Dynamic Random-dot Stereograms Research Articles

Differences in perceived depth for temporally correlated and uncorrelated dynamic random-dot stereograms

We investigated the influence of temporal frequency on binocular depth perception in dynamic random-dot stereograms (DRS). We used (i) temporally correlated DRS in which a single pair of images alternated between two disparity values, and (ii) temporally uncorrelated DRS consisting of the repeated alternation of two uncorrelated image pairs each having one of two disparity values. Our results show that disparity-defined depth is judged differently in temporally correlated and temporally uncorrelated DRS above a temporal frequency of about 3 Hz. The results and simulations indicate that (i) above about 20 Hz, the complete absence of stereomotion is caused by temporal integration of luminance, (ii) the difference in perceived depth in temporally correlated and temporally uncorrelated DRS for temporal frequencies between 20 and 3 Hz, is caused by temporal integration of disparity.

Stereomotion speed perception: Contributions from both changing disparity and interocular velocity difference over a range of relative disparities

The role of two binocular cues to motion in depth-changing disparity (CD) and interocular velocity difference (IOVD)- was investigated by measuring stereomotion speed discrimination and static disparity discrimination performance (stereoacuity). Speed discrimination thresholds were assessed both for random dot stereograms (RDS), and for their temporally uncorrelated equivalents, dynamic random dot stereograms (DRDS), at relative disparity pedestals of -19, 0, and +19 arcmin. While RDS stimuli contain both CD and IOVD cues, DRDS stimuli carry only CD information. On average, thresholds were a factor of 1.7 higher for DRDS than for RDS stimuli with no clear effect of relative disparity pedestal. Results were similar for approaching and receding targets. Variations in stimulus duration had no significant effect on thresholds, and there was no observed correlation between stimulus displacement and perceived speed, confirming that subjects responded to stimulus speed in each condition. Stereoacuity was equally good for our RDS and DRDS stimuli, showing that the difference in stereomotion speed discrimination performance for these stimuli was not due to any difference in the precision of the disparity cue. In addition, when we altered stereomotion stimulus trajectory by independently manipulating the speeds and directions of its monocular half-images, perceived stereomotion speed remained accurate. This finding is inconsistent with response strategies based on properties of either monocular half-image motion, or any ad hoc combination of the monocular speeds. We conclude that although subjects are able to discriminate stereomotion speed reliably on the basis of CD information alone, IOVD provides a precise additional cue to stereomotion speed perception.

Pupillary response induced by stereoscopic stimuli

Besides luminance change, the pupil responds to changes in spatial pattern, color content and target motion. Our experimental results show that transient pupillary constriction can also be elicited by dichoptically viewing a change in stereoscopic stimuli composed of dynamic random-dot stereograms from an initially flat surface to a stationary sinusoidal grating shown in depth. On the other hand, monocular viewing of these stimuli produced no obvious pupillary response. This indicates that the pupillary response in the dichoptic experiment was induced by the stereo information rather than by any change in the monocular stimuli. This finding presents a novel approach for the investigation of stereo perception that can also be applied in the clinical environment.

Spatial-scale interaction in human stereoscopic vision in response to sustained and transient stimuli

We investigated temporal properties of stereopsis at different spatial scales in dynamic random-dot stereograms (DRS) consisting of (i) the repeated presentation of two image pairs (i.e. sustained presentation) and (ii) single presentations of two image pairs (transient presentation). In dense stereograms perception of depth is possible if the levels of interocular correlation are above a certain threshold. Therefore, detection of correlation is an important step in stereopsis. Across different experiments, we varied the level of interocular correlation as a function of time by alternating correlated and un/anticorrelated random-dot stereograms. Also, we varied their spatial scale (coarse, fine and alternating between coarse and fine). The presence of stereopsis was examined using a forced-choice depth detection task. For both sustained and transient presentations of the stimuli, the results show that (i) stereopsis has similar temporal properties at coarse and fine spatial scales; (ii) interaction between spatial scales depends on their relative sizes. The results indicate a strong inhibitory influence of rivalry at a coarse scale on stereopsis at a fine scale, and just a weak inhibitory influence of rivalry at a fine scale on stereopsis at a coarse scale. This study provides experimental evidence for a hierarchical organisation of spatial scales in human stereoscopic vision based on neural interaction instead of vergence eye movements.

Stereoscopic visual evoked potentials in normal subjects and patients with open-angle glaucomas.

To evaluate stereoscopic visual evoked potentials (S-VEP) in normal controls and in patients with glaucomatous optic nerve damage. Computer-generated dynamic random-dot stereograms were used to elicit cortical visual evoked potentials using wireless electric liquid crystal shutter glasses. Normal subjects (n=22) and patients with glaucoma (n=22) were investigated using five different disparities from 9 to 40 arc min. Statistical dependency of measurements with different stimulus at identical patients was adjusted for. Peak times of onset and offset response of S-VEP can be significantly delayed in glaucomas. A general linear regression model confirmed that differences between patients and normals depend on disparity. S-VEP onset shows no significant difference between controls and glaucomas at 9 arc min disparity. At high disparities, however, peak time of the onset response was significantly (p<0.01) delayed in glaucomas when compared with normals (normals: 125.8+/-13 ms, glaucomas: 148.2+/-25.6 ms at 40 arc min). Visual evoked potential elicited by the onset of a random-dot stereogram can be used for objective measurement of stereoacuity in a clinical setting. Differences between controls and glaucomas in high and low disparities could indicate a stereo-specific deficit in glaucoma.

Unconscious adaptation: a new illusion of depth induced by stimulus features without depth

Here, we show a new illusion of depth induced by psychophysical adaptation to dynamic random-dot stereograms (RDS) that are interocularly anticorrelated (i.e., in which the images for the two eyes have reversed contrast polarity with each other). After prolonged viewing of anticorrelated RDS, the presentation of uncorrelated RDS (i.e., in which two images are mutually independent random-dot patterns) produces the sensation of depth, although both anticorrelated and uncorrelated RDSs are perceptually rivalrous with no consistent depth by themselves. Contrary to other aftereffects demonstrated in a number of visual dimensions, including motion, orientation, and disparity, this illusion results from unconscious adaptation; observers are not aware of what they are being adapted to during the process of adaptation. We further demonstrate that this illusion can be predicted from the simulated responses of disparity-selective neurons based on a local filtering model. Model simulations indicate that the inspection of anticorrelated RDS causes the adaptation of all disparity detectors except one sensitive to its disparity; therefore, those selectively unadapted detectors show relatively strong activation in response to the subsequent presentation of uncorrelated RDS and produce depth perception.

Sensitivity to horizontal and vertical disparity and orientation preference in areas V1 and V2 of the monkey.

It has been suggested that cells are most sensitive to disparities along the axis orthogonal to their orientation preference. To test this assumption we studied the orientation preference of 73 cells sensitive to retinal disparity, 44 from V1 and 29 from V2. Orientation preference and disparity sensitivity were not related in tuned excitatory and tuned inhibitory cells. We found 18 near/far cells with orientation preference. Of these, 10 (56%) had a preferred orientation less than 30% away from the orthogonal to the disparity axis whereas the remaining eight cells (44%) exceeded this value. Our data suggests that the neural mechanisms for encoding retinal disparities present in dynamic random dot stereograms may not be related to the preferred orientation of the cell.

Neurons in parafoveal areas V1 and V2 encode vertical and horizontal disparities.

Stereoscopic vision mainly relies on binocular horizontal disparity (HD), and its cortical encoding is well established in the foveal representation of the visual field. The role of vertical disparity (VD) is more controversial. Thus far, in the monkey, very few studies have investigated the HD sensitivity beyond 5 degrees of retinal eccentricity and no evidence of a real encoding of VD exists in the parafoveal representation of areas V1 and V2. Using dynamic random dot stereograms, we have tested both HD and VD selectivities in the parafoveal representation of V1 (calcarine V1) and V2 (eccentricities > 10 degrees ) in a behaving monkey. HD and VD selectivities have been characterized using fitting with Gabor function. A large proportion of the tested cells were both HD and VD selective (47%) and, to a lesser extent, HD selective only (8%) or VD selective only (23%). We found a real encoding of VD, with the same diversity in the tuning profiles as described for HD, that cannot be assimilated to a simple perturbation of the HD matching process. Moreover, the VD encoding had a finer scale than the HD one, which is coherent with the smaller range of naturally occurring VD. For the HD encoding, both the percentage of selective cells and the tuning parameters were close to those reported in foveal V1. These results show that, at parafoveal eccentricities in V1 and V2, disparity detectors are tuned to both horizontal and vertical dimensions of the positional disparity existing between matched features in both retinas.

The effect of disparity change on binocular visual evoked potential parameters elicited by convergent dynamic random-dot stereogram stimuli in humans.

Binocular visual evoked potentials (VEP) were recorded from the left and right occipital cortices of right-handed subjects in response to six different levels of convergent disparity using the stimulus of dynamic random-dot stereogram (DRDS). The VEP obtained consisted of a negative (N1) and a positive peak (P1) within intervals of 200-400 ms and 400-600 ms, respectively. The latency of the N1 wave decreased with disparity and the amplitude of the same wave displayed a concave-down curve. The tuning-curve of N1 showed that the cortical focus, which produced this wave, was sensitive to mid-disparities. The fact that the tuning-curve of the right hemisphere was sharper than that of the left implied that the right hemisphere was more disparity-selective. In contrast to N1, the change of the latency and the amplitude of the P1 with disparity consisted of concave-up curves. The inverse correlation between the amplitudes of N1 and P1 with disparity made me think that the activation of the neuron population, which gave rise to N1 wave, changed the synchronization level of the cortical focus responsible for the P1 wave. If we keep in mind that the perception of depth is the result of the disparity detection, then we may conclude that N1 is related to disparity detection and P1 is the electrophysiological correlate of a depth percept.

Mapping of cortical brain activity—the use of dynamic random dot stereograms

Three-dimensional perception (3-D) partly relies on the binocular fusion of horizontally disparate stimuli presented to the left and right eye. In humans, the input of the two eyes is fused only at the level of the visual cortex, thus, evoked potentials that are exclusively generated by cortical structures may be explored with dynamic random dot stereograms (dRDS). We determined the scalp topography of the dRDS-evoked brain activity in different groups of healthy subjects, and we found major differences between stereoscopic- and contrast-evoked brain activity: the strength of the potential fields, as well as their topography, significantly differs. Our data suggest that there are fewer neurons in the human visual cortex that are responsive to horizontal disparity, and that higher visual areas (like V2 and V3) are more engaged with stereoscopic processing than the primary visual cortex. The latencies of evoked components show no effect indicating that the binocular information flow to the visual cortex has a similar time course for both the processing of binocular dRDS and contrast information. In addition, we could show that healthy subjects improve to discriminate 3-D stimuli by training. Such changes in perceptual ability are paralleled by systematic alterations in the topography of stereoscopically evoked potential fields.

Quantitative analysis of the responses of V1 neurons to horizontal disparity in dynamic random-dot stereograms.

Horizontal disparity tuning for dynamic random-dot stereograms was investigated for a large population of neurons (n = 787) in V1 of the awake macaque. Disparity sensitivity was quantified using a measure of the discriminability of the maximum and minimum points on the disparity tuning curve. This measure and others revealed a continuum of selectivity rather than separate populations of disparity- and nondisparity-sensitive neurons. Although disparity sensitivity was correlated with the degree of direction tuning, it was not correlated with other significant neuronal properties, including preferred orientation and ocular dominance. In accordance with the Gabor energy model, tuning curves for horizontal disparity were adequately described by Gabor functions when the neuron's orientation preference was near vertical. For neurons with orientation preferences near to horizontal, a Gaussian function was more frequently sufficient. The spatial frequency of the Gabor function that described the disparity tuning was weakly correlated with measurements of the spatial frequency and orientation preference of the neuron for drifting sinusoidal gratings. Energy models make several predictions about the relationship between the response rates to monocular and binocular dot patterns. Few of the predictions were fulfilled exactly, although the observations can be reconciled with the energy model by simple modifications. These same modifications also provide an account of the observed continuum in strength of disparity selectivity. A weak correlation between the disparity sensitivity of simultaneously recorded single- and multiunit data were revealed as well as a weak tendency to show similar disparity preferences. This is compatible with a degree of local clustering for disparity sensitivity in V1, although this is much weaker than that reported in area MT.

Range and Mechanism of Encoding of Horizontal Disparity in Macaque V1

The responses of single cortical neurons were measured as a function of the binocular disparity of dynamic random dot stereograms for a large sample of neurons (n = 787) from V1 of the awake macaque. From this sample, we selected 180 neurons whose tuning curves were strongly tuned for disparity, well sampled and well described by one-dimensional Gabor functions. The fitted parameters of the Gabor functions were used to resolve three outstanding issues in binocular stereopsis. First, we considered whether tuning curves can be meaningfully divided into discrete tuning types. Careful examination of the distributions of the Gabor parameters that determine tuning shape revealed no evidence for clustering. We conclude that a continuum of tuning types is present. Second, we investigated the mechanism of disparity encoding for V1 neurons. The shape of the disparity tuning function can be used to distinguish between position-encoding (in which disparity is encoded by an interocular shift in receptive field position) and phase-encoding (in which disparity is encoded by a difference in the receptive field profile in the 2 eyes). Both position and phase encoding were found to be common. This was confirmed by an independent assessment of disparity encoding based on the measurement of disparity sensitivity for sinusoidal luminance gratings of different spatial frequencies. The contributions of phase and position to disparity encoding were compared by estimating a population average of the rate of change in firing rate per degree of disparity. When this was calculated separately for the phase and position contributions, they were found to be closely similar. Third, we investigated the range of disparity tuning in V1 as a function of eccentricity in the parafoveal range. We find few cells which are selective for disparities greater than +/-1 degrees even at the largest eccentricity of approximately 5 degrees. The preferred disparity was correlated with the spatial scale of the tuning curve, and for most units lay within a +/-pi radians phase limit. Such a size-disparity correlation is potentially useful for the solution of the correspondence problem.

Organisation of signals involved in binocular perception and vergence control

A novel type of dynamic random-dot stereogram (DRS) was used to study vergence movements and depth detection in response to temporal modulations of interocular correlation. Each DRS consisted of the repeated presentation of a pair of correlated images alternated by the presentation of a pair of uncorrelated images. The intervals of high ( T c) and low ( T u) correlation varied from 14 to 224 ms in steps of 14 ms. Depth detection and vergence responses behaved very different from each other as functions of T c and T u. The different behaviours suggest that depth and vergence most likely result from independent streams of disparity processing. It is speculated that magnocellular layers process disparities that drive vergence and that a parvocellular stream of disparity processing is involved in depth perception. This suggestion is discussed in relation to recent findings on binocularly perceived direction and depth. The discussion leads to suggesting a headcentric organisation of signals involved in binocular perception and a retinal organisation of signals involved in vergence control.

Modeling V1 Disparity Tuning to Time-Varying Stimuli

Most models of disparity selectivity consider only the spatial properties of binocular cells. However, the temporal response is an integral component of real neurons' activities, and time-varying stimuli are often used in the experiments of disparity tuning. To understand the temporal dimension of V1 disparity representation, we incorporate a specific temporal response function into the disparity energy model and demonstrate that the binocular interaction of complex cells is separable into a Gabor disparity function and a positive time function. We then investigate how the model simple and complex cells respond to widely used time-varying stimuli, including motion-in-depth patterns, drifting gratings, moving bars, moving random-dot stereograms, and dynamic random-dot stereograms. It is found that both model simple and complex cells show more reliable disparity tuning to time-varying stimuli than to static stimuli, but similarities in the disparity tuning between simple and complex cells depend on the stimulus. Specifically, the disparity tuning curves of the two cell types are similar to each other for either drifting sinusoidal gratings or moving bars. In contrast, when the stimuli are dynamic random-dot stereograms, the disparity tuning of simple cells is highly variable, whereas the tuning of complex cells remains reliable. Moreover, cells with similar motion preferences in the two eyes cannot be truly tuned to motion in depth regardless of the stimulus types. These simulation results are consistent with a large body of extant physiological data, and provide some specific, testable predictions.

The processing of stereoscopic information in human visual cortex: psychophysical and electrophysiological evidence.

Three-dimensional depth perception relies in part on the binocular fusion of horizontally disparate stimuli presented to the left and right eye. The mammalian visual system offers a unique possibility to study electrophysiologically cortical neuronal mechanisms: since the input of the two eyes remains separated up to the level of the visual cortex, evoked potential components that are generated exclusively by cortical structures may be explored when dynamic random-dot stereograms (dRDS) are presented. In a series of independent studies, we determined the scalp topography of dRDS evoked brain activity in different groups of healthy subjects, and we found consistent results. Major differences between stereoscopic and contrast evoked brain activity are seen in the strength of the potential fields as well as in their topography. Our findings suggest that there are fewer neurons in the human visual cortex that are responsive to horizontal disparity, and that higher visual areas like V2 are more engaged with stereoscopic processing than the primary visual cortex. On the other hand, component latencies of evoked brain activity show no effect signifying that the binocular information flow to the visual cortex has a similar time course for both the processing of contrast information and of dRDS stimuli. We could also verify that healthy subjects can learn to perceive 3D structure contained in dRDS. Changes in perceptual ability as measured with psychophysical tests are paralleled by systematic alterations in the topography of stereoscopically evoked potential fields. Stereoscopic VEP recordings may also be of clinical use: in patients with selectively disturbed depth perception but normal visual acuity there is a high correlation between clinical symptoms, perceptual deficiency, and altered VEP amplitudes and latencies.

The stereoscopic (cyclopean) motion aftereffect is dependent upon the temporal frequency of adapting motion

This study investigated whether the stereoscopic (cyclopean) motion aftereffect (induced by adaptation to moving binocular disparity information) is dependent upon the temporal frequency or speed of adapting motion. The stereoscopic stimuli were gratings created from disparity embedded in a dynamic random-dot stereogram. Across different combinations of stereoscopic spatial frequency, temporal frequency and speed of adapting motion, the duration of the aftereffect was dependent upon temporal frequency (maximal aftereffect=1–2 cyc s −1). These results support the idea that stereoscopic motion is processed by a cortical mechanism that computes cyclopean motion energy.

Response latencies to visual stimulation and disparity sensitivity in single cells of the awake Macaca mulatta visual cortex

The onset response latencies to dynamic random dot figures (solid figures) and dynamic random dot stereograms were measured in single units recorded from areas V1 and V2 of two awake Macaca mulatta monkeys. We studied 56 cells, 39 from V1 and 17 from V2. In 14 disparity sensitive and 13 disparity unsensitive cells from V1 the median latencies to solid figures were 59.8 and 73.6 ms, respectively, which were statistically different. In 26 disparity sensitive cells from V1 and 17 from V2 the median latencies to stereofigures were 85.6 and 97.9 ms, respectively, which were statistically different. These results indicate that V1 disparity sensitive cells may have shorter integration time than disparity unsensitive cells and that there is a transferring delay for disparity information between areas V1 and V2.

Binocular Interaction and Sensitivity to Horizontal Disparity in Visual Cortex in the Awake Monkey

We evaluated the binocular interaction and horizontal disparity sensitivity in neurons recorded from macaque visual cortex. Neurons from VI of three awake Macaca mulatta monkeys were isolated by means of extracellular recording and tested for disparity sensitivity with dynamic random dot stereograms. Neurons sensitive to horizontal disparities were stimulated both monocularly and binocularly with flashing bars and their responses quantified. ANOVA and regression tests were used for data analysis. Sixty-six cells out of 185 (66/185, 36%) showed sensitivity to horizontal disparity. Disparity sensitive cells were grouped into near (25/66, 38%), tuned inhibitory (16/66, 24%), far (13/66, 20%) and tuned excitatory (12/66, 18%). Receptive fields of tuned cells were located more centrally in the visual field than those of near and far cells. The binocular interaction in tuned inhibitory cells increased linearly along with ocular unbalance. Most of tuned excitatory cells (10/12, 83%) showed facilitatory binocular interaction, characterized by a stronger response to binocular stimulation than to the stimulation of the dominant eye. On the contrary, most of toned inhibitory cells (14/16, 88%) showed suppressory binocular interaction, characterized by a weaker response to binocular stimulation than to the stimulation of the dominant eye. Near and far cells showed both types of interaction in similar percentages. The binocular response showed a linear relationship with the sum of both monocular responses in tuned excitatory, tuned inhibitory and near cells, but not in far cells. Sensitivity to horizontal disparity may be a result of facilitatory and suppressive interactions between left and right inputs.

Stereopsis with persisting and dynamic textures

We measured the percept of changing depth from changing disparity in stereograms composed of random-dot textures that were either persistent or dynamically changed on every frame (a dynamic random-dot stereogram). Disparity was changed between frames to depict a surface undergoing smooth temporal changes in simulated slant. Matched depth was greater with dynamic random-dot stereograms than with persistent random-dot stereograms. These results confirm and extend earlier observations at depth threshold. We posit an explanation based on cue conflict between stereopsis and monocular depth cues.

Exposure duration affects the perceived direction of cyclopean Type II plaids

This study investigated the effect of exposure duration on the perceived direction of cyclopean Type I and Type II plaids moving in the X/ Y plane. The cyclopean plaids were created from grating components defined by binocular disparity embedded in a dynamic random-dot stereogram. The results showed that the cyclopean Type I plaid appeared to move in the intersection-of-constraints (IOC) direction across the range of exposures tested. However, the cyclopean Type II plaids appeared to move in a direction different from the IOC with short exposures but near the IOC with long exposures. This perceived directional shift was also obtained with luminance-defined Type II plaids. A common pattern-motion mechanism that processes cyclopean and luminance motion signals appears responsible for the perceived directional shift of the Type II plaids.