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PubMed2003

Stereoscopic processing in the human brain as a function of binocular luminance rivalry.

We investigated the neural substrates of a recent model of human stereodepth perception by obtaining measurements of regional cerebral blood flow (rCBF) using PET. Subjects experienced the perceptual properties of stereopsis by viewing rival-luminance stereograms displaying an identical random-dot pattern in their central portion while the backgrounds exhibited correspondent dots contrasting in black/white luminance. The stereoscopic vision induced by retinal luminance rivalry coincided with a significant elevation of rCBF in the dorsal visual pathway. Area V5 (MT) was activated bilaterally by the experimental condition while the remaining active loci were restricted to the right hemisphere. The neural sites that responded to this novel stereoscopic stimulus are similar to those activated by traditional stereograms containing horizontal disparities.

Adult↗
PubMed1997

Optometric management of patients with Alzheimer's disease.

BACKGROUND: The earliest symptoms of Alzheimer's disease (AD) may be visual. AD is an age-related, progressive dementia. At least 7% of Americans over 65 years of age are afflicted, and it ranks as the fourth leading cause of death in the United States. A review of the literature was performed to delineate the visual manifestations of Alzheimer's disease. RESULTS: Abnormal visual perception and visuospatial processing are common among patients with AD Visual acuity, pupillary light reflexes, and the ocular fundus do not appear to be specifically affected by AD. Whether AD impairs color vision is controversial. Stereopsis may also be affected. True visual-field defects are rarely found. Although not universally reported, some studies of contrast sensitivity functions show abnormal sensitivity possibly at low spatial frequencies. The electroretinogram is normal, while the visual evoked response is frequently abnormal. CONCLUSIONS: As primary eye care providers, optometrists must properly identify and refer individuals with suspected Alzheimer's disease to the appropriate specialist, as well as manage their visual welfare.

Aged↗
PubMed1996

Relative size disparities and the perception of surface slant.

Perceived slant produced by size disparities in random-dot displays was measured by tactile matching. For a 60 deg surface, slant produced by vertical-size disparity (the induced effect) was opposite to that produced by horizontal-size disparity. Overall-size disparity produced a little slant. With small displays, effects of horizontal and vertical disparities were reduced but not those of overall disparity. A zero-disparity surround increased effects of horizontal and overall disparities but reduced the induced effect. A mixture of horizontally disparate and zero-disparity dots produced two slanted surfaces. Vertically disparate and zero-disparity dots produced one slanted surface. Abutting opposite horizontal disparities produced surfaces with a sharp boundary. Abutting vertical disparities produced surfaces with a gradual boundary. Perceived slant depends on the difference between horizontal-size disparity detected locally and mean vertical-size disparity over a relatively large area.

Depth Perception↗
PubMed1993

Perception of local shape from shading.

Theoretically, metric solid shape is not determined uniquely by shading. Consequently, human vision has difficulty in categorizing shape when shading is the only cue. In the present research, subjects were required to categorize shaded quadric surfaces. We found that they were rather poor at this task; they confused hyperbolic and elliptic (both convex and concave) shapes easily. When a cast shadow visually indicated the direction of the illuminant, they were able to notice the concavity or convexity of elliptic shapes. However, they still confused elliptic and hyperbolic ones. Finally, when an animated sequence of eight intensity patterns belonging to one quadric shape had been displayed, the subjects were able to categorize the quadrics. However, the results are still quite moderate. Our experiments indicate that local shading structure is only a weak shape cue when presented in the absence of other visual cues.

Algorithms↗
PubMed1998

When stereopsis does not improve with increasing contrast.

It is well known that stereoacuity for conventional (1st-order) stimuli improves with increasing contrast with an approximate slope of -0.5 on log-log axes (Halpern DL, Blake RR. Perception 1988;17:483-495; Legge GE, Gu Y. Vis Res 1989;29:989-1004). In the experiments reported here a variety of stimuli were used (Gabor patches, amplitude modulated stimuli and 1D noise patches) and tasks (stereoacuity and Dmax) to determine if 2nd-order stereopsis shows a similar square root dependence. The results consistently demonstrate that the effect of contrast on stereopsis is quite different for the 2nd-order stimuli. Increases in stimulus contrast have little effect on performance; the resulting slopes are very shallow. The pattern of results is similar when the interocular contrast ratio is varied, demonstrating that 2nd-order processing is more resilient to stimulus differences in the two eyes than 1st-order.

Contrast Sensitivity↗
PubMed1996

A general model for the perception of space and motion.

The perception of space and motion involves successive transformations of signals with respect to different reference systems. The visual input is coded in terms of retinal coordinates. The retinocentric values from each eye require to be unified, and to be combined with signals for eye position and movement. This egocentric reference provides a signal for the angular size, motion, or orientation of the stimulus with respect to the observer. The egocentric signals are transformed to a coordinate system that is three-dimensional-the geocentric frame of reference. Further transformations can occur at earlier levels owing to patterncentric interactions within the visual field. When the geocentric signal corresponds to the physical dimensions of space and motion, this is referred to as perceptual constancy.

Acceleration↗
PubMed1999

An empirical explanation of the cornsweet effect.

A long-standing puzzle in vision is the assignment of illusory brightness values to visual territories based on the characteristics of their edges (the Craik-O'Brien-Cornsweet effect). Here we show that the perception of the equiluminant territories flanking the Cornsweet edge varies according to whether these regions are more likely to be similarly illuminated surfaces having the same material properties or unequally illuminated surfaces with different properties. Thus, if the likelihood is increased that these territories are surfaces with similar reflectance properties under the same illuminant, the Craik-O'Brien-Cornsweet effect is diminished; conversely, if the likelihood is increased that the adjoining territories are differently reflective surfaces receiving different amounts of illumination, the effect is enhanced. These findings indicate that the Craik-O'Brien-Cornsweet effect is determined by the relative probabilities of the possible sources of the luminance profiles in the stimulus.

Computer Graphics↗
PubMed1996

Effects of lighting on the perception of facial surfaces.

A series of experiments is reported that investigated the effects of variations in lighting and viewpoint on the recognition and matching of facial surfaces. In matching tasks, changing lighting reduced performance, as did changing view, but changing both did not further reduce performance. There were also differences between top and bottom lighting. Recognizing familiar surfaces and matching across changes in viewpoint were more accurate when lighting was from above than when it was from below the heads, and matching between different directions of top lighting was more accurate than between different directions of bottom lighting. Top lighting also benefited matching between views of unfamiliar objects (amoebae), though this benefit was not found for inverted faces. The results are difficult to explain if edge- or image-based representations mediate face processing and seem more consistent with an account in which lighting from above helps the derivation of 3-dimensional shape.

Adult↗
PubMed2001

Similarity in perception: a window to brain organization.

This paper presents a neural model of similarity perception in identification tasks. It is based on self-organizing maps and population coding and is examined through five different identification experiments. Simulating an identification task, the neural model generates a confusion matrix that can be compared directly with that of human subjects. The model achieves a fairly accurate match with the pertaining experimental data both during training and thereafter. To achieve this fit, we find that the entire activity in the network should decline while learning the identification task, and that the population encoding of the specific stimuli should become sparse as the network organizes. Our results, thus, suggest that a self-organizing neural model employing population coding can account for identification processing while suggesting computational constraints on the underlying cortical networks.

Brain↗
PubMed1994

Relative shear disparities and the perception of surface inclination.

A dichoptic display in which the images are cyclorotated in opposite directions does not appear inclined. This suggests that perceived inclination depends on the difference between horizontal-shear and vertical-shear disparity. Large random-dot stereoscopic displays were presented with various types of shear disparity. Perceived inclination was the same magnitude for horizontal and vertical shear disparities. Opposed horizontal and vertical shear produced greater inclination than a single-axis shear. Same-sign vertical and horizontal shear (rotation) produced no inclination. These results support the relative-shear hypothesis. Cyclovergence was measured and was insufficient to account for these effects. We conclude that perceived inclination depends on the difference between horizontal- and vertical-shear disparities. Perceived inclination was not based on vertical disparity within small displays or within large displays with a zero-disparity surround. Relative-shear disparities are therefore extracted globally rather than locally.

Adult↗
PubMed1998

Two-dimensional representations of the third dimension and their perception by infants.

Many researchers have examined the perception of the third dimension or the preference for three-dimensional versus two-dimensional stimuli in infants, but little is known about infants' representation of the third dimension in a two-dimensional display. Two experiments were conducted to study this capacity. The first experiment was a replication of earlier work, involving a television screen. No difference was observed between looking durations for the 'normal' and 'strange' events in 4-month-olds. As this situation was cognitively complex, a simpler interposition situation was displayed to 3-month-olds in the second experiment. Infants then looked longer at the 'strange' event than at the 'normal' event, suggesting that in a complex situation more perceptual indices must be given to infants. These different data are discussed.

Depth Perception↗
PubMed2000

Perception of partly occluded figures by baboons (Papio papio).

Comparative literature provides conflicting findings whether animals experience amodal completion. Five experiments were conducted to verify if baboons perceive partly occluded objects as complete. The first three experiments used a go/no-go procedure and a video monitor for stimulus presentation. These experiments failed to reveal amodal completion, suggesting that the stimuli were processed as 2-D images rather than 3-D objects. In contrast, completion was demonstrated in a fourth experiment with cardboard stimuli in a two-alternative forced-choice (2AFC) discrimination task presented in a Wisconsin General Test Apparatus. Although in experiment 5 the same 2AFC procedure was used as in experiment 4, completion was absent when the stimuli were shown with a computer graphic system. The results suggest that baboons share with humans the ability for amodal completion, but also underline some procedural factors that might affect the elicitation of this capacity.

Analysis of Variance↗
PubMed1983

Afterimages, binocular rivalry, and the temporal properties of dominance and suppression.

When different contours are presented to the two eyes, an unstable percept, binocular rivalry, is the result. Parts of each set of contours may be seen but the two sets are not seen in the same place at the same time. The contours need not be physically present. Afterimages will produce binocular rivalry. Normal rivalry can be prevented if intermittent stimulation is used. Previous work has shown that orthogonal gratings, flashed for less than 150 ms and separated by more than 150 ms, will appear to fuse into a plaid or checkerboard pattern. In the present experiment this phenomenon is examined with afterimages used to produce rivalry. This abnormal 'fusion' is seen when negative afterimages are stroboscopically illuminated at less than 5 Hz. The results obtained with afterimages are predictable from the previous results obtained with stimuli external to the eye.

Adult↗
PubMed2005

A laminar cortical model of stereopsis and 3D surface perception: closure and da Vinci stereopsis.

A laminar cortical model of stereopsis and 3D surface perception is developed and simulated. The model describes how monocular and binocular oriented filtering interact with later stages of 3D boundary formation and surface filling-in in the LGN and cortical areas V1, V2, and V4. It proposes how interactions between layers 4, 3B, and 2/3 in V1 and V2 contribute to stereopsis, and how binocular and monocular information combine to form 3D boundary and surface representations. The model includes two main new developments: (1) It clarifies how surface-to-boundary feedback from V2 thin stripes to pale stripes helps to explain data about stereopsis. This feedback has previously been used to explain data about 3D figure-ground perception. (2) It proposes that the binocular false match problem is subsumed under the Gestalt grouping problem. In particular, the disparity filter, which helps to solve the correspondence problem by eliminating false matches, is realized using inhibitory interneurons as part of the perceptual grouping process by horizontal connections in layer 2/3 of cortical area V2. The enhanced model explains all the psychophysical data previously simulated by Grossberg and Howe (2003), such as contrast variations of dichoptic masking and the correspondence problem, the effect of interocular contrast differences on stereoacuity, Panum's limiting case, the Venetian blind illusion, stereopsis with polarity-reversed stereograms, and da Vinci stereopsis. It also explains psychophysical data about perceptual closure and variations of da Vinci stereopsis that previous models cannot yet explain.

Animals↗
PubMed1994

Perceived motion in structure from motion: pointing responses to the axis of rotation.

We investigated the ability to match finger orientation to the direction of the axis of rotation in structure-from-motion displays. Preliminary experiments verified that subjects could accurately use the index finger to report direction. The remainder of the experiments studied the perception of the axis of rotation from full rotations of a group of discrete points, the profiles of a rotating ellipsoid, and two views of a group of discrete points. Subjects' responses were analyzed by decomposing the pointing responses into their slant and tilt components. Overall, the results indicated that subjects were sensitive to both slant and tilt. However, when the axis of rotation was near the viewing direction, subjects had difficulty reporting tilt with profiles and two views and showed a large bias in their slant judgments with two views and full rotations. These results are not entirely consistent with theoretical predictions. The results, particularly for two views, suggest that additional constraints are used by humans in the recovery of structure from motion.

Attention↗
PubMed1992

Prime time: fatigue and set effects in the perception of reversible figures.

Subjects viewed unambiguous versions of both stationary and rotating Necker cube illusions for varying durations prior to the presentation of the standard ambiguous figure. In each case, the subjects were more likely to report the ambiguous figure to be (1) in the same configuration as that of the preceding prime following brief preexposure periods and (2) in the opposite configuration from that of the preceding prime following long preexposure periods. In addition, the number of reversals of the figure during the test period was also strongly related to the duration of the preexposure period, with progressively fewer reversals reported following longer preexposure periods. The results are interpreted as revealing the concurrent roles of "set" effects in the brief preexposure conditions and neural fatigue effects in the long preexposure conditions. Furthermore, the ability of the proposed two-process model to integrate the myriad of empirical effects in the reversible-figure literature is emphasized.

Adult↗
PubMed1995

Object classification for human and ideal observers.

We describe a novel approach, based on ideal observer analysis, for measuring the ability of human observers to use image information for 3D object perception. We compute the statistical efficiency of subjects relative to an ideal observer for a 3D object classification task. After training to 11 different views of a randomly shaped thick wire object, subjects were asked which of a pair of noisy views of the object best matched the learned object. Efficiency relative to the actual information in the stimuli can be as high as 20%. Increases in object regularity (e.g. symmetry) lead to increases in the efficiency with which novel views of an object could be classified. Furthermore, such increases in regularity also lead to decreases in the effect of viewpoint on classification efficiency. Human statistical efficiencies relative to a 2D ideal observer exceeded 100%, thereby excluding all models which are sub-optimal relative to the 2D ideal.

Depth Perception↗
PubMed1994

A theory of shape constancy based on perspective invariants.

Shape constancy refers to the phenomenon in which the percept of the shape of a given object remains constant despite changes in the shape of the object's retinal image. The phenomenon of shape constancy is considered from historical, theoretical and empirical perspectives in this paper. First, four prior theories are discussed; specifically, (1) Helmholtzian theory, which assumes that shape constancy is achieved by taking an object's orientation into account, (2) Gestalt theory, which assumes that shape constancy involves a relationship between the perceived shape and perceived orientation of an object, (3) Gibsonian theory, which assumes that shape constancy is based on projective invariants and (4) multiple view theory, which assumes that shape constancy is achieved by memorizing a large set of different views of the object. It is shown, by an analysis of the prior literature, that none of these theories can actually explain the phenomenon of shape constancy. A new theory, which is based on new perspective invariants of a flat shape, is then proposed. The new Perspective Invariants Theory can account for all prior shape constancy experiments. New experiments, testing predictions of the Perspective Invariants Theory are then described. These experiments showed that: (1) a novel shape can be matched with its single perspective image in the absence of depth cues, (2) perceptual processing of shape is impaired when the range of possible values of tilt is wide, (3) perceptual processing of shape is not affected by the width of the range of possible values of slant. These results support predictions of Perspective Invariants Theory.

Algorithms↗