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PubMed2000

Neurons in the posteromedial lateral suprasylvian area of the cat are sensitive to binocular positional depth cues.

Single units in the posteromedial lateral suprasylvian area of the cat are known to be very sensitive to movement. A proportion of these cells can encode movement in depth, but it is unclear whether posteromedial lateral suprasylvian cells only rely upon motion cues to evaluate stimulus depth or whether they can also code for spatial cues. The present study aims at assessing the sensitivity to spatial disparity of binocular cells, in the postero-medial lateral suprasylvian area, in order to determine whether these units are tuned to positional depth cues. A total of 126 single cells located in the posteromedial lateral suprasylvian area of anesthetized, paralyzed cats were examined. As recordings were performed in the central visual field representation, receptive fields were small. A third of the receptive fields were surrounded by an inhibitory region and almost three-quarters of the cells were direction-selective. Most cells (110/114) were binocular, and a large proportion of single neurons responded to stimuli appearing on the fixation plane by increasing (tuned excitatory cells, 43%) or decreasing (tuned inhibitory cells, 14%) their response rate. A smaller proportion of cells increased their firing rate in response to crossed (near cells, 10%) or uncrossed (far cells, 6%) spatial disparities, hence demonstrating respective preference for stimuli presumably appearing in front of or behind the fixation plane. As compared to primary visual cortex, the proportion of disparity-sensitive cells in posteromedial lateral suprasylvian area is similar, but selectivity is significantly coarser. As the posteromedial lateral suprasylvian area can code for both spatial and temporal aspects of stimuli, this area might be involved in the spatiotemporal integration of depth cues, a process that may also participate in the control of accommodation and vergence.

Animals↗
PubMed1998

Visual test of Listing's law during vergence.

A simple visual test was used to measure how much Listing's plane rotates as a function of the vergence angle. This test measured the elevation-dependent torsional disparity of horizontal and vertical lines during three tasks: vergence on a near target, vergence through prisms that remained fixed, and through prisms that rotated with eye elevation. Consistent with our previous search-coil measurements, the results here suggest that the angle between the Listing's planes of the two eyes is somewhat less than the vergence angle.

Convergence, Ocular↗
PubMed2006

Vergence dynamics and variability of fixation disparity in children and adults.

With dichoptic nonius lines presented repetitively on a computer monitor for only short moments in time (e.g., 80 ms), temporal aspects of vergence can be assessed: the variability of fixation disparity with a stationary central fusion stimulus and the velocity of convergence or divergence changes. Results and test-retest correlations of these psychophysical vergence measures are shown for children and adults. In most children older than about 7 years, the estimation of vergence velocity turned out as well as in adults in terms of amount and reliability.

Adolescent↗
PubMed2006

The binocular coordination of eye movements during reading in children and adults.

Recent evidence indicates that each eye does not always fixate the same letter during reading and there has been some suggestion that processing difficulty may influence binocular coordination. We recorded binocular eye movements from children and adults reading sentences containing a word frequency manipulation. We found disparities of significant magnitude between the two eyes for all participants, with greater disparity magnitudes in children than adults. All participants made fewer crossed than uncrossed fixations. However, children made a higher proportion of crossed fixations than adults. We found no influence of word frequency on children's fixations and on binocular coordination in adults.

Adolescent↗
PubMed2005

V1 partially solves the stereo aperture problem.

If a bar stimulus extends beyond a cell's receptive field, then alterations in binocular disparity parallel to the bar's orientation leave the portion of the stimulus within the cell's receptive field unchanged. This makes it hard for the cell to respond correctly to the bar's disparity. Likening the cell's receptive field to an aperture through which the cell views the world, this issue has been called the "aperture problem" and is a specific form of the more general stereo correspondence problem. We found no cells in macaque primary visual cortex (V1) that, when faced with this situation, were sensitive to the disparity of the bar. However, we did find a number of cells that showed sensitivity to parallel disparity shifts, but these cells responded only to the ends of the long bar. The ability to respond selectively to such tracking features could be the first step towards solving the stereo aperture problem. The second step would require either that the disparity information that in V1 is associated only with the ends of the stimulus be associated with the rest of the stimulus or that subsequent stages of visual processing respond preferentially to the end-selective cells. As this second step does not appear to occur in V1, we conclude that V1 only partially solves the stereo aperture problem.

Animals↗
PubMed1998

Role of spatial and temporal coincidence in depth organization.

The linking of spatial information is essential for coherent space perception. A study is reported of the contribution of temporal and spatial alignment for the linkage of spatial elements in terms of depth perception. Stereo half-images were generated on the left and right halves of a large-screen video monitor and viewed through a mirror stereoscope. The half-images portrayed a black vertically oriented bar with two brackets immediately flanking this bar and placed in crossed or uncrossed disparity relative to the bar. A pair of thin white 'bridging lines' could appear on the black bar, always at zero disparity. Brackets and bridging lines could be flickered either in phase or out of phase. Observers judged whether the brackets appeared in front of or behind the black bar, with disparity varied. Compared to conditions when the bridging lines were absent, depth judgments were markedly biased toward "in front" when bridging lines and brackets flashed in temporal phase; this bias was much reduced when the bridging lines and brackets flashed out of phase. This biasing effect also depended on spatial offset of lines and brackets. However, perception was uninfluenced by the lateral separation between object and brackets.

Computer Graphics↗
PubMed2000

Cross-domain adaptation reveals that a common mechanism computes stereoscopic (cyclopean) and luminance plaid motion.

Across three experiments, this study investigated the visual processing of moving stereoscopic plaid patterns (plaids created with cyclopean components defined by moving binocular disparity embedded in a dynamic random-dot stereogram). Results showed that adaptation to a moving stereoscopic plaid or its components affected the perceived coherence of a luminance test plaid, and vice versa. Cross-domain adaptation suggests that stereoscopic and luminance motion signals feed into a common pattern-motion mechanism, consistent with the idea that stereoscopic motion signals are computed early in the motion processing stream.

Adaptation, Ocular↗
PubMed2006

Temporal dynamics of stereo correspondence bi-stability.

Periodic stereoscopic stimuli offer multiple viable solutions to the stereo correspondence problem. When viewing such stimuli for prolonged periods of time, observers continually switch their perceptual state between alternative correspondence solutions. We examine the temporal dynamics of this correspondence bi-stability. Participants were presented with an ambiguous stereogram comprised of regularly spaced dots. This stimulus was perceived as a fronto-parallel plane situated either behind or in front of fixation, depending on the achieved correspondence solution. The stimulus was presented continuously for one minute, with participants instructed to report the sign of the perceived depth at the sound of an auditory prompt presented, on average, every 2 s. Inter-ocular contrast and available disparities were varied so as to manipulate preferred correspondence. We find that participants are initially biased to perceive the stimulus as having an uncrossed disparity. Furthermore, we find that following an initial period of change, perceptual preference and perceptual stability (measured as the probability of an observer's percept changing between consecutive responses) remain constant over the presentation period. Finally, we find that manipulations of matching preference affect both the transient preference for, and stability of, one percept over another. Our results suggest two distinct phases of biasing in the correspondence matching process, one early, the other sustained.

Depth Perception↗
PubMed2004

Perceptual "read-out" of conjoined direction and disparity maps in extrastriate area MT.

Cortical neurons are frequently tuned to several stimulus dimensions, and many cortical areas contain intercalated maps of multiple variables. Relatively little is known about how information is "read out" of these multidimensional maps. For example, how does an organism extract information relevant to the task at hand from neurons that are also tuned to other, irrelevant stimulus dimensions? We addressed this question by employing microstimulation techniques to examine the contribution of disparity-tuned neurons in the middle temporal (MT) visual area to performance on a direction discrimination task. Most MT neurons are tuned to both binocular disparity and the direction of stimulus motion, and MT contains topographic maps of both parameters. We assessed the effect of microstimulation on direction judgments after first characterizing the disparity tuning of each stimulation site. Although the disparity of the stimulus was irrelevant to the required task, we found that microstimulation effects were strongly modulated by the disparity tuning of the stimulated neurons. For two of three monkeys, microstimulation of nondisparity-selective sites produced large biases in direction judgments, whereas stimulation of disparity-selective sites had little or no effect. The binocular disparity was optimized for each stimulation site, and our result could not be explained by variations in direction tuning, response strength, or any other tuning property that we examined. When microstimulation of a disparity-tuned site did affect direction judgments, the effects tended to be stronger at the preferred disparity of a stimulation site than at the nonpreferred disparity, indicating that monkeys can selectively monitor direction columns that are best tuned to an appropriate conjunction of parameters. We conclude that the contribution of neurons to behavior can depend strongly upon tuning to stimulus dimensions that appear to be irrelevant to the current task, and we suggest that these findings are best explained in terms of the strategy used by animals to perform the task.

Animals↗
PubMed1989

Recording of disparity vergence in comitant esotropia.

Vergence movements induced by base-out prisms were recorded, with an infrared eye movement recording system, in 5 patients with comitant esotropia and anomalous retinal correspondence and in 5 normal volunteers. The findings support the hypothesis that vergence movements in strabismus are induced by disparity, since accommodative as well as proximal vergence had been ruled out. Vergence movements are much slower and with different characteristics in strabismic patients with respect to normals. It may be argued that vergence movements represent the motor fusion component left over in strabismus. They may be considered the objective representation of the well-known phenomenon of prism compensation or adaptation, found in many strabismic patients.

Adolescent↗
PubMed2002

Monocular alignment in different depth planes.

We examined (a) whether vertical lines at different physical horizontal positions in the same eye can appear to be aligned, and (b), if so, whether the difference between the horizontal positions of the aligned vertical lines can vary with the perceived depth between them. In two experiments, each of two vertical monocular lines was presented (in its respective rectangular area) in one field of a random-dot stereopair with binocular disparity. In Experiment 1, 15 observers were asked to align a line in an upper area with a line in a lower area. The results indicated that when the lines appeared aligned, their horizontal physical positions could differ and the direction of the difference coincided with the type of disparity of the rectangular areas; this is not consistent with the law of the visual direction of monocular stimuli. In Experiment 2, 11 observers were asked to report relative depth between the two lines and to align them. The results indicated that the difference of the horizontal position did not covary with their perceived relative depth, suggesting that the visual direction and perceived depth of the monocular line are mediated via different mechanisms.

Adolescent↗
PubMed2003

Reading a population code: a multi-scale neural model for representing binocular disparity.

Although binocular neurons in the primary visual cortex are sensitive to retinal disparity, their activity does not constitute an unambiguous disparity signal. A multi-spatial-scale neural model for disparity computation is developed to examine how population activity might be interpreted to overcome ambiguities at the single neuron level. The model incorporates a front end that encodes disparity by a family of complex cell-like energy units and a second stage that reads the population activity. Disparity is recovered by matching the population response to a set of canonical templates, derived from the mean response to white noise stimuli at a range of disparities. Model predictions are qualitatively consistent with a variety of psychophysical results in the literature, including the effects of spatial frequency on stereoacuity and bias in perceived depths, and the effect of standing disparity on increment thresholds. Model predictions are also consistent with data on qualitative appearance of complex stimuli, including depth averaging, transparency, and corrugation. The model also accounts for the non-linear interaction of disparities in compound grating stimuli. These results show that a template-match approach reduces ambiguities in individual and pooled neuronal responses, and allows for a broader range of percepts, consistent with psychophysics, than other models. Thus, the pattern of neural population activity across spatial scales is a better candidate for the neural correlate of depth perception than the activity of single neurons or the pooled activity of multiple neurons.

Depth Perception↗
PubMed2003

A laminar cortical model of stereopsis and three-dimensional surface perception.

A laminar cortical model of stereopsis and later stages of 3D surface perception is developed and simulated. The model describes how initial stages of monocular and binocular oriented filtering interact with later stages of 3D boundary formation and surface filling-in in the lateral geniculate nucleus and cortical areas V1, V2, and V4. In particular, it details how interactions between layers 4, 3B, and 2/3A in V1 and V2 contribute to stereopsis, and clarifies how binocular and monocular information combine to form 3D boundary and surface representations. Along the way, the model modifies and significantly extends the disparity energy model. Neural explanations are given for psychophysical data concerning: 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, da Vinci stereopsis, and various lightness illusions. By relating physiology to psychophysics, the model provides new functional insights and predictions about laminar cortical architecture.

Contrast Sensitivity↗
PubMed2005

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.

Adult↗
PubMed1994

Precise placement of nonius lines on a personal computer screen for measuring fixation disparity.

A method is described for measuring fixation disparity psychophysically using the conventional monitor of a personal computer. Both the nonius test stimuli and the fusion stimulus were presented on the same screen. To be able to adjust the offset of the nonius lines in very small steps, special electronic circuitry was developed that allows up to 16 steps within the pixel separation of the monitor. The method is flexible in that various geometrical dimensions of the visual stimuli and different psychometric procedures can be used.

Fixation, Ocular↗
PubMed2005

Effect of interocular delay on disparity-selective v1 neurons: relationship to stereoacuity and the pulfrich effect.

The temporal properties of disparity-sensitive neurons place important temporal constraints on stereo matching. We examined these constraints by measuring the responses of disparity-selective neurons in striate cortex of awake behaving monkeys to random-dot stereograms that contained interocular delays. Disparity selectivity was gradually abolished by increasing interocular delay (when the delay exceeds the integration time, the inputs from the 2 eyes become uncorrelated). The amplitude of the disparity-selective response was a Gaussian function of interocular delay, with a mean of 16 ms (+/-5 ms, SD). Psychophysical measures of stereoacuity, in both monkey and human observers, showed a closely similar dependency on time, suggesting that temporal integration in V1 neurons is what determines psychophysical matching constraints over time. There was a slight but consistent asymmetry in the neuronal responses, as if the optimum stimulus is one in which the right stimulus leads by about 4 ms. Because all recordings were made in the left hemisphere, this probably reflects nasotemporal differences in conduction times; psychophysical data are compatible with this interpretation. In only a few neurons (5/72), interocular delay caused a change in the preferred disparity. Such tilted disparity/delay profiles have been invoked previously to explain depth perception in the stroboscopic version of the Pulfrich effect (and other variants). However, the great majority of the neurons did not show tilted disparity/delay profiles. This suggests that either the activity of these neurons is ignored when viewing Pulfrich stimuli, or that current theories relating neuronal properties to perception in the Pulfrich effect need to be reevaluated.

Action Potentials↗
PubMed1996

Changes in oculomotor functions before and after loading of a 3-D visually-guided task by using a head-mounted display.

Changes in visual and oculomotor functions were tested in eight young volunteer subjects after performing a 3-D visually-guided task for 25 min. The visual stimuli were given by using a head-mounted display. No significant change was detected in the AC/A ratio and the stereo acuity. Changes were found in refraction and ocular convergence. Slight but significant hyperopic changes were detected in refraction after the task. The mean amplitude of convergence eye movement elicited by test stimuli after the task was significantly changed in the pooled data. They were significantly reduced when the subjects were tested by the disparity step of 0.7 degrees, but significantly increased when tested by the step of 6 degrees. The mean amplitudes were not significantly changed after the task when the subjects were tested by the intermediate disparity steps of 1.5 and 3 degrees. In data obtained for each subject, the amplitude of ocular convergence evoked by test stimuli after the task was reduced significantly in most subjects. In the majority of subjects, however, the results were not consistently significant when they were tested by step stimuli with different disparities. In only a few subjects, the changes were consistently significant except when the largest disparity was tested. On the other hand, the mean peak velocities of ocular convergence after the task were not significantly changed in the pooled data. In individual subjects, the changes of velocities of ocular convergence evoked by test stimuli after the task were more variable: they increased in some subjects but decreased in other subjects. Consequently, in only one subject, both amplitudes and peak velocities of ocular convergence tested by the disparity steps of 0.7, 1.5 and 3 degrees were consistently reduced after the task. The changes in refraction or ocular convergence found in this study were relatively small, and were not in the pathological range. The significance of these data are discussed. The results of the present study provide the basis for the more detailed analyses of the human factor in the head-mounted display.

Adult↗
PubMed2003

Stereothresholds with simulated vergence variability and constant error.

Stereothresholds are elevated by vergence constant error (fixation disparity), vergence noise, or both. This study investigated the separate and combined effects of simulated vergence constant error and variability on stereothresholds in four normal observers. Targets were 30 arc min bright vertical lines presented separately to the two eyes for 150 ms in darkness. Vergence constant error, simulated as a pedestal disparity, was induced by altering the screen positions of the stereo half-images relative to a previously visible binocular fixation point. Vergence variability was simulated simultaneously by disconjugate motion (amplitude=0-0.5 deg per eye; frequency=2 or 4 Hz) of a pair of scanning mirrors in a Wheatstone stereoscope that was used to present the images to each eye. Various combinations of pedestal disparity and simulated vergence variability produce equivalent stereothresholds whenever the calculated mean deviation (sigma/instantaneous vergence errors//N) of the stimulus from the fixation plane is the same. In particular, stereothresholds are optimal for mean deviations up to approximately 1.4 arc min and then increase according to a power function with an exponent of 0.61. The results imply that vergence constant errors and vergence variability impair stereothresholds because of the resulting mean deviation from the horopter.

Adult↗