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PubMed2000

Surface integration influences depth discrimination.

Image fragments arising from partial occlusion may be perceptually unified by a surface integration process on the basis of similar color or texture. In a new objective measure pitting surface feature similarity against binocular disparity, observers discriminated whether a colored circle had either crossed or uncrossed disparity relative to a surrounding gray rectangle. Sensitivity to disparity was impaired only when (1) the configuration of the other surface fragments in the display supported the integration of a surface behind the rectangle and circle, and (2) matched the color of the central circle. Results were consistent with the hypothesis that a surface integration process integrated similarly-colored surface fragments into a smooth surface, even when those fragments were at different depths. Surface integration caused small and reliable effects on depth perception despite unambiguous disparity information. Perceived depth does not depend solely upon disparity, and may be determined after three-dimensional figural unity is established.

Color Perception↗
PubMed1995

Neuronal mechanisms underlying stereopsis: how do simple cells in the visual cortex encode binocular disparity?

Binocular neurons in the visual cortex are thought to form the neural substrate for stereoscopic depth perception. How are the receptive fields of these binocular neurons organized to encode the retinal position disparities that arise from binocular parallax? The conventional notion is that the two receptive fields of a binocular neuron have identical shapes, but are spatially offset from the point of retinal correspondence (zero disparity). We consider an alternative disparity-encoding scheme, in which the two receptive fields may differ in shape (or phase), but are centered at corresponding retinal locations. Using a reverse-correlation technique to obtain detailed spatiotemporal receptive-field maps, we provide support for the latter scheme. Specifically, we show that receptive-field profiles for the left and right eyes are matched for cells that are tuned to horizontal orientations of image contours. However, for neurons tuned to vertical orientations, the left and right receptive fields are predominantly dissimilar in shape. These results show that the striate cortex possesses a specialized mechanism for processing vertical contours, which carry the horizontal-disparity information needed for stereopsis. Thus, in a major modification to the traditional notion of the neural basis of stereopsis, we propose that binocular simple cells encode horizontal disparities in terms of phase at multiple spatial scales. Implications of this scheme are discussed with respect to the size-disparity correlation observed in psychophysical studies.

Algorithms↗
PubMed1996

Pooling of vertical disparities by the human visual system.

Two experiments are described in which the effects of scaling vertical disparities on the perceived amplitudes of dome-shaped surfaces depicted with horizontal disparities were examined. The Mayhew and Longuet-Higgins's theory and the regional-disparity-correction theory of Garding et al predict that scaling should generate a change in perceived depth appropriate to the viewing distance simulated by the scaled vertical disparities. Significant depth changes were observed, by means of a nulling task in which the vertical-disparity-scaling effect was cancelled by the observer choosing a pattern of horizontal disparities that made the dome-shaped surface appear flat. The sizes of the scaling effects were less than those predicted by either theory, suggesting that other cues to fixation distance such as oculomotor information played an appreciable role. In conditions in which 50% of the texture elements were given one value of vertical-disparity scaling and the remaining 50% were left unscaled, the size of the scaling effect on perceived depth could be accounted for by equally weighted pooling of the vertical-disparity information unless the two scalings were very dissimilar, in which case the lower scaling factor tended to dominate. These findings are discussed in terms of a Hough parameter estimation model of the vertical-disparity-pooling process.

Attention↗
PubMed2004

Why is spatial stereoresolution so low?

Spatial stereoresolution (the finest detectable modulation of binocular disparity) is much poorer than luminance resolution (finest detectable luminance variation). In a series of psychophysical experiments, we examined four factors that could cause low stereoresolution: (1) the sampling properties of the stimulus, (2) the disparity gradient limit, (3) low-pass spatial filtering by mechanisms early in the visual process, and (4) the method by which binocular matches are computed. Our experimental results reveal the contributions of the first three factors. A theoretical analysis of binocular matching by interocular correlation reveals the contribution of the fourth: the highest attainable stereoresolution may be limited by (1) the smallest useful correlation window in the visual system, and (2) a matching process that estimates the disparity of image patches and assumes that disparity is constant across the patch. Both properties are observed in disparity-selective neurons in area V1 of the primate (Nienborg et al., 2004).

Adult↗
PubMed1993

On the perceptual identity of dynamic stereopsis and kinetic depth.

This paper presents a set of experiments demonstrating novel interactions between kinetic depth (depth-from-motion) and dynamic stereopsis (depth-from-disparity). Previous research has shown that adaptation to a moving stereoscopic figure influences the subjective percept of a subsequently viewed kinetic depth figure. In this paper the interactions between kinetic depth and dynamic stereopsis are shown to be very robust and to occur in situations involving perceptual priming. It is also found that kinetic depth and dynamic stereo stimuli are indistinguishable when the stereoscopic stimulus has small, but perceptually salient, disparity. These results are consistent with the hypothesis that stimuli for kinetic depth and for dynamic stereopsis engage a common neural network.

Depth Perception↗
PubMed1998

The area of spatial integration for initial horizontal disparity vergence.

We investigated over what central area disparity in a random dot stereogram is integrated to stimulate an initial vergence response. Vergence was measured subjectively, with a forced choice dichoptic nonius vernier task following a brief (230 msec) stimulus presentation. Stimuli were random-dot stereograms showing a central circular disc of 12.5 min arc crossed retinal disparity in front of, and occluding, a same density fixation plane surround. The size of the disc was varied. All ten observers responded to the brief stimulus. Initial vergence increased with increasing disc diameter and, for nine out of ten subjects, reached a maximum with the disc ca 6 deg, suggesting this is the extent of the spatial integration region. Below 6 deg diameter, surround and target disparities were averaged together. Initial horizontal vergence responds automatically to a cyclopean target presented in the centre of gaze by pooling disparities within a limited but surprisingly large area.

Adult↗
PubMed2000

Three-dimensional shape coding in inferior temporal cortex.

Neurons in the rostral lower bank of the superior temporal sulcus (TEs), part of the inferior temporal cortex, respond selectively to three-dimensional (3D) shapes. We have investigated how these neurons represent disparity-defined 3D structure. Most neurons were selective for either first-order (disparity gradients) or second-order (disparity curvature) disparities. The latter selectivity proved remarkably vulnerable to disparity discontinuities, such as sharp edges or steps in disparity. The majority of the neurons remained selective for small disparity variations within the stimulus. 3D shape selectivity was preserved when the frontoparallel position or the stimulus size was altered. Thus, in TEs, 3D shape is coded by first- and second-order disparity-selective neurons, which are highly sensitive to spatial variations of disparity.

Action Potentials↗
PubMed1999

Does binocular disparity facilitate the detection of transparent motion?

Recent physiological studies have established that cortical cells that are tuned for the direction of motion may also exhibit tuning for binocular disparity. This tuning does not appear to provide any advantage in discriminating the direction of global motion in random-dot kinematograms. Here we investigated the possibility that this tuning may be important in the perception of transparent motion. Random-dot kinematograms were presented which contained coherent motion in a single direction or in two opposing directions. A greater proportion of signal dots was required for the detection of transparent motion than of motion in a single direction. This difference vanished when the two opposite directions of motion were presented with different disparities. These results suggest that the direction of global motion can be computed separately for surfaces which are clearly segregated in depth.

Humans↗
PubMed1996

Encoding of binocular disparity by simple cells in the cat's visual cortex.

1. Spatiotemporal receptive fields (RFs) for left and right eyes were studied for simple cells in the cat's striate cortex to examine the idea that stereoscopic depth information is encoded via structural differences of RFs between the two eyes. Traditional models are based on neurons that possess matched RF profiles for the two eyes. We propose a model that requires a subset of simple cells with mismatched RF profiles for the two eyes in addition to those with similar RF structure. 2. A reverse correlation technique, which allows a rapid measurement of detailed RF profiles in the joint space-time domains, was used to map RFs for isolated single neurons recorded extracellularly in the anesthetized paralyzed cat. 3. Approximately 30% of our sample of cells shows substantial differences between spatial RF structure for the two eyes. Nearly all of these neurons prefer orientations between oblique and vertical, and are therefore presumed to be involved in processing horizontal disparities. On the other hand, cells that prefer orientations near horizontal have matched RF profiles for the two eyes. Considered together, these findings suggest that the visual system takes advantage of the orientation anisotropy of binocular disparities present in the retinal images. 4. For some cells, the spatial structure of the RF changes over the time course of the response (inseparable RF in the space-time domain). In these cases, the change is similar for the two eyes, and therefore the difference remains nearly constant at all times. Because the difference of the RF structure between the two eyes is the critical determinant of a cell's relative depth selectivity for the proposed model, space-time inseparability of RFs is not an obstacle for consistent representation of stereoscopic information. 5. RF parameters including amplitude, RF width, and optimal spatial frequency are generally well matched for the two eyes over the time course of the response. The preferred speed and direction of motion are also well matched for the two eyes. These results suggest that the encoding of motion in depth is not likely to be a function of simple cells in the striate cortex. 6. The results presented here are consistent with our model, in which stereoscopic depth information is encoded via differences in the spatial structure of RFs for the two eyes. This model provides a natural binocular extension of the current notion of monocular spatial form encoding by a population of simple cells. Note, however, that our findings do not exclude the possibility that positional shifts of RFs also play a role in determining the disparity selectivity of cortical neurons.

Animals↗
PubMed2004

Receptive field size in V1 neurons limits acuity for perceiving disparity modulation.

Disparity selectivity in the striate cortex has generally been studied with uniform disparity fields covering the receptive field (RF). In four awake behaving monkeys, we quantitatively characterized the spatial three-dimensional structure of 55 V1 RFs using random dot stereograms in which disparity varied as a sinusoidal function of vertical position ("corrugations"). At low spatial frequencies, this produced a modulation in neuronal firing at the temporal frequency of the stimulus. As the spatial frequency increased, the modulation reduced. The mean response rate changed little and was close to that produced by a uniform stimulus at the mean disparity of the corrugation. In 48 of 55 (91%) neurons, the modulation strength was a lowpass function of spatial frequency. These results are compatible with a response determined only by the weighted mean of the disparities of the dots (the weights being set by the RF envelope) and suggest that there is no disparity-based surround inhibition or selectivity for disparity gradients. This simple weighting scheme predicts a relationship between RF size and the high-frequency cutoff. Comparison with independent measurements of RF size was compatible with this. All of this behavior closely matches the binocular energy model. The mean cutoff frequency, 0.5 cycles per degree, is similar to equivalent measures of decline in human psychophysical sensitivity for such depth corrugations as a function of frequency (Tyler, 1974; Prince and Rogers, 1998; Banks et al., 2004). This suggests that human cyclopean acuity for disparity modulations is limited by the selectivity of V1 neurons. This in turn is primarily limited by the RF size, because we find no sensitivity for disparity gradients or other disparity differences within the RFs.

Action Potentials↗
PubMed2002

Computing relief structure from motion with a distributed velocity and disparity representation.

Recent psychophysical experiments suggest that humans can recover only relief structure from motion (SFM); i.e., an object's 3D shape can only be determined up to a stretching transformation along the line of sight. Here we propose a physiologically plausible model for the computation of relief SFM, which is also applicable to the related problem of motion parallax. We assume that the perception of depth from motion is related to the firing of a subset of MT neurons tuned to both velocity and disparity. The model MT neurons are connected to each other laterally to form modulatory interactions. The overall connectivity is such that when a zero-disparity velocity pattern is fed into the system, the most responsive neurons are not those tuned to zero disparity, but instead are those having preferred disparities consistent with the relief structure of the velocity pattern. The model computes the correct relief structure under a wide range of parameters and can also reproduce the SFM illusions involving coaxial cylinders. It is consistent with the psychophysical observation that subjects with stereo impairment are also deficient in perceiving motion parallax, and with the physiological data that the responses of direction- and disparity-tuned MT cells covary with the perceived surface order of bistable SFM stimuli.

Cues↗
PubMed1999

Global motion processing is not tuned for binocular disparity.

An important goal of the visual system is the segmentation of image features into objects and their backgrounds. A primary cue for this is motion: when a region shares the same pattern of motion it is segregated from its surround. Three experiments were carried out to investigate whether the segmentation of image features on the basis of motion information is facilitated by the addition of binocular disparity. Coherence thresholds were measured for the discrimination of the global direction of motion of random dot kinematograms (RDKs) in which the relative disparity of the signal and noise dots was manipulated. When the signal dots were embedded in a three dimensional cloud of noise dots, coherence thresholds were similar to those measured when signal and noise dots were both presented with zero disparity. However, when the signal dots were separated from the noise dots in depth, global motion processing was strongly facilitated. These results were considered in terms of two models, one in which global motion is processed by disparity tuned mechanisms, the other in which the discrimination of the direction of motion is mediated by an attention-based system. It was concluded that global motion processing is not tuned for binocular disparity and that the facilitation of the discrimination of direction provided by binocular disparity in certain circumstances reflects the rôle of an attention-based system.

Depth Perception↗
PubMed1991

A neural network model of kinetic depth.

We propose a network model that accounts for the kinetic depth in structure from motion phenomena. Using plausible neural mechanisms, the model accounts for (1) fluctuations in perception when viewing a simple kinetic depth stimulus, (2) disambiguation of this stimulus with stereoscopic information, and (3) subsequent bias of the percept of this stimulus following stereoscopic adaptation. The model comprises two levels: a layer of monocular directionally selective motion detectors that provide input to a second layer of disparity-selective and direction-selective binocular mechanisms. The network of facilitatory and inhibitory connections between binocular mechanisms gives rise to fluctuations in network activity that mimic the fluctuations in perception of kinetic depth in the absence of disparity information. The results of a psychophysical experiment are consistent with the nature of the proposed interactions.

Adaptation, Ocular↗
PubMed2005

Colour, polarity, disparity, and texture contributions to motion segregation.

We measured how different cues are combined in motion-segregation processes by using motion stimuli where randomly distributed target dots were organised in global revolving motion while the remaining noise dots performed random motion. Target dots were cued with a different colour, polarity, disparity depth, or texture orientation than the noise dots, or they were the same as the noise dots. The stimuli were presented with a prolonged static cue preview which provided position cues to target dots or, briefly with static pre-target and post-target noise frames, which provided false position cues (no preview). All cues efficiently facilitated global motion segregation in cued-preview conditions. Colour completely failed to facilitate global motion segregation in no-preview conditions. Polarity and disparity facilitated segregation in no-preview conditions, although sensitivities were lower than in the preview conditions. Remarkably, texture orientation largely facilitated motion segregation by the same amount in both cued-preview and no-preview conditions. So, colour provides only position cues to the motion-segregation task whereas texture orientation, disparity, and to a lesser extent polarity are integrated with the segregation process.

Color Perception↗
PubMed2001

The combined influence of binocular disparity and shading on pictorial shape.

The combined influence of binocular disparity and shading on pictorial shape was studied. Stimuli were several pairs of stereo photographs of real objects. The stereo base was 0, 7, or 14 cm, and the location of the light source was varied over three positions (one from about the viewpoint of the camera, one about perpendicular to the line of sight, and one in between the two). Therefore, in total, nine different combinations were studied. Subjects had to perform surface attitude settings at about 300 positions in the image plane. From the settings, depth maps were calculated on which a principal components analysis was performed. It was found that three components were enough to account for at least 97.8% of the variance in the data. The first component accounted for shape constancy. The effects of the two cues could be isolated as a linear combination of the other two components. The effects of the disparity and the shading cue variation were found to combine in almost linear fashion.

Contrast Sensitivity↗
PubMed1993

Stereo and motion disparities interfere with positional averaging.

If required to judge the spatial separation between features that are sub-parts of larger objects, observers are biased in the direction of reporting the separation between the centroids of the objects. We predicted that this bias would be overcome by separating the target features from the larger objects, either by stereoscopic disparity or by relative motion. We measured the observer's perceptual biases for the "cluster illusion" with the target dots specified by: (1) contrast alone, (2) contrast and disparity, (3) disparity alone, (4) contrast and motion, and (5) motion alone. Disparity had the effect predicted, but the effects of motion were less clear.

Contrast Sensitivity↗
PubMed2000

Global factors that determine the maximum disparity for seeing cyclopean surface shape.

A disparity gradient limit explains why the maximum amplitude of sinusoidal disparity gratings increases with decreasing disparity spatial frequency. It also explains why the largest disparity for binocular fusion (diplopia threshold) varies directly with stimulus element separation. Does a disparity gradient limit also apply to the detection of cyclopean shape? A previous study addressed this question and concluded that it does not. We examined this question by measuring the largest disparity amplitude (dmax) at which observers could judge the shape of cyclopean disparity gratings. We used trapezoidal, triangular, sinusoidal, and square wave gratings in order to dissociate the effects of disparity gradient and disparity spatial frequency. Gabor micropatterns were used to minimize potential scale-dependent interactions with luminance processing. Our results support a disparity gradient limit for cyclopean shape perception, with additional factors being involved at high disparity spatial frequencies. Combining the gradient limit hypothesis with lowpass disparity filtering describes the pattern of dmax for both smooth and discontinuous surface shapes.

Depth Perception↗
PubMedDate not supplied

Effect of disparity in the peripheral field on short-latency ocular following responses.

Ocular following responses induced by brief movements of the visual scene were examined in monkeys for their dependence on disparity in the peripheral field. A random dot pattern was projected onto a tangent screen and partitioned into central and peripheral regions. Test stimuli were velocity steps applied in the central region, while stimuli in the periphery were stationary. The visual images in the central region were seen always in the plane of the screen, while stimuli in the periphery could be seen in front, behind, or in the plane of the screen (achieved by a system of polarizing filters). Initial ocular following responses were larger when the peripheral stimuli were presented with an uncrossed disparity than without disparity. On the other hand, responses were smaller when the peripheral stimuli were presented with crossed disparity (< 5.0 deg) than without disparity. The result is consistent with the idea that ocular following responses are dependent on the perceived viewing distance.

Animals↗