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DEPTH PERCEPTION.
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Scientific analysis of the eye movement system.... More...
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Paper Abstract:
Scientific analysis of the eye movement system. Underlying mechanism of perception and distinct brain areas. Binocular vision. Monocular vision. Stereopsis. Monocular clues for depth perception. Discusses various research studies and experiments, and their findings. Development of depth perception in infants.
Paper Introduction:
Depth perception is the ability to determine the relative positions of objects in space (Yanoff, 1999). It may occur with or without binocular vision, and depends on both monocular and binocular clues. Stereopsis is a form of depth perception which requires binocular vision and sensory fusion of the input from both eyes. Under certain conditions, stereopsis may be stimulated by rivalrous objects whose images cannot be fused. Stereopsis is the perception of depth stimulated by objects with horizontal disparity, one object usually located before the fixation point, and one located after it. Stereopsis can be stimulated by horizontal disparity alone. Whole visual contours are not necessary for stereopsis, and random dots can simulate disparity.
Stereoacuity is the disparity at which a depth difference can j
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The architectural changes in themagnocellular pathways suggest changes in information processing duringhuman evolution. Stereoacuity dissipates rapidly towardsthe periphery of the visual field of the eye, and with increasing objectdistance. Visualacuity, measured by visual evoked potentials using graded targets, isdetermined by the commonly used Snellen test to be between 2 /4 and2 /6 at birth. Under idealconditions, foveal stereoacuity is 1 seconds of arc. Othercortical areas were activated later. B. Journal ofExperimental Child Psychology, 8 , pp. The anteromedial cuneus has the temporal position which allowsit to interact with the primary visual cortex and so to modify informationtransferred via the V1 extrastriate cortices. This also gives information to the brainabout depth. The eye movement system is not completely functional at birth, butdevelops rapidly, its growth paralleling that of vision (Mills, 1998).Fixations is present at birth, but not the ability to track, and visualacuity is between 2 /4 and 2 /6 . Human layer 4A contains a large amount of magnocellular-like tissue distributed in a complex mesh pattern, whereas in otheranthropoid primates these cells are in simple vertical arrays. It is necessary to understand the order of distinct brain areas tounderstand the underlying mechanisms of perception (Vanni, Tanskanen,Seppa, Uutela and Hari, 2 1). 6, Section 5, pp. Stereoacuity is the disparity at which a depth difference can just bedetected. They could make the distinction in three-dimensional displays ofblocks, but not in a two-dimensional display. D. No preference for EE was seen. Infants' perception ofkinetic depth and stereokinetic displays. One occurs when the muscles of the eyeare used to change the shape of the lens to focus on an object. Infants looking at thewhole display spent significantly longer looking at a novel cube than atthe familiar cube after habituation. They used KDE, SKE and EE (elasticityeffect) displays. Individual three-dimensional estimates were aligned with anonlinear transformation according to individual brain shapes and theindividual and group average minimum current estimates were compared withexisting maps of human visual cortices. Proceeds of theNational Academy of Sciences, USA, 96, pp. They quoteevidence that has been presented that infants as young as 8 weeks of agecan perceive three-dimensional forms, and recognize a rectangular formwhatever the slant of the form presented to them. It may occur with or without binocularvision, and depends on both monocular and binocular clues. These results suggest that the direct parvocellular-geniculateprojection in layer 4A may have been reduced early in the evolution fromapes to humans; the magnocellular-component of this layer was modified andpossibly enhanced in human evolution, and the honeycomb model does notcharacterize the human layer 4A. The cubes either had orlacked an interior corner, but were otherwise identical. This shows that visual stimuliactivate two cortical areas from the very beginning of the corticalresponse. PerceptualPsychophysiology, 62, pp. The results ofthese experiments have to be evaluated bearing in mind that infants thisyoung cannot communicate and so many assumptions have to be made fromobservations of their behavior. Stereopsis is aform of depth perception which requires binocular vision and sensory fusionof the input from both eyes. Binocular vision and disparity sensitivity are two factors which aidin depth perception. Perception of three-dimensional cues in early infancy. American FamilyPhysiology, 6 , pp. Under binocular conditions, those infants who were disparity-sensitive reached for the nearer object more consistently than those whowere disparity-insensitive. R. N. Humans and apes were found to lack adense band of cytochrome oxidase staining in layer 4A that marks thestratum of parvocellular-geniculate afferents in monkeys. Childhood and Adolescence. Horizontal following to midline isusually present by one month of age, with visual acuity at about 2 /3 .Vertical following begins at two months, with visual acuity improved to2 /2 . GaleEncyclopedia. Visual function is limited at birth. The first experiment examined thesubjects' sensitivity to various types of information from motion incontinuously transforming two-dimensional displays, which give between-contour and within-contour data. 1-2.Philadelphia, PA: 1999. Journal of Experimental Human PerceptualPerformance, 12, pp. Young infants perception ofillusory contours in dynamic displays. 2 7-224. Similarly, 12-week-oldinfants have been shown to recognize this constancy of shape. (2 1). A study by Granrud (1986), looked at infants of fourand five months of age to examine the effects of binocularity and disparitysensitivity on spatial perception. Stereopsisis the perception of depth stimulated by objects with horizontal disparity,one object usually located before the fixation point, and one located afterit. 2776-278 . Humans alsodisplayed a unique arrangement of magnocellular-related cells and dendritesin this layer. (1998). This suggests disparity-sensitivity aids in depthperception. Analyzing the results, the researchers found that activationonset in both the primary posterior visual areas and in the anteromedialpart of the cuneus occurred 55 to 7 ms after stimulus onset. The infants preferred to look atthe first display in the three-dimensional condition and the second one inthe two-dimensional display. Bringing both eyes to focus on one object gives muscularcues to the brain about the distance of the object. 9 7-916. The amountof muscular tension involved gives the brain feedback about distance. Proceeds of the National Academy of Sciences, USA, 98,pp. Bhatt, R. The above studies show that depth perception in infants occurs veryearly in life, possibly as early as two months of age. E. Infants were exposed to computer-generatedimages of two rods behind a box. In a third experiment, theillusory boundaries could only be detected by spatiotemporal integrationbecause of the enhancement and deletion of sparse background textures. Yanoff, M. The second visual cueis caused by retinal disparity: the retina of each eye sees the object froma slightly different perspective. In their study, Schmuckler and Proffitt used 5-month-old-infants.Four experiments were performed. Depth perception is the ability to determine the relative positions ofobjects in space (Yanoff, 1999). Infants who had only been exposed tothe center of the display did not differentiate between the cubes, lookingat both familiar and novel cubes for the same amount of time. The retina and its photoreceptor cells develop, optic nervesand tracts become myelinated, and the synaptic visual cortex increases indensity, so that by six months of age, visual acuity on the Snellen testhas reached 2 /3 . The authors point out the difficulties of doingexperiments in such young children in that they cannot answer questions andmuch has to be assumed from their actions. There was no difference under monocularviewing conditions. Stereopsis can be stimulated by horizontal disparity alone. Vanni, S., Tanskanen, T., Seppa, M., Uutela, K., & Hari, R. References Arterberry, M. At three monthsof age they can already recognize three-dimensionality, but cannot detectit in two-dimensional images. Mills, M. There were five male and five female subjects. 55 -556. Looking at the development of depth perception in infants, Arterberryand Yonas (2 ) subjected infants 8 weeks old to kinetic random dotdisplays that depicted three-dimensional cubes. Perception, 27, pp. 341-353. They citeother studies that showed infants of four months of age could onlyrecognize three-dimensionality if it was associated with motion. The rapid anatomic development of the eye and central visualpathways during the first six months of life is paralleled by a rapidimprovement in visual acuity, contrast sensitivity and the development ofcolor vision. Depth perception. The fourth experiment again looked atpreferences between SKE and EE displays, but this time with equal viewingtimes. In a second experiment, the infants were shown two displays: onecontained one novel element among 15 familiar ones, and the other containedone familiar element among 15 novel ones. Kagan, J., & Gall, S. Binocularity is just detectable at this age,and so infants this young probably do not have good disparity-sensitivityyet. This shows that in experiment one, the reasonthey looked longer at EE displays was because they found them different,whereas they found KDE and SKE displays alike. (1998). 122-13 . With an integrated system of voluntary reflexeye movements which allows normal alignment of the eyes, fixation onobjects of interest and smooth tracking rapidly develop so that by the ageof three months an infant can follow an object closely both vertically andhorizontally, with the eyes remaining aligned. Visual fixation ispresent in full-term infants, but it is not until two months of age thatthey are capable of tracking, i.e. Good horizontal and vertical tracking are usually present by threemonths of age, when visual acuity has improved to 2 /1 and accommodationbegins. (1986). At this stage,binocularity has not been proved to be fully established, giving furtherevidence for the fact that binocular vision is not essential for depthperception, but it does appear to help. It is proportional to the interpupillary distance. following a moving object. The infants were exposed to SKE and EE images, and the results showed thatthey looked at both equally. Binocularity is detectable at this age. Preuss, Qi and Kaas (1999) examined the primary visual area inhumans, chimpanzees, an orangutan, Old World monkeys, and New World monkeysto see if they had the same layer. Johnson, S. This shows that they weresensitive to three-dimensional clues, which are necessary for depthperception, in static displays. (2 1).Coinciding early activation of the human primary visual cortex andanteromedial cuneus. This ruledout the idea that the infants who looked at the full display merelyconcentrated on the difference in motion in the center of the display.This experiment suggests that infants as young as eight weeks of age canperceive the three-dimensional shape of objects from the optical flow, andso exhibit depth perception. Inall three experiments, the infants were able to separate the rods from thebox, showing they used illusory contours to perceive depth. In a secondexperiment, these boundaries were illusory because of motion shear, and thebackground texture being enhanced and deleted. By the time an infant is able to move around under its own power, itsvisual system is highly developed, and is ready for the transition fromtactile-based movement cues to visual ones when the infant learns to walk.Although the visual system does not mature until about 1 years of age, itdevelops very rapidly in the first few months of life. The eye in childhood. Binocular vision, the ability to perceive vision with both eyesimultaneously, stereopsis, the merging of the images from the two eyes togive a single image, and depth perception develop normally as long as theeyes are properly aligned. Journal of Experimental ChildPsychology, 7 , pp. (1998). In one experiment, the surfaces andboundaries were differentiated by dense surface textures. Half of theinfants tested were exposed to the full array of cubes, whereas the otherhalf were exposed to the central area of the display where the flow withthe presence or absence of the corners differed. (1998). Wholevisual contours are not necessary for stereopsis, and random dots cansimulate disparity. It is best at the fovea, the focal point of the eye, and dependson the visual acuity of each eye. Possibly they saw them as two-dimensional, or they may just havefound them more interesting. (1994). Schmuckler, M. This suggeststhat even at four months of age, infants have cognitive contributions tothe perception of illusory boundaries of objects. Comparison with otherspecies suggests that the anteromedial cuneus is either analogous to the V6area in the monkey, or is an area unique to humans. H. These experiments provide evidencethat 5-month-old infants can detect the presence of between-contour motioninformation in KDE and SKE displays, but do not recognize within-contourmotion of KDE and EE displays. After six months of age, development slows down.Myelination of the central visual pathways continues for the first fouryears of life, and the visual cortex continues to develop for the firstdecade of life. Thesecond type of clue relates to external visual stimuli, and is used to maketwo-dimensional representations look three-dimensional. Bhatt and Waters (1998) examined three-month-old infants' processingof three-dimensional static images. S., & Waters, S. The results of thisexperiment showed that KDE and SKE patterns resulted in the perception ofthree-dimensionality, but the EE patterns were not seen as the same byinfants. Journal of ExperimentalPsychology, 2 , pp. Schmuckler and Proffitt (1994) looked at the development ofkinetic depth using kinetic depth effect (KDE) displays, two-dimensionalprojections of a three-dimensional form revolving on an axis other than theline of sight, and the stereokinetic effect (SKE) with a stereokineticdisplay consisting of a set of nested circles with a constant eccentricityto determine their ability to detect depth from motion. Another two experiments were used to determine whether the superiorityof depth perception in four-month-olds was related to binocular disparitysensitivity. Structure alone is not enough: the timingof various activations is necessary to follow visual processes.Researchers used neuromagnetic signals recorded while subjects viewedpattern reversal or luminance stimuli directed at the four visual field`quadrants. 116 1-116 6. These techniqueswere not recognized by the art world until the 16th century. Experiment two tried to answer this question. By six months, visualacuity has improved to between 2 /2 and 2 /3 , and binocularity is welldeveloped. Evolutionary changes may account for changes in the organizationof the parvocellular geniculate inputs. In their first experiment, the infants could distinguish amisoriented shape in a three-dimensional array, but not one depicting two-dimensional patterns containing all of the trilinear junctions but not thelines. P., & Aslin, R. The results overall showed thatbinocular disparity sensitivity contributes greatly to depth perception ininfants. M., Qi, H., & Kaas, J. S., & Bertin, E. Distinctivecompartmental organization of human primary visual cortex. In one experiment, they were able todetect a block which appeared to be illuminated from the bottom in an arraydepicting three-dimensional blocks which appeared to be illuminated fromthe top. Binocular vision and spatial perception in 4-and 5-month-old infants. Binocular depth cues require the use of both eyes, and also uses twotypes of cues. It is between-contour motion that leads todepth perception. Color detection and contrast are poorly developed atbirth. (1999). The primary visual layer of macaques and other monkeys contains alayer (4A), which is a mosaic of separate tissue compartments related tothe parvocellular and magnocellular layers of the lateral geniculatenucleus. Bhatt, R. Normal vision develops rapidly during the first six months after birth(Mills, 1998). 36-49. The study shows that human brainevolution affected sensory as well as higher order functions. Again, like adults, they were drawn todiscrepancies in three-dimensional cues. These results tie in with reports that magnocellular-related retinal ganglion cells of humans have larger dendritic fields thanmacaques, and humans are more sensitive to luminance contrast thanmacaques. Ophthalmology, 1st Ed., Ch. Psychologists have identified two kinds of monocular clues for depthperception (Kagan and Gall, 1998). Retinal disparity only works for objects within 5 yards ofthe viewer because the distance from one pupil to another is relativelysmall: only about 6.5 centimeters. E., & Yonas, A. E. The first two experiments compared theinfants for monocular and binocular depth perception and found that infantsat both ages consistently reached for the nearer of two objects more oftenwhen using binocular viewing conditions than under monocular viewingconditions. The disparity-sensitive infants showed size constancy, but thedisparity-insensitive infants did not. Experiment three examined if height differences of the displays wasimportant, and found that again, the results for KDE and SKE displays weresimilar and for EE were different. Perception of three-dimensional shapespecified by optic flow by 8-week-old infants. Left and right visualfield quadrants were stimulated in random order in the same session, butthe upper and lower visual field quadrants were stimulated in separatesessions. Like adults, they were sensitive to the holistic combinations ofjunctions and not just the junctions themselves. 315-332. Depth perception appears toprecede visual acuity in its development in human infants. A., & Proffitt, D. Under certain conditions, stereopsis may bestimulated by rivalrous objects whose images cannot be fused. In another experiment, the infants did notshow a preference for a display containing one novel element among severalfamiliar elements and one in which there was one familiar element andseveral novel elements, suggesting they did not recognize discrepancies inthree-dimensional cues. Preuss, T. A final experiment accustomed the infants to an object andthen presented it with that object and an identical object differing onlyin size. Pectoral cues and three-dimensional information processing in early infancy. Granrud, C. In adults, between-contour information in KDE and SKEprovides the perception of three-dimensions. These results differ from theones discussed previously in which three-month-old infants did notdiscriminate between these two types of displays. Bhatt and Bertin (2 1) studied infants three months of age to examinewhether they are sensitive to line junction cues for depth perception usedby adults. A complex set of experiments to look at the ability of four-month-oldinfants to perceive illusory contours in dynamic designs was conducted byJohnson and Aslin (1998).
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