Dynamic Random-dot Stereograms Research Articles

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.

Learning to see 3-D: psychophysics and brain electrical activity.

We investigated human perceptual learning with stereoscopic stimuli presented below threshold. Different visual patterns were shown as dynamic random dot stereograms in a forced-choice design in order to determine the psychophysical thresholds of 16 adults. Brain electrical activity was recorded from 30 electrodes over parieto-occipital areas while stereograms were presented with horizontal disparities below threshold. During the observation of sub-threshold stimuli, we tested repeatedly whether implicit perceptual learning occurred. More than half of the subjects learned to see stereoscopic targets. This was accompanied by topographic changes in the pattern of activation of neural assemblies in the visual cortex where the center of activity shifted towards the right hemisphere. Subjects who did not improve in perception, displayed no such effects.

A real-time video pattern generator for use in ophthalmology

An automated real-time microcomputer-based video pattern generator for use in optometry and ophthalmology is presented. The system can generate various vision pattern tests including a static and dynamic random dot stereogram that can be used to test depth perception. The patterns are generated in real time, which provides the ability to generate programmable images with objects that can move at different speeds. This feature is very useful in testing depth perception among infants and noncommunicative people by correlating the movement of the eye with the movement of the object. The system also can generate other patterns such as checkerboards, vertical and horizontal bars, and provide the ability to sweep the size of the checkers and bars. These patterns are also useful for testing visual acuity. The system hardware is based on the TMS34010 graphics processor and hardware circuits and is connected to a host computer through a RS-232C serial communication port. Both control and application programs are written in assembly language. The system is fast, versatile and flexible with affordable cost.

Occlusion contributes to temporal processing differences between crossed and uncrossed stereopsis in random-dot displays

Stereoscopic depth discrimination was investigated in crossed and uncrossed directions using stimuli defined by binocular disparity differences embedded in dynamic random-dot stereograms. Across three experiments, fixation was directed to a point on the display screen (which placed crossed stimuli in front of, and uncrossed stimuli behind, the background dots of the stereogram), to a point in front of the display screen (which placed both crossed and uncrossed stimuli in front of the background dots), and to a point behind the display screen (which placed both crossed and uncrossed stimuli behind the background dots). Results showed that depth discrimination was always good when the stimuli appeared in front of the background dots of the stereogram, whereas discrimination was always poor when the stimuli appeared behind the background dots. These results suggest that differences between crossed and uncrossed stereopsis as reported in past research arose, in part, from effects related to occlusion.

The effect of target size and eccentricity on reflex disparity vergence.

This study examined the effects of stimulus size and eccentricity on reflex disparity vergence: the small, involuntary corrections of eye alignment which serve to minimize the binocular disparity of fixated targets. Subjects were instructed to fixate steadily on a small, stationary mark superimposed on the center of a dynamic random dot stereogram. The stereogram was binocularly uncorrelated except for a fully correlated patch whose size and eccentricity were varied systematically across trials. The disparity of the patch was varied sinusoidally over time to stimulate vergence following movements. The overall purpose was to determine the relative contributions of various field loci in controlling binocular fixation by finding the smallest patch which would reliably drive vergence against the effort to fixate steadily. Psychophysical thresholds for detection of the correlated patch stimuli were also measured for comparison to the oculomotor results. Results showed that the smallest effective patch increased with eccentricity similarly for both vergence responses and psychophysical detection, suggesting they depend on a common, presumably cortical matching process. The dependence of response on eccentricity is roughly consistent with changes in the cortical magnification factor, suggesting that the area of cortex stimulated may be the determining factor in vergence responses to this class of stimulus.

The cyclopean (stereoscopic) barber pole illusion.

Across two experiments, this study found that the barber pole illusion (i.e. grating pattern appearing to move in the direction of the long axis of a rectangular aperture) is perceived with stereoscopic (cyclopean) motion. The grating and aperture comprising the barber pole display were created from binocular disparity differences embedded in a dynamic random-dot stereogram or from luminance differences. In Experiment 1, observers viewed a square-wave grating moving through a rectangular aperture of 2:1 or 4:1 aspect ratio and indicated whether the grating appeared to move in a direction perpendicular to its orientation or in the direction of the long axis of the aperture. For both stereoscopic and luminance stimuli equally, the grating appeared to move in the direction of the aperture (i.e. the barber pole illusion) more often with the larger aspect ratio than with the smaller aspect ratio. The condition for which a stereoscopic grating moved through a luminance rectangular aperture was also examined: the grating appeared to move in the direction of the aperture (inter-attribute barber pole illusion). In Experiment 2, observers viewed a square-wave grating moving through a rectangular aperture of 3:1 aspect ratio whose sides were indented in order to change the local direction of motion of the line terminators. For both stereoscopic and luminance stimuli, the grating appeared to move more frequently in a direction perpendicular to its orientation with the indented aperture (i.e. the illusion was diminished). Thus, local velocity signals from moving stereoscopic line terminators play a role in the production of the barber pole illusion similar to that of luminance motion signals. This suggests that the generation and propagation of motion signals at cyclopean levels of vision play a part in the representation of coherently-moving rigid surfaces.

Modulation of cell responses to horizontal disparities by ocular vergence in the visual cortex of the awake macaca mulatta monkey

Horizontal retinal disparity is the most important cue for stereopsis. However, accurate stereoscopic perception requires additional information on fixation distance. The ocular vergence angle may provide information on fixation distance and therefore may be used to calibrate horizontal disparities. We studied the responses of cells from cortical visual area V1 of one macaca mulatta monkey to dynamic random dot stereograms at different ocular vergence angles. We observed that in about half of cells sensitive to horizontal disparity the vergence angle modifies the cell responses to horizontal disparities. These results suggest that vergence angle may be used to calibrate horizontal disparities for fixation distance.

Use of Dynamic and Colored Stereogram to Measure Stereopsis in Strabismic Patients

The effectiveness of movement or color has not been well studied in assessing stereopsis in patients with strabismus. We developed a new stereotest equipped with both a monochromatic dynamic random dot stereogram (DRDS) and a static-colored stereogram (SCS) and examined the stereopsis of patients with strabismus. Three-dimensional (3D) images were displayed on a liquid crystal display equipped with a parallax barrier system, allowing 3D images to be seen independently by each eye without glasses. A DRDS with maximum disparity of 3200 seconds of arc was displayed having front-rear movement. An SCS displaying cartoon characters with disparities of 400 seconds of arc was also tested and compared with the Titmus stereotest. A total of 52 strabismic patients were tested. The DRDS showed a significantly higher (P = 0.02) detection rate of stereopsis (39/52, 75%) as compared with the Titmus fly test (28/52, 54%). The SCS did not show any difference in the stereopsis detection rate (24/521, 46%) when compared with the Titmus animal test (20/52, 38%). Thus, the DRDS was useful in detecting stereopsis in patients without stereopsis on the conventional Titmus fly test, while the SCS did not show any difference when compared with the Titmus animal test. The DRDS may examine a different aspect of stereopsis from the static stereopsis measured by the Titmus stereotest or SCS.

Stereoscopic segregation of transparent surfaces and the effect of motion contrast.

Stereoscopic segregation in depth was studied using two superimposed frontoparallel surfaces displayed in dynamic random dot stereograms. The two patterns were positioned symmetrically in front of and behind a binocular fixation point. They were either stationary, or they could move relative to each other. Sensitivity for segregation was established by adding gaussian distributed disparity noise to the disparities specifying the two planes, and finding the noise amplitude that gave threshold segregation performance. Observers easily segregate the two surfaces for disparity differences between approximately 6 and 30-40 arcmin. Motion contrast, which by itself provides no cue to perform the task, greatly improves sensitivity for segregation. Noise tolerance rises by a factor of two or more when the patterns move at different speeds, or in different (frontoparallel) directions. The effect increases with directional difference, but the optimal directional difference deviated from 180 deg. The optimal speed varies with disparity difference. Thus, motion and disparity must interact in order to resolve the two transparent planes.

Direction discrimination of cyclopean (stereoscopic) and luminance motion

This study compared direction discrimination of cyclopean (stereoscopic) and luminance motion involving stimuli equated for effective strength. The stimuli were random-walk cinematogram (RWC) displays whose signal and noise discs were created from binocular disparity differences embedded in a dynamic random-dot stereogram or from luminance differences. Experiment 1 measured global motion detection thresholds for cyclopean and luminance stimuli by manipulating the proportion of signal to noise discs. Detection thresholds for cyclopean motion were about 25% whereas detection thresholds for luminance motion were 5%, thus five times more cyclopean motion events than luminance events were necessary to elicit threshold responding. Experiment 2 measured thresholds for discriminating the direction of cyclopean and luminance motion under conditions of equal stimulus strength by presenting the motion displays at equal multiples of detection threshold. Direction discrimination thresholds (ranging from about 5–30 deg, depending upon conditions) were similar for cyclopean and luminance motion, thus the precision with which the pooling of local motion events in one direction can be discriminated from the pooling of events in a slightly different direction is the same for cyclopean and luminance stimuli. The finding that cyclopean motion information is pooled is consistent with the idea that the direction of cyclopean motion is coded in the responses of a population of directionally selective mechanisms.

Perceived Speed of Moving Cyclopean Corrugations Increases with Increasing Disparity Amplitude

In order to investigate how cyclopean motion is coded by the visual system, the points of subjective equality (PSEs) were measured for (i) speed, (ii) spatial frequency (SF), and (iii) temporal frequency (TF) as a function of peak-to-trough disparity amplitude for cyclopean corrugations. Two panels (3.0 deg × 7.0 deg) of dynamic random-dot stereograms were located 0.5 deg on either side of a central fixation spot. Each panel contained a horizontally oriented sinusoidal cyclopean corrugation whose SF, TF, and disparity amplitude were under experimental control. On each trial, the cyclopean corrugations were displaced vertically in opposite directions. Subjects judged which panel contained the higher SF, TF, or speed depending on condition. The reference stimulus was a sinusoidal corrugation with SF=0.4 cycles deg−1, TF=0.8 Hz, speed of 2.0 deg s−1, and peak-to-trough disparity amplitude of 8 min arc around fixation. We found that, as the peak-to-trough disparity amplitude of the test stimulus increased from 2 min arc to 32 min arc, the PSE for speed decreased from 2.21 deg s−1 to 1.67 deg s−1, compared to a reference speed of 2.00 deg s−1. However, across the same levels of disparity amplitude, the PSE for SF remained constant and the PSE for TF varied but with no consistent pattern. Thus, perceived speed increases with increased disparity amplitude. As all levels of disparity amplitude were above threshold, cyclopean speed cannot be detected by a purely ‘feature-tracking’ mechanism. These metamers and the poor TF matching performance suggest that cyclopean speed is coded by a sparse number of temporal mechanisms.

A clinical evaluation of stereopsis required to see 3-D images.

Some children with esotropia who have been diagnosed as 'stereoblind' on the basis of conventional stereotests (which principally use static images with small disparity) may nevertheless enjoy stereopsis for three-dimensional (3-D) animations that use dynamic images with large disparity. The purpose of this study was to develop a new stereotest equipped with dynamic random-dot stereogram (DRDS) which has larger disparity with movement and use it with esotropic children to see if they can attain stereopsis for such images. Subjects were 17 esotropic children between 5 and 10 years old (mean age 6.8 years) who had failed to demonstrate stereopsis with the Titmus fly test. Seven children had infantile esotropia and 10 had partially accommodative esotropia. The test images were DRDS that were presented on a fluorescent screen (12 x 9 cm) viewed through a lenticular lens. The dots were displayed in circle and triangle patterns that have counterphase front-rear movement with maximum disparity of 800 s. Patients were placed with eyes at a distance of 60 cm from the screen and were instructed to point out the pattern (circle or triangle) that was produced. The average angle of strabismus was 7.7 delta by alternating prism cover test (5 m). Seven patients passed and 10 failed the DRDS test. There was no significant difference in the mean angle of strabismus or the age of examination between the two groups; however, the age at onset of strabismus was significantly higher in those who passed the DRDS test. These results suggest that the DRDS test is useful in evaluating the potential stereopsis in children with esotropia who do not pass conventional stereotests.

Enduring Stereoscopic Motion Aftereffects Induced by Prolonged Adaptation

This study investigated the effects of prolonged adaptation on the recovery of the stereoscopic motion aftereffect (adaptation induced by moving binocular disparity information). The adapting and test stimuli were stereoscopic grating patterns created from disparity embedded in dynamic random-dot stereograms. Motion aftereffects induced by luminance stimuli were included in the study for comparison. Adaptation duration was either 1,2,4,8,16,32 or 64 min and the duration of the ensuing aftereffect was the variable of interest. The results showed that aftereffect duration was proportional to the square root of adaptation duration for both stereoscopic and luminance stimuli; on log-log axes, the relation between aftereffect duration and adaptation duration was a power law with a slope near 0.5 in both cases. For both kinds of stimuli, there was no sign of adaptation saturation even at the longest adaptation duration. Copyright © 1996 Elsevier Science Ltd.

Judgments by monkeys of apparent depth in dynamic random-dot stereograms

Young macaques discriminated apparent depths of targets embedded in dynamic random dot stereograms; a test of stereopsis. In a ‘same/different’ paradigm, the discrimination took longer if the pair of stimuli appeared to be in same depth plane, than when they appeared to be located in a different depth plane. The decision time was an inverse function of the disparity difference. Apparent depth discrimination performance decreased as a function of disparity, with no differences in judgments regarding crossed or uncrossed disparities.

Excitatory binocular neurons are lost following prismatic binocular dissociation in infant monkeys

Four infant rhesus monkeys had prismatic dissociation of binocular vision by viewing the world through prisms. Those monkeys tested previously for ability to utilize horizontal disparity cues in detection of dynamic random dot stereograms, were found here to have few excitatory binocular neurons in visual cortex (V1). Each eye was well represented in the monocular ability to drive cortical neurons, whilst stimulus orientation tuning appeared normal in the monocular neurons, but somewhat less sensitive in the remaining binocular neurons. Binocular dissociation early in life constitutes conditions unfavorable for the maintenance of neural connections delivering binocular excitation to the visual cortex.

Loss of stereopsis in monkeys following prismatic binocular dissociation during infancy

Prismatic binocular dissociation was used during infancy to mimic conditions of strabismus in macaque infants. Prisms worn continuously produce a diplopia unfavorable for the maintenance and development of the binocular visual system. Prism-reared monkeys were tested as young adults and found to be permanently stereoblind for dynamic random dot stereograms. Control monkeys did comparably to humans on such tests. It is concluded that short periods of diplopia attendant with strabismus are sufficient to produce permanent stereoblindness.

Behavioural and Visual Evoked Potential Measures of the Role of Disparity in Stereo Detection and Discrimination

The initial experiment in this research used psychophysical responses to dynamic random-dot stereograms (DRDS) to examine disparity detection and disparity or depth discrimination in stereopsis. Using identical crossed and uncrossed disparity stimuli across conditions at 150 ms exposure, we found that while disparity could be detected and form discriminated, observers were unable to discriminate depth direction. These results suggest that disparity detection is a faster process than disparity discrimination and that form can be discriminated prior to the assignment of depth direction. In a second experiment we examined disparity (correlation) and noncorrelation in stereopsis and used visual evoked potentials (VEPs) in response to three DRDS displays: (a) a target containing disparity on a correlated background, (b) an uncorrelated target on a correlated background, and (c) a target containing disparity on an uncorrelated background. At 200 ms stimulus exposures, VEPs to regions of disparity and regions of noncorrelation on correlated backgrounds were largely undifferentiated. There was not, however, any discernible VEP to stimuli incorporating disparity on an uncorrelated background. These findings imply that while form may be discriminated on the basis of a region of disparity (correlation) in stereopsis, the uncorrelated background condition suggests this is a slow process, since there was no apparent response to form or depth in the recording epoch. Early processes may regard the entire DRDS as a region of noncorrelation with both target and background being mismatched, and hence initially undifferentiated. A faster process may discriminate form on the basis of a region of noncorrelation, as suggested by the correlated background conditions.

Integration of Disparity Information across Multiple Fixations

We measured stereo thresholds for observers viewing noisy, dynamic random-dot stereograms (DRDS) containing either a spatial or a temporal step in depth. Stereograms were presented either with or without a small fixation cross at zero disparity. When the fixation marker was present observers were instructed to maintain fixation throughout the presentation. For spatial steps in depth, the lowest thresholds were found when there was no fixation marker and observers were free to look around the stimulus. Thresholds were higher with a fixation marker in the stimulus. The opposite pattern of results was found for stimuli that contained a temporal step in depth, ie the lowest thresholds were found with a fixation marker. Furthermore, presentation of a single depth region with a fixation marker gave the lowest thresholds of any configuration tested. The results with temporal changes in depth are reminiscent of the findings of Westheimer (1979 Experimental Brain Research36 585 – 597). Westheimer argued that sequential presentation in the same location might depress sensitivity to changes in depth, but in DRDS no single dot changes in depth without also changing in retinal location, so this explanation is not applicable. We conclude that, with a spatial step in depth, it is advantageous to foveate both sides of the step, so multiple fixations help. With a temporal step in depth, the spatial structure of the stimulus is uniform, so multiple fixations may not necessarily help, especially if they result in uncertainty about vergence position.

Disparity tuning of the stereoscopic (cyclopean) motion aftereffect

Across five experiments this study investigated the disparity tuning of the stereoscopic motion aftereffect (adaptation from moving retinal disparity). Adapting and test stimuli were moving and stationary stereoscopic grating patterns, respectively, created from dynamic random-dot stereograms. Observers adapted to moving stereoscopic grating patterns presented with a given disparity and viewed stationary test patterns presented with the same or differing disparity to examine whether the motion aftereffect is disparity contingent. Across experiments aftereffect duration was greatest when adapting motion and test pattern both were presented with zero disparity and in the plane of fixation. Aftereffect declined as disparity of adapting motion and/or test pattern increased away from fixation, even under conditions in which depth position of adapt and test was equal. This argues against a relative depth separation explanation of the decline, and instead suggests that the amount of adaptable substrate decreases away from fixation.

Differences between stereopsis, interocular correlation and binocularity

In normal human subjects, evoked potentials in response to depth reversing two-color dynamic random-dot stereograms disappeared or were greatly reduced at equiluminance, whereas responses to shifts between patterns that were correlated and anticorrelated (for the two eyes) were, for most subjects, actually larger at equiluminance than at non-equiluminance. Responses were only slightly diminished at equiluminance to similar texture-shifting patterns that were identical to the two eyes. These results suggest that a significant fraction of cells with input from both eyes can respond to correlation/anticorrelation shifts, yet are not involved in stereopsis. Also, binocular rivalry may gate the responses of these binocular-nonstereoscopic units.