Form and depth in global stereopsis

Three experiments were performed to investigate the role of vergence and the relationship between form and depth processes in global steropsis by comparing global and classical stereopsis. In the first experiment, the speed of stereoscopic resolution as a function of initial fixation-target distance was measured to discover the role of vergence in stereopsis with the random-dot and contoured stereograms. In the second experiment, the accuracy of form and depth discrimination as a function of fixation-target distance was measured using brief stimulus exposure (150 ms) to examine the nature of form and depth processing in global stereopsis. In the third experiment, the speed of resolving random-dot stereograms in the presence or absence of juxtaposed contoured stereograms was observed to examine the interaction of global stereopsis and classical stereopsis. The conclusions of these studies are summarized as follows: First, vergence plays a critical role in the solution of the random-dot stereograms but not in the solution of contoured stereograms. Second, performance with the contoured stereograms is better than with the random-dot stereograms in terms of both speed and accuracy. Third, in random-dot stereograms, discrimination of form is independent of and more accurate than discrimination of depth. Fourth, again for random-dot stereograms, the disparity of target relative to fixation systematically affects discrimination of form but not discrimination of depth. Fifth, a rapid reduction in reaction time over practice occurs for both types of stereograms. Finally, strong interference with the solution of the random-dot stereograms by the monocular contour occurs when the two kinds of stimuli are present simultaneously.     

Cerebral asymmetries in random-dot stereopsis: reversal of direction with changes in dot size

Visual field differences in stereoscopic form recognition using Julesz-type random dot stereograms were investigated. Dot size was varied in order to test the possibility that variations in the carrier dimension have contributed to past estimates of visual field differences. Twelve male and twelve female subjects, all right-handed, appeared for three test sessions-one with each different dot size. In each session the stimuli were flashed twenty-four times in each visual field, for 120 ms. Results showed no overall visual field effect, but a highly significant interaction between visual field and dot size. For small dots, left visual field superiority was observed, as previously reported by Durnford and Kimura. With large dots, however, the right visual field was superior. This reversal of visual field differences as a function of dot size implies that there is no consistent cerebral hemispheric specialization for stereopsis or stereoscopic form recognition per se. Instead, it appears that there is relative hemispheric specialization for responding to the carrier of stereoscopic information.     

Perceiving surfaces in depth beyond the fusion limit of their elements

An isolated dot appears double outside a small disparity range called Panum's fusional area. In random-dot stereograms (RDSs), however, this doubling, or diplopia of dot elements is not evident at any disparity. Nevertheless, depth is perceived up to disparities that greatly exceed Panum's fusional limit. Either one is unaware of dot diplopia at disparities exceeding Panum's fusional limit or the fusion limit is extended. To examine these possibilities, we developed a novel RDS in which dichoptically color-coded dots have a distinctive color when fused, and return to their intrinsic colors when diplopic. We measured the fusion limit of dots in this RDS, and compared it to the patent stereopsis limit of the perceived surface in similar RDSs. We found that the fusional area of dots in the RDS was comparable to Panum's fusional area. Furthermore, there was clear dissociation between the fusion limit and the patent stereopsis limit in the RDS. We conclude that the elements composing a surface are not necessarily fused when a large disparity surface is perceived in depth.     

Temporal properties of disparity processing revealed by dynamic random-dot stereograms

In studies of the temporal flexibility of the stereoscopic system, it has been suggested that two different processes of binocular depth perception could be responsible for the flexibility: tolerance for interocular delays and temporal integration of correlation. None has investigated the relationship between tolerance for delays and temporal integration mechanisms and none has revealed which mechanism is responsible for depth perception in dynamic random-dot stereograms. We address these questions in the present study. Across five experiments, we investigated the temporal properties of stereopsis by varying interocular correlation as a function of time in controlled ways. We presented different types of dynamic random-dot stereograms, each consisting of two pairs of alternating random-dot patterns. Our experimental results demonstrate that (i) disparities from simultaneous monocular inputs dominate those from interocular delayed inputs; (ii) stereopsis is limited by temporal properties of monocular luminance mechanisms; and (iii) depth perception in dynamic random-dot stereograms results from cross-correlation-like operation on two simultaneous monocular inputs that represent the retinal images after having been subjected to a process of monocular temporal integration of luminance.     

Binocular vision in infancy: Responsiveness to uncrossed horizontal disparity

This research explored the onset of stereopsis, the ability to perceive depth from the different views provided by the two eyes. In a longitudinal study, infants were tested weekly from 6 to 20 weeks of age. The primary goal of the study was to establish the onset and the early development of sensitivity to uncrossed horizontal disparity. The infant participants were shown dynamic random dot stereograms displaying two squares, one with uncrossed horizontal disparity (0.5°) and one with vertical disparity (0.5°). The stimuli were presented on an autostereoscopic monitor. We used two methods, the forced-choice preferential looking (FPL) method and the classical natural preference (CNP) method, to measure whether the infants preferred the uncrossed over the vertical disparity display. According to the FPL data, the mean relative preferences for horizontal over vertical disparity were significantly greater than chance probability (0.50) from 13 weeks of age onward. With the CNP method we found significant preferences for uncrossed horizontal disparity from 15 weeks onward. The FPL method was hence more sensitive than the CNP method as it indicated an earlier onset of responsiveness to stereoscopic information.     

Ecologically invalid monocular texture leads to longer perceptual latencies in random-dot stereograms

Two experiments were conducted to explore Gillam and Borsting's (1988, Perception 17 603-608) report that uncorrelated monocular texture facilitates stereopsis by shortening the latency to see depth in random-dot stereograms. Experiment 1 used stereograms similar, in pattern but not disparity, to Gillam and Borsting's with monocular texture present or absent. A third condition, where monocular texture was dissimilar to the binocular panels and background, was also used. We were unable to generalize the findings of Gillam and Borsting for a depth step of 6 min of arc to a larger depth step of 24 min of arc. That is, we observed no significant difference in latencies between the conditions with monocular texture absent and present at a disparity of 24 min of arc. We found latencies to be significantly longer in the monocular-texture-different condition than the monocular-texture-absent condition, however. We account for this, ad hoc, by arguing that the monocular-texture-different stereogram depicts a rare or 'accidental' visual scenario. This account was supported by the results of experiment 2 which showed that stereograms depicting accidental views yielded longer latencies than those depicting generic views. We conclude that the ecological validity of monocular texture must also be considered when assessing the effects of monocular texture on stereopsis.     

Colour inputs to random-dot stereopsis.

Recently it has been claimed by Livingstone and Hubel that, of three anatomically and functionally distinct visual channels (the magnocellular, parvocellular interblob, and blob channels), only the magnocellular channel is involved in the processing of stereoscopic depth. Since the magnocellular system shows little overt colour opponency, the reported loss of the ability to resolve random-dot stereograms defined only by colour contrast seems consistent with this view. However, Julesz observed that reversed-contrast stereograms could be fused if correlated colour information was added. In the present study, 'noise' (non-corresponding) pixels were injected into random-dot stereograms in order to increase fusion time. All six subjects tested were able to achieve stereopsis in less than three minutes when there was only correspondence in colour and not in luminance, and three when luminance contrast was completely reversed. This ability depends on information about the direction of colour contrast, not just the presence of chromatic borders. When luminance and chromatic contrast are defined in terms of signal-to-noise ratios at the photoreceptor mosaic, chromatic information plays at least as important a role in stereopsis as does luminance information, suggesting that the magnocellular channel is not uniquely involved.     

Detection of glass patterns by pigeons and humans: implications for differences in higher-level processing

Glass patterns have been used to examine mechanisms underlying form perception. The current investigation compared detection of Glass patterns by pigeons and humans and provides evidence for substantial species differences in global form perception. Subjects were required to discriminate, on a simultaneous display, a random dot pattern from a Glass pattern. Four different randomly presented Glass patterns were used (concentric, radial, parallel-vertical, and parallel-horizontal). Detection thresholds were measured by degrading the Glass patterns through the addition of random noise. For both humans and pigeons, discrimination decreased systematically with the addition of noise. Humans showed detection differences among the four patterns, with lowest thresholds to radial and concentric patterns and highest thresholds to the parallel-horizontal pattern. Pigeons did not show a detection difference across the four patterns. Implications for differences in neural processing of complex forms are discussed.     

Patent stereopsis with diplopia in random-dot stereograms

Fusional limits for an RD (random-dot) stereogram with overall horizontal retinal disparity due to the temporalward pulling of its two constituent RD patterns beyond the divergence limits of the eyes have been determined by using an afterimage method. Two criteria for fusion were used, viz, the perception ofsingle local RD elements and the perception of stereoscopic depth in the “hidden” square of side 1.38 deg with 8-min relative horizontal disparity. The diplopia thresholds were found to be in the range of 0.15–0.3 deg, and hence do not exceed the “classical” upper limit of about 0.3 deg, which has been reported for elementary line stereograms. The stereoscopic limits were found to be in the range of 0.5–1.3 deg, which is compatible with the precision of patent stereopsis from double images which has been reported for elementary line stereograms. The results of the present third look at the experiments performed by Fender and Julesz (1967) suggest that there are no special neuronal processes raising the fusional limits for RD stereograms above those for elementary line stereograms. Previous claims that such special neuronal processes may occur seem to be based on an evaluation of fusional limits obtained with different criteria for fusion. It is further argued that the major hysteresis effect that has been observed for fusional limits for RD stereograms should not be ascribed to a raising of the classical size of the diplopia threshold due to special neuronal processes initiated by RD stereograms, but to a lowering of the classical limits of patent stereopsis from double images due to the increased difficulty of solving the correspondence problem in RD stereograms.     

Temporal integration of monocular images separated in time: stereopsis, stereoacuity, and binocular luster

We evaluated stereopsis and binocular luster using electronically controlled shutter glasses with alternating monocular stimulation. In Experiment 1, we used the standard method for testing stereoacuity to obtain a gradual measure of stereopsis. Stereo thresholds decreased with increasing alternating frequency of two monocular half-images without a delay between them. Increasing delays led to increasing thresholds. In Experiment 2, we compared stereopsis resulting from two monocular half-images of a random-dot stereogram and binocular luster with respect to the minimum alternating frequency of the two half-images and the maximum interocular delay that were tolerated without a breakdown of the impression. Below 3 Hz, no stereopsis occurred. Binocular luster was observed only above 10 Hz. The mean threshold of interocular delay for detecting the global figure in a random-dot stereogram was about 51 msec, but for binocular luster it was about 20 msec. Overall, temporal integration was better for stereopsis than for binocular luster.     

Visual processing and dyslexia

Magnocellular-pathway deficits have been hypothesized to be responsible for the problems experienced by dyslexic individuals in reading. However, research has yet to provide a detailed account of the consequences of these deficits or to identify the behavioural link between them and reading disabilities. The aim of the present study was to determine the potential consequences of the magnocellular-pathway deficits for dyslexics in a comprehensive range of visual tasks. Dyslexics and nondyslexics were compared on their ability to (i) perform vernier-acuity and orientation-acuity tasks; (ii) perceive motion by using a range of measures common in the psychophysical literature (Dmin, Dmax, and global coherence); and (iii) perceive shapes presented in random-dot stereograms at a range of disparity pedestals, thereby dissociating stereopsis from vergence control. The results indicated no significant differences in performance between the dyslexic and nondyslexic subjects in terms of the visual-acuity measures. In general, dyslexics performed relatively poorly on measures of motion perception and stereopsis, although when considered individually some of the dyslexics performed better than some of the controls. The poor performance of the dyslexics in the stereo-gram tasks was attributable to a subgroup of dyslexics who also appeared to have severe difficulty with the motion-coherence task. These data are consistent with previous evidence that some dyslexics may have deficits within the magnocellular visual pathway.     

Attention-based texture segregation

Luminance- or color-defined +/- 45 degrees-oriented bars were arranged to yield single-feature or double-conjunction texture pairs. In the former, the global edge between two regions is formed by differences in one attribute (orientation, or color, or luminance). In the color/orientation double-conjunction pair, one region has +45 degrees red and -45 degrees green textels, the other -45 degrees red and +45 degrees green textels (the luminance/orientation double-conjunction pair is similar); such a pair contains a single-feature orientation edge in the subset of red (or green) textels, and a color edge in the subset of +45 degrees (or -45 degrees) textels. We studied whether edge detection improved when observers were instructed to attend to such subsets. Two groups of observers participated: in the test group, the stimulus construction was explained to observers, and they were cued to attend to one subset. The control group ran through the same total number of sessions without explanations/cues. The effect of cuing was week but statistically significant. Feature cuing was more effective for color/orientation than for luminance/orientation conjunctions. Within each stimulus category, performance was nearly the same no matter which subset was attended to. On average, a global performance improvement occurred over time even without cuing, but some observers did not improve with either cuing or practice. We discuss these results in the context of one-versus two-stage segregation theories, as well as by reference to signal enhancement versus noise suppression. We conclude that texture segregation can be improved by attentional strategies aimed to isolate specific stimulus features.     

Two forms of retinal disparity

The retinal disparities in stereograms where the vertical alignment of pairs of homologous points in one eye differs from that in the other eye were found to be more effective than disparities that do not involve that kind of binocular difference. The presence of such “transverse disparities” was found to shorten the time elapsed until perceived depth was reported in four instances, in two simple stereogram pairs and in two different pairs of random dot pattern stereograms. In an experiment where binocular parallax was in conflict with an effect of past experience, the presence of transverse disparities caused binocular parallax to prevail. The presumption that the amount of perceived depth depends only on the amount of disparity (provided distances from the eyes are unchanged) and not on the configuration in which it manifests itself was found not to hold in stereograms containing transverse disparities.     

Attention and cue-producing behavior in the monkey

Explicit cue-producing responses were employed to study attending behavior in the monkey. The subjects learned a discrimination based on compound stimuli, a vertical bar embedded on a red ground versus a horizontal bar on a green ground. On some trials only one of the two stimulus components was presented (red versus green or vertical versus horizontal bar), and the animals had the option of responding on the basis of the component presented or transforming it to the compound discriminanda by means of a cue-producing response. Analysis of the choice and cue-producing response behavior showed that (a) both monkeys acquired the discrimination between the compound cues solely on the basis of the color component, (b) mastery of this discrimination did not confer any "habit loading" (discriminative control) on the bar component, and (c) the monkey may prefer to respond on the basis of one component (color) even though it is capable of using both components equally effectively.     

Aging and the perception of 3-D shape from dynamic patterns of binocular disparity

In two experiments, we investigated the ability of younger and older observers to perceive and discriminate 3-D shape from static and dynamic patterns of binocular disparity. In both experiments, the younger observers' discrimination accuracies were 20% higher than those of the older observers. Despite this quantitative difference, in all other respects the older observers performed similarly to the younger observers. Both age groups were similarly affected by changes in the magnitude of binocular disparity, by reductions in binocular correspondence, and by increases in the speed of stereoscopic motion. In addition, observers in both age groups exhibited an advantage in performance for dynamic stereograms when the patterns of binocular disparity contained significant amounts of correspondence "noise." The process of aging does affect stereopsis, but the effects are quantitative rather than qualitative.     

Second-order avoidance behavior in monkeys

Two monkeys were trained on a three-component multiple schedule using discrete trials. In one component (food), a response terminated a red stimulus and produced food and S(Delta). In the second component (avoidance), a response terminated a green stimulus and avoided shock. In the third component (optional), a response terminated a blue stimulus and produced S(Delta). The consequence of not responding in the blue stimulus, however, was the production of either a food or an avoidance trial. Manipulation of these consequences showed that when food trials were available only a small percent of the time after optional trials, the subjects tended not to respond in the blue, even though this led to an increase in the total number of avoidance trials per day. If only avoidance trials followed as a consequence of not responding in the optional component, the animals terminated the blue stimulus and avoided the avoidance trials. Throughout the experiment both subjects maintained consistently low shock rates (about 10 per day) and these were not affected by the various manipulations. The data suggest that a stimulus associated with avoidance can be a conditioned aversive stimulus and will maintain a more remote avoidance response under certain conditions.     

Postfusional latency in stereoscopic slant perception and the primitives of stereopsis

Random dot stereograms of slanted surfaces were constructed, each representing one or two slanted surfaces in different relative arrangements and with different axes. Latency to fusion and from fusion to stereoscopic resolution was measured for each stimulus. It was found that latency to fusion was always very brief but that latency to stereoscopic resolution varied markedly, depending upon the orientation and arrangement of the stereoscopic surfaces. A gradient of discontinuities at a surface boundary produced an instant slant response for that surface, whereas a gradient of absolute disparities across the surface did not, except under conditions where vertical declination (a form of orientation disparity) was present. We conclude that stereopsis is not based on the primitives used in matching the images for fusion and that it is, at least initially, a response to disparity discontinuities which play no role in the fusion process. We also conclude that vertical declination is responded to globally as a slant around a horizontal axis but that other forms of orientation disparity are ineffective. The evidence from our experiments does not support the existence of a stereoscopic ability to respond globally to differences in magnification (or spatial frequency). It is suggested that stereoscopic perception of slant around a vertical axis is slow because it results from the integration of local processes.     

Temporal integration differences between crossed and uncrossed stereoscopic mechanisms

In this study, we investigated temporal integration of disparity information for crossed and uncrossed stereopsis. Across three experiments, exposure duration thresholds were measured for stereoscopic stimuli created from dynamic random-dot stereograms. In Experiment 1, an investigation of disparity detection showed that detection thresholds were equal for the crossed and uncrossed directions. In Experiment 2, an examination of duration limits for depth perception showed that critical durations were lower, and depth more veridical, for crossed depth than for uncrossed depth. In Experiment 3, an investigation of depth discrimination revealed that discrimination thresholds were lower for crossed depth than for uncrossed depth. These results suggest that crossed and uncrossed mechanisms differ in terms of their temporal integration properties at processing levels involving the computation and discrimination of depth.     

An ecological analysis of binocular vision

Summary Underlying the classic binocularity problems of singleness and three-dimensionality is a theory that the stimulus for binocular vision constitutes two two-dimensional images and metric differences between parts of those images. This characterization of the stimulus is criticized here and in its stead an ecologically-based characterization is presented wherein a binocular transformational invariant is shown to specify absolute (body-scaled) size, shape, and distance. The transformation is characterized as a rotation and its specificity to distance assumes a constant interocular distance and either homogeneously textured or extended surfaces. Four experiments demonstrate perceivers' abilities to detect this information and report (verbally or by reaching) the absolute distances of surfaces in stereograms. A fifth experiment revealed that accurate performance did not depend on oculomotor information. The assumptions of texture extent and distribution and constant interocular distance and their possible violations were discussed. A sixth experiment demonstrated that violations of interocular distance are absorbed by surface shapes. The existence and detection of a binocular rotation dissolves the putative problems of singleness and stereopsis, indicates that the importance of having two frontal eyes is for perception of absolute distance, and reformulates the problems for an algorithmic (physiological) theory of vision.     

Maternal gestational stress alters adaptive and social behavior in adolescent rhesus monkey offspring

Data from rodent studies have indicated that stress experienced by pregnant mothers may result in behavioral and biological abnormalities in their offspring. However, few studies have examined the effects of prenatal stress on the offspring beyond the childhood period. In this study, 7 prenatally stressed (PNS) monkeys and 7 monkeys from undisturbed pregnancies were tested under mildly challenging conditions at 4 years of age. Following separation from cagemates and group formation, PNS monkeys showed more locomotion, abnormal and disturbance behavior than controls. Controls showed approximately six times more play than PNS animals. The PNS males showed the most clinging to others and the largest increase in contact with other animals over the period. Group differences were also found when the monkeys were observed as groups or alone in a playroom. Controls showed more explanatory behavior in the playroom, whereas PNS monkeys showed more inactivity. Control animals showed a decrease in distress vocalizations over time in the playroom, whereas PNS animals showed the opposite pattern. Control animals spent more time in proximity to and contact with cagemates than PNS animals. These results indicate that prenatal stress can have effects on adaptive and social behavior that persist into adolescence.