mach bands
lane ej, proto av, phillips tw. mach bands and density perception .
mach bands were discovered by the austrian physicist ernst mach. mach is known for his work in optics, mechanics, and wave dynamics, as well as notable advances in the field of supersonic travel (the mach number is also named for him). mach discovered the physiological phenomenon of mach bands during the 1860s. [ 1 ]
knowledge of the phenomenon may help prevent mach bands being mistaken for a true abnormality such as pneumomediastinum .
ratliff, f. : why mach bands are not seen at the edges of a step? vision res .
daffner rh. pseudofracture of the dens: mach bands. ajr am j roentgenol 1977; 128: 607–12 .
discovered in the 1860s by ernst mach (hence the name), mach bands are actually a set of interrelated phenomena. take a look at this image :
lane ej, proto av, phillips tw. mach bands and density perception. radiology 1976; 121: 9–17 .
ratliff, floyd. mach bands: quantitative studies on neural networks in the retina. holden day, 1965. isbn 0816270457
we see mach bands everywhere on opening our eyes. look closely at the shadows cast on the ground, there are mach bands of light and dark strips near the edge of each shadow. in discovering biological psychology, the biological processes are elegantly explained .
mach bands, as well as numerous other visual and perceptual illusions, help scientists study the way the eye and brain process visual information .
du buf, j. : ramp edges, mach bands, and the functional significance of the simple cell assembly. biol. cybern .
3. gordenne wh, malchair fl. mach bands in mammography. radiology. 1988; 169 (1): 55 - 8 .
ross, j. , morrone, m. , burr, d. : the conditions under which mach bands are visible. vision res .
morrone, m. , ross, j. , burr, d. , owens, r. : mach bands depend on spatial phase. nature
mach bands are an optical illusion consisting of an image of two wide bands, one light and one dark, separated by a narrow strip with a light - to - dark gradient. the human eye perceives two narrow bands of different brightnesses either side of the gradient that are not present in the original image .
( 1 )
zdravković s. (2015) mach bands. in: luo r. (eds) encyclopedia of color science and technology. springer, berlin, heidelberg
lateral inhibition explains a famous visual illusion known as mach bands, named after their discoverer, physicist ernst mach (1838–1916). lateral inhibition accentuates the edges of the stimulus. mach noticed that when two bars, one dark and one bright, are next to each other, you see little bands of extra dark at the edge of the dark band and extra light at the edge of the light bar. these bands do not exist but are an illusion caused by lateral inhibition via our center - surround receptive fields. in this illustration, you will be able to manipulate some images to make mach bands appear and disappear. examine the figure to the right .
the following figure has a motion effect that some have described as
flowing electricity
. watch out. some people feel dizzy. the illusion combines mach bands and stripes (5 )
the bottom black line shows the physical intensity of light distributed across whilst the red line indicates our perception of light and dark mach band field with the dip indicating the dark mach band and the protrusion indicating the light mach band .
these optical illusions are largely just a bit of fun. the mach bands are relevant to plain film radiography because of the potential to produce the illusion of an enhanced / accentuated interface between two structures .
mach band is a perceptual phenomenon. when the human eye looks at two bands of colours, one light and one dark, side - by - side, a the eye perceives a narrow strip of gradient light to dark light, in the middle separating the two solid bands. however, this is not the actual image .
daffner rh. visual illusions in computed tomography: phenomena related to mach effect .
mach bands are an optical illusion where a band of gradients will appear in places to be lighter or darker than they actually are. mach bands, as well as numerous other visual and perceptual illusions, provide a valuable way to investigate how the eye and brain process visual information. equally, they are used by artists for visual effect, entertaining and satisfying the endless fascination human beings have with novelty and creativity .
mach bands are an optical illusion consisting of an image of two wide bands, one light and one dark, separated by a narrow strip with a light - to - dark gradient. the human eye perceives two narrow bands of different brightnesses either side of the gradient that are not present in the original image .
( 1) even when this image is magnified, the illusion persists - the solid grey appears to change where it meets the graduated ctripes
when viewing plain film images we tend to assume that what we see is a real and undistorted anatomical reality. mach bands are an optical illusion that leads us to perceive densities that do not exist in reality .
mach bands or the mach effect refers to an optical phenomenon from edge enhancement due to lateral inhibition of the retina 2. this is an inbuilt edge enhancement mechanism of the retina, where the edges of darker objects next to lighter objects will appear lighter and vice versa, creating a false shadow 4 .
↑ institute of physics, belgrade, ernst mach. retrieved october 12, 2007 .
the hering grid illusion, simultaneous contrast, and mach bands, described below, are three examples of the visible effects of lateral inhibition (usually you are quite unaware of it). the diagram below shows the hering grid .
lotto, r. b. , s. m. williams, and d. purves. mach bands as empirically derived associations. proceedings of the national academy of sciences. 96, 9 (1999): 5245 - 50 .
1 lotto, r. b. , williams, s. m. , & purves, d. (1999). mach bands as empirically derived associations. proceedings of the national academy of sciences, 96, 5245 - 5250 .
an alternative explanation of mach bands has to do with the way the eye and brain perceive highlights and lowlights, creating harmonics in the visual waveforms that merge in certain areas and create the illusion of darker or lighter sections. [ 2 ]
wallis sa, georgeson ma (2012) mach bands and multiscale models of spatial vision: the role of first, second, and third derivative operators in encoding bars and edges. journal of vision (2012) 12 (13): 18, 1–25
when viewing plain film images we tend to assume that what we see is a real and undistorted anatomical reality. this page starts with an illusion known as mach bands and continues with other optical illusions which question whether we really see what we think we see. these are not all just quaint and quirky optical tricks (well... some are). it is widely considered that mach bands, for example, can produce an effect that mimics rigler' s sign (pneumoperitoneum) on abdominal plain film .
when the image is cropped along a line between the triangles the mach band is no longer visible .
stimulus type: change stimulus to a series of bars you can add and remove or a stimulus with a gradual change in luminance. each stimulus has different ways you can change it. number of bars: (bar stimulus only) change the number of bars on the screen starting from the darkest bar on the left. center: (gradient stimulus only) change the intensity of the region in the middle of the stimulus area from bright to dark. indicate mach bands: draw arrows to highlight where the mach bands may be found .
von bekesy, g. : mach - and hering - type lateral inhibition in vision. vision res .
pessoa, l. : mach band attenuation by adjacent stimuli: experiment and filling - in simulations. perception
the individual bands should appear as gradients, and they may even appear to be curved. in fact, they are all solid colors. now look at this one :
figure 7. von bersky, g. (1967) mach band type lateral inhibition in difference sense organs .
when the cropped image is duplicated, flipped horizontally, and juxtaposed, the mach band is similarly not visible .
ernst mach, a famous physicist published many papers such as the principles of supersonics, famously known as mach 1 which represents the speed that an object is travelling at the speed of sound. however, interestingly, he published many papers relating to the neurophysiological theories behind the phenomenon on visual effect which is named after him, mach band. mach’s theories were researched on only after world war ii by floyd ratliff and thereafter, became the basis of many other research studies. one major contribution on mach band theory is to radiology (papageorges, et al. , 1989) .
). austrian physicist and philosopher ernst mach (1838–1916) introduced this, now famous, optical illusion in 1865 .
hold your hand over the image on your computer screen so that you can only see the grey bands on the left on their own. they are clearly the same shade of grey !
named after the austrian physicist ernst waldfried josef wenzel mach (1838 - 1916) who first described it in 1865 5 .
mach bands are (nearly) not related to the hermann grid. their explanation is classically linked to processing of the visual input in the retina (e. g. lateral inhibition). that is not really wrong, but the details are markedly more involved, see wallis & georgeson for a current in - depth account .
mach bands are old hat: 1865, also they' re weak, but interesting all the same. in brief: mach bands appear at points of inflection in a luminance (or colour) gradient. the figure on the right (modified from mach 1920, x, 11, fig. 27) shows a pattern which is to be rapidly moved (in a repeat pattern affixed to a drum) so fast, that the white and black mix to form a gradient: the gradient starts light on top (a), slowly getting darker, at the inflection point α getting darker faster, at the second inflection point β getting darker more slowly again. my demo above differs in that it is rotated by 90° and the shallow outer gradients are simply on level luminance .
mach e (1886, 1920) die analyse der empfindungen und das verhältnis des physischen zum psychischen, 9. aufl. 1922) .
the explanation of mach bands is often based on the concept of lateral inhibition. lateral inhibition is a process by which the collection of light by retinal cells in the eye is affected by the light collection of neighboring cells. in nature, this helps boost the perception of object edges, making it easier to see the edge of an object that might otherwise not be noticed .
the sliders allow to control the width of the ramp and the luminance contrast. i find it surprising how shallow the ramp can be with the mach band remaining visible .
two phenomena (cervical burnout and mach band) create radiolucent areas that can mimic carious lesions. this paper discusses these phenomena and provides a technique to aid in interpretation .
1. daffner rh. visual illusions in computed tomography: phenomena related to mach effect. ajr am j roentgenol. 1980; 134 (2): 261 - 4 .
look carefully at the grey bars near the middle of the luminance grating below. you should be able to see a darker region just to the left (on the darker side) of the edge between two adjacent regions and a lighter region just to the right (i. e. , on the lighter side) of the same edge. these are called mach bands, and even though you can see them, they do not physically exist in the stimulus .
when looking at mach bands, one sees a central band of a light to dark gradient surrounded on one side by the lightest color and on the opposite side by the darkest color. when looking at this gradient, one sees a band along both the light edge that appears to be lighter than the solid field surrounding it, and a band along the dark edge that appears darker than its surroundings. in reality, there is no color darker or lighter than the surrounding fields .
the appearance of a light or dark line on a radiograph where there is a concave or convex interface in the subject, a physiologic optical form of edge enhancement. see also: mach band .
lateral inhibition is also responsible for mach bands. the bright halo you can see around a blurred shadow is a mach band. you can create a blurred shadow on a bright sunny day (winter days are best) by holding your hand about 5 or 6 feet from a sunlit area of the floor. many people notice a bright halo around the edge of the blurred shadow. you can enhance the halo by moving your hand makes back and forth at about 6 inches per second. the bright halo is not present in the physical stimulus. it is created in the retina by lateral inhibition. the bright halo has the effect of sharpening the edge of the blurred shadow. the same process occurs with a sharp edge, bit it is not easily noticeable .
syrkin, g. , yinon, u. , gur, m. : simple cells may be the physiological basis of the mach band phenomenon: evidence from early monocularly deprived cats. soc. neurosci. abstr .
4. panikkath r, panikkath d. mach band sign: an optical illusion. proc (bayl univ med cent). 2014; 27 (4): 364 - 5. free text at pubmed - pubmed citation
in figure 1 below, we see the two bands as gradient however the colours are solid colours. each solid colour reflects different amount of light with the darker band reflecting lesser than the white band. the borders appear ‘blurred’ due to focus of receptive fields on to the edge. this is caused by competing reaction in the receptor cells in the retina known as lateral inhibition .
if you look closely at this grey scale it appears that each band of grey is not a consistent density. the left side of each band appears slightly darker than the right. this is another example of the mach band optical illusion .
a similar illusion, again caused by lateral inhibition in the retina, is the background contrast effect (3). the radiographic density of an image gets altered in comparison with its surrounding structures. this is similar to the mach band effect, but it covers a larger area than the mach effect, which creates only a narrow bandlike effect. cornsweet illusion is a similar optical illusion, where an object of uniform intensity might appear to have different intensities if separated by a line. this effect is similar to the background contrast effect, in that large areas can be affected, but unlike the mach effect, here the region adjacent to lighter structures will appear lighter and the region near darker areas will appear darker .
diagnosis of pseudofractures by the radiologist can be challenging in the emergency setting. this review describes the common pseudofractures, which are attributable to normal variants and anomalies, mach band formation, subjective contour formation, extraneous materials, and previous injuries .
mach e (1865) über die wirkung der räumlichen verteilung des lichtreizes auf die netzhaut [ on the effect of the spatial distribution of the light stimulus on the retina ]. sitzungsberichte der mathematischnaturwissenschaftlichen classe der kaiserlichen akademie der wissenschaften 52: 303–322 that source is not easily accessible. find mach' s own summary here (a page from the fabulous leipzig wundt collection) in sectionx - 11. he even displays the second derivative formula, which corresponds to the points of inflection i mentioned above .
blackmore, j. t. , r. itagaki, and s. tanaka. ernst mach' s vienna 1895 - 1930: or phenomenalism as philosophy of science (boston studies in the philosophy of science). springer, 2001. isbn 0792371224
5. mach e. über die wirkung der räumlichen verteilung des lichtreizes auf die netzhaut [ on the effect of the spatial distribution of the light stimulus on the retina ]. sitzungsberichte der mathematischnaturwissenschaftlichen classe der kaiserlichen akademie der wissenschaften. 1865; 52: 303–322 .
lateral inhibition and mach band experiments can be done even on the skin. by just using your nail to press into skin of your hand, you will form an obvious imprint caused by the pressure of your nail. the pressure caused a depression followed by a protusion on your skin. there is also an inhibitory effect in the stimulus and sensation effect. however, figure 7 below shows a side view of the imprint on the skin, indicating the light intensity effect causing the dark and white mach band on your skin .
the way in which lateral inhibition could produce the perception of mach bands is shown below. if the receptive field of a retinal ganglion cell lies entirely within the boarders of a band (e. g. , cells 1 and 2 below), then both the excitatory center and inhibitory surround are uniformly illuminated with little net effect on their firing (i. e. , the excitatory and inhibitory firing cancel each other). compare this to cell 3, whose inhibitory surround is partially in the darker area to its left. as a result, cell 3 will be less inhibited by the surround and fire at a faster rate, resulting in the brain interpreting this as brighter .
physicians rely on radiographs and computed tomography for diagnosis. however, they must realize that radiographs provide shadows of objects, and several artifacts arising from our visual perception can mimic pathologic lesions. this case report illustrates the importance of recognizing one of the most common and important illusions, the mach band effect .
in the graph on the bottom, the black line represents the actual luminance of the figure. the red line reflects the perceived luminance. the red line’s deviation from the black line represents the mach band phenomena, and the little spikes over and above the black line represent the bright and dark lines you see in the second figure above .
this is a supine chest x - ray image. the arrowed structure is the left paraspinal stripe. this stripe appears to be less dense along it' s edge (arrowed) - this is not real, it is a mach band optical illusion. the left paraspinal stripe is approximately uniform density. the appearance of the band being less dense along it' s edge is anatomically ilogical .
the chest radiograph of a 63 - year - old woman who was found unresponsive was concerning for pneumopericardium, but lacked the other corroborative features suggestive of pneumopericardium. none of the follow - up chest radiographs showed evidence of air around the heart. the radiolucent shadow that mimicked pneumopericardium in this case was due to an artifact known as mach band sign, an illusion created by lateral inhibition in the light receptors in the retina .
in figure 4, you will see that lateral inhibition process causes the eye to perceive the mach band effect at the edge of each strip of the different shades from light to dark grey and the contouring effect when you scan your eyes across the figure. however, if you were to block of the surrounding strips and focus on one strip of colour, you will notice that each strip is just one solid colour by itself .
the receptive field over the bright mach band gives a stronger response in the center because part of the surround is in the darker area. therefore it receives less inhibition from the surround than did the center at the extreme left and right ends. the receptive field over the dark band receives more surround inhibition because part of the surround is in the brighter area. therefore, the excitatory response is less and this results in our seeing that the area as darker .
a 63 - year - old woman was found unresponsive by her husband. she was intubated on the scene by paramedics and transferred to the intensive care unit. the chest radiograph showed opacities in the mid and lower zones of the right lung (figure). a linear hypodensity at the lateral margin of the right atrium was suspicious for pneumopericardium. no radiolucent line was visible on the left side of the heart or around the great vessels. the patient was treated with broad - spectrum antibiotics for possible aspiration pneumonia. she later improved and was extubated. no chest radiographs during her subsequent hospital course showed pneumopericardium. the radiolucent shadow seen lateral to the right atrium was due to the mach band sign .
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each band appears to vary in brightness as shown on the left; in reality each band is the same shade across its whole width .
, a neural process in the retina visual system that enhances the sharpness of edges. lateral inhibition refers to the fact that neurons caused to fire by a light stimulus will inhibit firing of neighboring neurons .
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the receptive fields are represented as a disk (+) and annulus (-). the center disk is an excitatory area and the annulus an inhibitory area. the receptive fields in the uniformly white and uniformly black areas receive about the same stimulation in their excitatory centers and inhibitory surrounds. therefore the center excitations are in balance with the surround inhibitions .
on the right there is a dark area and a light area, the gap filled by a luminance ramp. the luminance ramp goes up, and then stays constant (visible in the green profile). the visual impression, however, is a bright band a the right end of the ramp, and a darkish band a the left end of the ramp (the latter being less pronounced) .
if you look closely at the area above the center two arrows, you should see a thin bright line (left - middle arrow) and a thin dark line (right - middle arrow). once again, this is despite the fact that each of the three areas (dark, light, and in between) are solid colors .
back to real blogging soon, but before then, i wanted to post this. you probably saw…
in chapter 3, we finally get to read all about the strict father and nurturant parent. …
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notice the dark band that appears immediately to the right and the light band that appears immediately to the left of the gradient .
↑ colin blakemore, k. adler, and m. pointon. vision: coding and efficiency (cambridge university press 1993). isbn 0521447690
fineman, mark. the nature of visual illusion. dover publications, 1996. isbn 0486291057
robinson, j. o. the psychology of visual illusion. dover publications, 1998. isbn 978 - 0486404493 .
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the center surround receptive fields that we find in our eye have both excitatory and inhibitory regions. review how they operate in isle 3. 11 (a) and isle 3. 11 (b). as a result of having both excitatory and inhibitory areas, these receptive fields exhibit lateral inhibition. see the text for more information about lateral inhibition .
the goal of lateral inhibition is to facilitate edge detection. edge detection is the process of distinguishing where one object ends and the next begins. that is, it works to make edges as clear as possible. edge detection is important—think about trying to see a camouflaged predator, perhaps a lioness—in the tall dry grass. the lioness' s fur and the grass are roughly the same color, important for the lioness as she slowly stalks her prey (you, in this case). however, the fur and the grass are not identical, and you want your visual system to pick up where the grass ends and where the lioness begins. this involves edge detection and gestalt figure ground discrimination. it is for this reason that edge detection evolved .
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these are the same grey blocks separated by a white strip. they now appear as solid shades of grey .
the two grey blocks appear to be different shades of grey - the left block appears lighter than the right block. this is an example of the
these are the same two blocks of grey with a black line superimposed over the centre. now the two blocks of grey appear to be the same shade of grey
this is a more extreme example of the cornsweet effect. the top and bottom greys are the same shade of grey. i didn' t believe they were the same shade of grey when i first saw this image. to prove the point, i extended the grey areas as shown below .
look at the center of a figure and try moving closer and further from the figure semi - quickly. the effect is a change in the apparent brightness
( 4 )
this one is just weird and has absolutely nothing to do with radiography (if you find a connection please let me know) .
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in this brightness illusion the physical contrast on the edge between the adjacent shades of gray is exaggerated (fig .
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ratliff, f. , hartline, h. : the responses of limulus optic nerve fibers to patterns of illumination on the receptor mosaic. j. gen. physiol .
davidson, m. : a perturbation analysis of spatial brightness interaction in flashed visual fields. ph. d. dissertation (unpublished). university of california, berkeley (1966 )
fiorentini, a. : foveal and extra foveal contrast threshold data point of a non uniform field. atti delta fondazione giorgio ronchi
heinemann, e. : simultaneous brightness induction. in: jameson, d. , hurvich, l. (eds .) handbook of sensory physiology, vol. vii - 4, pp. 146–169. springer, berlin (1972 )
tolhurst, d. : on the possible existence of edge detector neurons in the human visual system. vision res .
morrone, m. , burr, d. : feature detection in human vision: a phase - dependent energy model. proc. r. soc. lond. b
watt, r. , morgan, m. : a theory of the primitive spatial code in human vision. vision res .
fiorentini, a. , baumgartner, g. , magnussen, s. , schiller, p. , thomas, j. : the perception of brightness and darkness: relations to neuronal receptive fields. in: spillmann, l. , werner, j. (eds .) visual perception: the neurophysiological foundations, pp. 129–161. academic, san diego (1990 )
kingdom, f. , moulden, b. : a multi - channel approach to brightness coding. vision res .
refers to the inhibition that neighboring neurons in brain pathways have on each other. for example, in the visual system, neighboring pathways from the receptors to the optic nerve, which carries information to the visual areas of the brain, show lateral inhibition. this means that neighboring visual neurons respond
if they are activated at the same time than if one is activated alone. so the fewer neighboring neurons stimulated, the more strongly a neuron responds .
you might expect that such inhibition would decrease the visual system' s ability to represent information. in fact, this process greatly increases the visual system' s ability to respond to edges of a surface. this happens because neurons responding to the edge of a stimulus respond more strongly than do neurons responding to the middle. the
edge
neurons receive inhibition only from neighbors on one side - - the side away from the edge. neurons stimulated from the middle of a surface get inhibition from all sides .
the pictures you see taken from space craft are computer enhanced by a process just like lateral inhibition. computer transformation of the raw image makes neighboring points of the computerized image inhibit each other. this makes the very faint edges in the original image much sharper. this is why lateral inhibition is important: it makes edges stand out
look steadily at the center of the left grid. most people notice faint grey clouds at the intersections of the light bars (except the one you look straight at). the bottom panel of the drawing shows the explanation for this effect. the faint clouds appear because intersections have lateral inhibitions from white areas on 4 sides, whereas the middle of the bars have lateral inhibition from white areas on only 2 sides. so the intersections receive (roughly) twice the lateral inhibition that the bars do. this makes the neural signal triggered by the intersection weaker than the signal triggered by a section between intersections. this difference produces the perception of faint clouds at the intersection .
simultaneous contrast is illustrated in the two rectangles in the figure below. most people see the center rectangle on the left as darker than the one on the right, even though physically they are identical .
the center rectangle on the left gets little lateral inhibition from its dark surround. the center rectangle at the right gets considerable lateral inhibition from its light surround. therefore, the light from the center rectangle on the left sends a stronger neural signal to the brain than does the same light from the center of the right rectangle, so the center rectangle on the right appears brighter .
horizontal cells. the centre is excitatory and the surround inhibitory. they compete with each other to activate the bipolar cells in the retina .
figure 3 shows a process of lateral inhibition, d gets more light in the inhibitory surround, thus the excitatory centre will respond at a slower rate. b and c receives same intensity of light, but c receives less light in it’s inhibitory surround, more light in the excitatory, having a faster response than b. the contour effect (concave and convex curves) is perceived when in low intensity areas, the amount of inhibition is constant, resulting in darkening and whereas in high intensity areas, there is less inhibition, resulting in a lightening .
figure 4 above shows light entering the eye through the cornea and lenses and projected on to the retina, at the back of the eye. 5 types of neurons; the rods and cones, bipolar cells, ganglion cells, amacrine cells and horizontal cells form the retinal layers. the receptors made up of rods and cons, bipolar cells and ganglion cells transmit the signals vertically through the retina, whilst the horizontal and amacrine cells transmit signals horizontally across the retina .
light is changed to energy in the receptive fields as electrical signals and sent to the visual cortex neurons of the brain via the lgn (lateral geniculate nucleus) through the firing of the ganglion cells when light passes through the receptive field. the ganglion cells respond to the antagonistic receptive photoreceptors. the receptive field consists of on - centre, off - surround ganglion cells and off - centre, on - centre ganglion cells as in figure 5 .
the amacrine and horizontal cells sends signals horizontally across the receptive fields and convergence takes place in the retina. as there are approximately 120 million rods and 6 million cones in the retina, more rods have to come together to deliver their signals to one ganglion cell as opposed to one single cone having access to one ganglion cell .
this process of pulling together allows the rods to be more sensitive to changes in the visual field whilst the cones “zooms” into focus .
figure 5a shows light passing through the on - centre and off - surround receptive field. the firing rate is moderate as light passes through the off - surround receptive field and the firing rates is faster when light reaches the on - centre receptive field .
figure 5b shows that when light passes through the middle off - centre, on - surround receptive filed, the firing rate is faster at the on - surround and is moderately fired at the off - centre, and again at a faster firing rate at the on - surround receptive field .
the eyes see objects and defines boundaries and edges through the firing of the ganglion cells in the receptive fields. in figure 6a, the on - centre, off - surround array of receptor cells is activated when bright light is shone of the on - centre, the firing rate is rapid and when the surround is lighted, the the ganglion cells fire at a moderate rate. our visual system responds to edges in objects in this say. the lgn generates a signal to the visual cortex and sees the edge at a specific angle .
figure 7 shows an example of lateral inhibition and simultaneous contrast. there is same amount of light is reflected on these two squares, however, the grey box in the right square looks darker than the grey box in the black square. both grey boxes are the same colour. simultaneous contrast occur when when the electrical signals received from the receptors in surrounding black box is less intense than the signals received in the surrounding white box. there is more inhibition in the surround of the white box causing less firing in the grey square, thus the grey look darker as compared to the grey square in the black box .
2. sugita r, sugimura e, itoh m et - al. pseudolesion of the bile duct caused by flow effect: a diagnostic pitfall of mr cholangiopancreatography. ajr am j roentgenol. 2003; 180 (2): 467 - 71 .
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from the department of internal medicine, texas tech university health sciences center school of medicine, lubbock, texas .
corresponding author: ragesh panikkath, md, dm, texas tech university health sciences center school of medicine, 3601 4th street, ms 9410, lubbock, tx 79430 (e - mail: ude. cshutt @ htakkinap. hsegar; moc. liamg @ phsegarrd) .
in patients with pneumomediastinum, usually several other findings substantiate the presence of air in the mediastinum. the “continuous diaphragm sign” is due to air tracking on the inferior surface of the heart, giving the appearance of the diaphragm as a continuous line below the heart. radiolucencies may be seen surrounding great vessels due to the air tracking alongside them. moreover, a bright white line (of parietal pericardium) bordering the free margin of the radiolucent shadow of air is also seen in true cases of pneumopericardium. these findings were not present in our patient. apart from the phenomenon of optical illusion, the presence of radiolucent lung between the cardiac border and the descending branch of the right pulmonary artery is also believed to be responsible for this appearance .
( a) chest radiograph showing opacities in the right lower zone and a radiolucent shadow bordering the right atrial silhouette. (b) magnified view of the cardiac silhouette showing a curvilinear radiolucent shadow bordering the right atrium .
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the appearance of a light or dark line on a radiograph where there is a concave or convex interface in the subject, a physiologic optic form of edge enhancement .
an optical illusion in radiography. wherever there is a boundary between two objects of different luminosity the retina enchances the image. it leads to the misdiagnosis of fractures in the metacarpal and metatarsal bones .
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copyright © 1983 american dental association. published by elsevier inc. all rights reserved .
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mach bands is an optical illusion named after the physicist ernst mach . it exaggerates the contrast between edges of the slightly differing shades of gray , as soon as they contact one another , by triggering edge-detection in the human visual system . 
mach bands is an optical illusion named after the physicist ernst mach. it exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual system.
mach bands is an optical illusion named after the physicist ernst mach. it exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual system.