As per my comment above, it would seem also that any anatomical prediction would have to include the extensive work on conflict, inhibition etc, and this reviewer wonders whether the current task would reveal anything surprising here.\n- Related, if the authors want to advocate the use of this task, it may be useful to anticipate modifications. One could vary the likelihood of incongruent/congruent trials, for example, which may help to address the previous issue about preparation/prior expectation. The authors use a fixed interval of 100ms between cue and target stimulus, which may not be optimal (nb. How was this interval chosen?). In any case, it may be useful to explore the temporal nature of the process, by varying this interval similar to SSRT tasks. Note also that there is debate about the nature of these processes, e.g. whether the selection and stop process occur in parallel or not, and what the dynamics of this process are.\nExperimental design\nsee above\nValidity of the findings\nsee above\nAdditional comments\nsee above\nCite this review as\nAnonymous Reviewer (2013) Peer Review #2 of \"The controlled imitation task: a new paradigm for studying self-other control (v0.1)\". PeerJ https://doi.org/10.7287/peerj.161v0.1/reviews/2\nReview 5: Hugo Th\u00e9oret \u00b7 Aug 1, 2013\nBasic reporting\nNo comments\nExperimental design\nNo comments\nValidity of the findings\nNo comments\nAdditional comments\nIn this paper, the authors present a novel task that requires participants to suppress self-activated motor reponses following an initial cue. The authors argue that this task, which complement the well known \u00ab automatic imitation task \u00bb, may be used to study self-other representations.\n\nThe paper presents a good argument for the development of such a task, and the protocol and task appear well thought. I do not have concerns regarding the manuscript as it stands. I wonder, however, how performance on the CIT correlates with performance on the classical AIT\u2026 It would have been quite interesting to have participants perform both tasks\u2026\nCite this review as\nTh\u00e9oret H (2013) Peer Review #3 of \"The controlled imitation task: a new paradigm for studying self-other control (v0.1)\". PeerJ https://doi.org/10.7287/peerj.161v0.1/reviews/3\nReview 6: \n",
    "LLM_extracted_review": "['Review 1: The addition of Experiment 2 clarifies the results in Experiment 1 and provides strong evidence of how temporal dynamics modulate these effects. An excellent study that will inspire more research on this important topic.\\n\\nReview 2: The task lacks an important control condition for self-other control processes. Without this control task, the RT costs observed may only reflect reprogramming an action. The manipulation of the 100 ms interval and the consideration of orthogonal spatial s-r effects are suggested for further exploration.\\n\\nReview 3: The experiment is well-executed but lacks a control that would allow conclusions about imitation. The results may simply reflect slower responses when instructions change mid-trial, rather than any specific imitation effect. The small sample size and handedness of participants could impact results. The measurement of reaction time needs clarification.\\n\\nReview 4: The rationale of the task is useful, but the authors should elaborate on why AIT effects are not explained by other processes. The CIT may be similar to other inhibition tasks, and its construct validity is questioned. The temporal structure of both tasks should be matched for better comparison. Effect size measures should be reported, and anatomical inferences should be approached cautiously.\\n\\nReview 5: The authors present a well-thought-out task for studying self-other representations. However, it would be interesting to see how performance on the CIT correlates with the classical AIT.']"
}