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PubMed1999

Short term modification of disparity vergence eye movements.

Dynamics of disparity vergence eye movements can be modified by adaptive stimuli that generate large transient disparities. These modifications were observed for convergence as well as divergence eye movements. After modification, the peak velocities of the step responses for convergence and divergence were substantially higher than in normal baseline responses, a change observed in all four subjects studied. The change in peak velocity of a step response occurred very rapidly after presentation of the adaptive stimuli. Main sequence plots showed that first-order dynamic characteristics increased for post-adaptive responses with respect to normal step responses. Hence, response modification could be quantified as a change in gain accompanied with an increase in the effective response time constant. The adaptive responses to convergent and divergent 'disappearing' step stimuli revealed that the adaptation process modifies the high-velocity component of both disparity convergence and divergence eye movements. Moreover, a gain change in this component alone could account for both the gain and the time constant modifications seen in the overall response. A process of recovery or de-adaptation was also observed for both convergence and divergence eye movements. This observed short-term modification demonstrates a unique control mechanism for vergence eye movements that is effective in either direction.

Adaptation, Ocular↗
PubMed1999

Stereopsis from contrast envelopes.

We report two experiments concerning the site of the principal nonlinearity in second-order stereopsis. The first exploits the asymmetry in perceiving transparency with second-order stimuli found by Langley et al. (1998) (Proceedings of the Royal Society of London B, 265, 1837-1845) i.e. the product of a positive-valued contrast envelope and a mean-zero carrier grating can be seen transparently only when the disparities are consistent with the envelope appearing in front of the carrier. We measured the energy at the envelope frequencies that must be added in order to negate this asymmetry. We report that this amplitude can be predicted from the envelope sidebands and not from the magnitude of compressive pre-cortical nonlinearities measured by other researchers. In the second experiment, contrast threshold elevations were measured for the discrimination of envelope disparities following adaptation to sinusoidal gratings. It is reported that perception of the envelope's depth was affected most when the adapting grating was similar (in orientation and frequency) to the carrier, rather than to the contrast envelope. These results suggest that the principal nonlinearity in second-order stereopsis is cortical, occurring after orientation- and frequency-selective linear filtering.

Adaptation, Ocular↗
PubMed1994

Perceiving textures: beyond filtering.

Classical texture segregation theory and current visual filtering models suggest that rapid texture segregation is determined by the properties of early cortical filtering. To examine this assumption, we manipulated the binocular disparity of texture elements (an L vs a bar) relative to adjoining squares. Such a manipulation has little effect on the early cortical filtered representations of the stimuli but can have a strong influence on surface representation. In particular, it can determine whether an image patch is seen as a distinct surface, in front, or part of a larger occluded surface, in back. When seen as part of an occluded surface, the distinctive shape of each texture element is reduced. Accordingly, we found that texture discrimination performance was impaired when the texture elements had uncrossed disparities ("back" case). Additional experiments indicated that this result was not due to the disparity manipulation alone, but occurred only when each texture element was perceived as part of an occluded surface. We conclude that in rapid texture discrimination, the visual system cannot ignore information regarding surface layout. This suggests that for purposes of texture perception, either: (a) the visual system has no access to the filtered image or (b) one cannot entertain the concept of a filtered image uninfluenced by higher order surface processing.

Depth Perception↗
PubMed1995

Binocular disparity processing with opposite-contrast stimuli.

Stereoscopic perception of relative depth with reversed-contrast half images differs in several important respects from stereopsis with matched-contrast half images. Thus, reversed-contrast images show no correlated shift in visual direction, indicating that the sensory-fusion mechanism ignores opposite-sign edges; one experiment addressed this aspect of the problem. Mainly, this was a quantitative study of opposite-contrast stereopsis, in which stereoacuity was measured as a function of bar width by means of narrow-band stimuli. Acuity was about an order of magnitude worse for reversed-contrast than for matched stimuli, but the ability to see valid (disparity-dependent) depth was not altogether lost even with wide (1 cycle deg-1) reversed-contrast bars. It is generally believed that depth with opposite-contrast stimuli is mediated by interaction between binocular stimuli components that have the same sign of contrast. Perceived depth was measured as a function of disparity and thus one of the predictions of that 'same-sign hypothesis' was tested experimentally; then, the magnitude of same-sign components was manipulated within the reversed-contrast stimuli, and thus the general prediction of the same-sign hypothesis was tested. The results show conclusively that the same-sign hypothesis cannot account for opposite-contrast stereopsis; its mechanism remains unknown.

Contrast Sensitivity↗
PubMed2001

The perception of texture on folded surfaces.

Do judgments of texture similarity reflect surface texture or image texture? To find out, we had observers view a rectangular surface that was folded into three panels, much like a brochure. Each panel was textured with an oriented noise pattern and the observers' task was to determine which side panel matched the center panel in surface texture. Information about surface geometry was conveyed by binocular disparity and by the boundaries of the rectangular surface. We found that observers were often consistently wrong, selecting the texture that differed in the image and not on the surface. In sharp contrast, when observers judged the texture orientation on each panel individually, their judgments were accurate reflections of the surface texture. So even when observers can recover surface texture, their judgments of texture similarity may still be based on image texture.

Adolescent↗
PubMed2001

Stereopsis-processing regions in the human parieto-occipital cortex.

We performed fMRI on the human parieto-occipital cortex in order to identify the neural processing regions of stereopsis. Visual stimulation for stereopsis was performed with a random-dot stereogram displayed in the image guides of a new binocular visual stimulation device that we developed. Interestingly, regions from the dorsal portion of the occipital lobe to the superior parietal lobule were activated by binocular disparity, while the inferior parietal lobule was not activated. Moreover, these regions were shown as dominant in the right hemisphere. Functional brain mapping revealed that the processing regions of stereopsis were dorsally located in parieto-occipital cortex, and that the superior parietal lobule is an important region for neural processing of human stereopsis.

Adult↗
PubMed2004

Can misalignments in typical infants be used as a model for infantile esotropia?

PURPOSE: To investigate the nature of early ocular misalignments in human infants to determine whether they can provide insight into the etiology of esotropia and, in particular, to examine the correlates of misalignments. METHODS: A remote haploscopic photorefraction system was used to measure accommodation and vergence in 146 infants between 0 and 12 months of age. Infants underwent photorefraction immediately after watching a target moving between two of five viewing distances (25, 33, 50, 100, and 200 cm). In some instances, infants were tested in two conditions: both eyes open and one eye occluded. The resultant data were screened for instances of large misalignments. Data were assessed to determine whether accommodative, retinal disparity, or other cues were associated with the occurrence of misalignments. RESULTS: The results showed that there was no correlation between accommodative behavior and misalignments. Infants were more likely to show misalignments when retinal disparity cues were removed through occlusion. They were also more likely to show misalignments immediately after the target moved from a near to a far position in comparison to far-to-near target movement. DISCUSSION: The data suggest that the prevalence of misalignments in infants of 2 to 3 months of age is decreased by the addition of retinal disparity cues to the stimulus. In addition, target movement away from the infant increases the prevalence of misalignments. These data are compatible with the notion that misalignment are caused by poor sensitivity to targets moving away from the infant and support the theory that some forms of strabismus could be related to failure in a system that is sensitive to the direction of motion.

Accommodation, Ocular↗
PubMed1991

Stereopsis in the cat: behavioral demonstration and underlying mechanisms.

The neural substrates subserving stereopsis were investigated behaviorally and electrophysiologically in the cat. In one set of studies, we examined behaviorally the ability of normal cats to perceive depth on the sole basis of spatial disparity using random-dot stereograms. Results showed that the animals were able to carry out this discrimination. We then evaluated the contribution of the optic chiasm, the corpus callosum and the primary visual cortex to this function. Results indicated that: (1) chiasma transection drastically reduced the ability of the animals to solve the random-dot problem; (2) a callosal split had little or no effect on their ability to relearn the same discrimination; (3) a section of both the corpus callosum and optic chiasm abolished this ability; and (4) bilateral lesions of areas 17-18 also abolished it. In another set of studies, we examined electrophysiologically the properties of neurons in the various visual cortical areas where disparity-based depth discrimination processes are presumed to take place. We recorded from areas 17, 18 and 19 of normal and split-chiasm cats. Results showed that: (1) the primary visual cortex of the normal cat contained cells sensitive to stimulus disparity; (2) these disparity sensitive neurons were also present in area 19 although in a much lower proportion and were more widely tuned than those in areas 17-18; and (3) following the section of the optic chiasm, there was a significant decrease in the number of disparity sensitive cells in areas 17-18, whereas in area 19 they were nearly completely absent. The results obtained from the lesion studies and from the single unit recording experiments indicate that stereoscopic depth perception is highly dependent in the cat upon the integrity of the through-the-chiasm geniculo-striate pathway and its target primary visual cortex.

Animals↗
PubMed1993

The effect of disparity on motion coherence.

Many moving plaid stimuli are ambiguous, and perception switches between a coherent plaid pattern and two transparent gratings. Here, experiments are reported that examined the effect of stereodepth between the two gratings of the moving plaid stimulus on the perception of coherence or motion transparency. Increasing disparity increased the percentage of time that two independently drifting transparent gratings were perceived. This was studied for plaids with various levels of intersection luminance. Using intersection luminances beyond conditions of physical transparency increased the percentage of time that one coherent plaid was seen. These two opposing influences could be pitted against each other to achieve constant levels of coherence. An adaptation paradigm was also used in which observers adapted to a stationary stimulus with either zero, crossed or uncrossed disparity between the gratings, and then indicated the occurrence of coherence and motion transparency in test stimuli of drifting plaids with zero, crossed or uncrossed disparity. Adaptation to crossed and uncrossed stereo-depth increased relative perceived coherence equally, especially for zero test disparity. An analysis of the length of the episodes of coherence and motion transparency indicated that the effect of adaptation was to decrease the length of motion transparency episodes, while the length of coherence episodes did not change. It is concluded that mechanisms involved in the processing of stereo-depth must have an input to the integration stage of the motion channel and that pattern and component motion mechanisms can operate quite independently.

Adaptation, Ocular↗
PubMed2004

Binary and ternary textures containing higher-order spatial correlations.

A quantitative method is presented for creating a large number of classes of binary (256) and ternary (7.62 x 10(12)) textures. The binary textures are presented as black and white (contrasts -1 and 1). The ternary textures have three levels: black, white and the mean luminance gray (contrasts -1, 0 and 1). The ternary patterns in particular display a wide variety of properties, including depth cues from disparity and lighting. Given the very large number of ternary patterns, we present guidelines and analytical methods for selecting sets of textures with particular image qualities and/or nonlinear relationships between pixels. The second- and third-order correlation functions of several thousand examples were examined to reveal patterns that are functionally isotrigon with other textures and or with uniformly distributed noise patterns.

Contrast Sensitivity↗
PubMed1995

Comparing depth from motion with depth from binocular disparity.

The accuracy of depth judgments that are based on binocular disparity or structure from motion (motion parallax and object rotation) was studied in 3 experiments. In Experiment 1, depth judgments were recorded for computer simulations of cones specified by binocular disparity, motion parallax, or stereokinesis. In Experiment 2, judgments were recorded for real cones in a structured environment, with depth information from binocular disparity, motion parallax, or object rotation about the y-axis. In both of these experiments, judgments from binocular disparity information were quite accurate, but judgments on the basis of geometrically equivalent or more robust motion information reflected poor recovery of quantitative depth information. A 3rd experiment demonstrated stereoscopic depth constancy for distances of 1 to 3 m using real objects in a well-illuminated, structured viewing environment in which monocular depth cues (e.g., shading) were minimized.

Adult↗
PubMed2000

Stereo correspondence in one-dimensional Gabor stimuli.

Previous data [Prince, S.J.D., & Eagle, R.A., (1999). Size-disparity correlation in human binocular depth perception. Proceedings of the Royal Society of London B, 266, 1361-1365] have demonstrated that the upper disparity limit for stereopsis (DMax) is considerably smaller in filtered noise stereograms than in isolated Gabor patches of the same spatial frequency. This discrepancy is not currently understood. Here, the solution of the correspondence problem for bandpass stereograms was further examined. On each trial observers were presented with two one-dimensional Gabor stimuli containing disparities of equal magnitude but opposite sign. Subjects were required to indicate which interval contained the crossed disparity stimulus. It was found that matching behaviour changed as a function of Gabor envelope size. As a function of disparity magnitude, performance cycled between mostly correct and mostly incorrect at large envelope sizes but was always correct at small envelope sizes. At intermediate envelope sizes performance was cyclical at small disparities but always correct at large disparities. The critical envelope size at which performance changed from mostly correct to mostly incorrect at 270 degrees phase disparity was used as a measure of the matching performance as other parameters of the Gabor were varied. Both absolute and relative contrast were shown to influence the perceived sign of matches. Critical envelope size was also found to decrease as a function of spatial frequency, but more slowly than a phase-based limit would predict. These data cannot be predicted by current models of stereopsis, and can be used to constrain future models.

Contrast Sensitivity↗
PubMed1997

Neural network model of short-term horizontal disparity vergence dynamics.

We present a neural network model of short-term dynamics of the human horizontal vergence system (HVS) and compare its predictions qualitatively and quantitatively with a large variety of horizontal disparity vergence data. The model consists of seven functional stages, namely: (1) computation of instantaneous disparity; (2) generation of a disparity map; (3) conversion of the disparity into a velocity signal; (4) push-pull integration of velocity to generate a position signal; (5) conversion of the position signal to motoneuron/plant activity for each eye; (6) gating of velocity overdrive signal to motoneuron/plant system; and finally (7) discharge path for position cells. Closed-loop (normal binocular viewing) symmetric step and staircase disparity vergence data were collected from three subjects and model parameters were determined to quantitatively match each subject's data. The simulated closed-loop as well as open-loop (disparity clamped viewing) symmetric step, sinusoidal, pulse, staircase, square and ramp wave responses closely resemble experimental results either recorded in our laboratory or reported in the literature. Where possible, the firing pattern of the neurons in the model have been compared to actual cellular recordings reported in the literature. The model provides insights into neural correlates underlying the dynamics of vergence eye movements. It also makes novel predictions about the human vergence system.

Convergence, Ocular↗
PubMed1991

Image segmentation and VEP topography.

VEP scalp fields were measured for four pattern onset-offset stimuli identical except for the single contrast attribute that allowed a square-wave grating to be seen. The contrast attribute used for each stimulus was either texture, binocular disparity, luminance, or color. All stimuli were presented as dynamic random dot displays. Scalp potentials were sampled at 1.5 cm intervals by 66 electrodes over the posterior scalp. Customized rigid helmets were constructed for each subject to ensure reproducible electrode placement. The scalp fields due to the four stimuli were significantly different in all cross-comparisons. This implies that in the human visual cortex there exists regional selectivity for the processing of the four different contrast attributes.

Brain Mapping↗
PubMed1989

Is there a constancy for velocity?

Human observers are unable to use disparity information to transform the angular velocity signal into a precise object-based code. The Weber fraction for discriminating changes in objective velocity (cm/sec) is about twice the Weber fraction for discriminating changes in angular velocity (deg/sec), and is substantially higher than predicted from a combination of the errors in judging disparity and angular velocity. By comparison, judgments of the distance traversed by the moving target show excellent size constancy. The discrimination of changes in objective size (cm) is as precise as the discrimination of changes in angular size (deg). The angular velocity signal is useful without transformation into an object-centered signal; it guides eye and body movements, and is the basis of motion parallax judgments. The need to retain this angular signal may explain why there is no efficient mechanism for velocity constancy.

Discrimination, Psychological↗
PubMed2001

Macaque inferior temporal neurons are selective for three-dimensional boundaries and surfaces.

The lower bank of the superior temporal sulcus (TEs), part of the inferior temporal cortex, contains neurons selective for disparity-defined three-dimensional (3-D) shape. The large majority of these TEs neurons respond to the spatial variation of disparity, i.e., are higher-order disparity selective. To determine whether curved boundaries or curved surfaces by themselves are sufficient to elicit 3-D shape selectivity, we recorded the responses of single higher-order disparity-selective TEs neurons to concave and convex 3-D shapes in which the disparity varied either along the boundary of the shape, or only along its surface. For a majority of neurons, a 3-D boundary was sufficient for 3-D shape selectivity. At least as many neurons responded selectively to 3-D surfaces, and a number of neurons exhibited both surface and boundary selectivity. The second aim of this study was to determine whether TEs neurons can represent differences in second-order disparities along the horizontal axis. The results revealed that TEs neurons can also be selective for horizontal 3-D shapes and can code the direction of curvature (vertical or horizontal). Thus, TEs neurons represent both boundaries and surfaces curved in depth and can signal the direction of curvature along a surface. These results show that TEs neurons use not only boundary but also surface information to encode 3-D shape properties.

Animals↗
PubMed2001

A hierarchy of the functional organization for color, form and disparity in primate visual area V2.

By combining optical imaging, single unit electrophysiology and cytochrome oxidase (CO) histology, we sought to reveal in greater detail the functional organization within the CO stripes of visual area V2 of primates. To visualize the disparity selective regions of V2, the imaging of binocular interaction was employed. These imaging maps guided single unit penetrations that then revealed a columnar organization for disparity. Our studies also showed a pattern of intermixing between the color and disparity pathways of V2, including the existence of single cells tuned for both color and disparity. While previous studies have suggested that the CO stripes of V2 constitute the fundamental organizational unit within V2, our results show a further level of organization consisting of functionally distinct subcompartments, 0.7-1.5 mm in diameter, within individual stripes. These subcompartments, which are not clearly revealed by CO histochemistry, lie within each of the thin, pale, and thick CO dense stripes in V2 and are specific for aspects of color, orientation and retinal disparity, respectively. The present results favor an architectural view of V2, not unlike that of V1, as a collection of functionally distinct subcompartments or modules situated within each of the V2 stripes. These modules also support the notion that for each cortical area (e.g. V1, V2, V4), there exists a stereotyped cortical module with a geometry that is characteristic for each area. These modules exist as a middle tier in a hierarchy of functional organization within V2.

Animals↗
PubMed1997

Direction-selective coding of stereoscopic (cyclopean) motion.

This study employed a selective adaptation paradigm and investigated thresholds for direction discrimination of translational stereoscopic motion (moving binocular disparity information). The stimuli were moving arrays of randomly positioned stereoscopic discs created from disparity embedded in dynamic random-element stereograms. When discrimination thresholds were measured across a range of base directions following adaptation in a fixed direction, discrimination thresholds were maximally elevated 20-30 deg away from adaptation and reduced in the same direction as adaptation. These results are consistent with a distributed-channel model of direction coding and indicate that the direction of stereoscopic motion is encoded by adaptable direction-selective mechanisms similar to those proposed for luminance-defined motion.

Adaptation, Ocular↗