Dynamic Random Dot Stimuli Research Articles

Increments in visual motion coherence are more readily detected than decrements.

Understanding the circuits that access and read out information in the cerebral cortex to guide behavior remains a challenge for systems-level neuroscience. Recent optogenetic experiments targeting specific cell classes in mouse primary visual cortex (V1) have shown that mice are sensitive to optically-induced increases in V1 spiking but are relatively insensitive to decreases in neuronal spiking of similar magnitude and time course. This asymmetry suggests that the readout of signals from cortex depends preferentially on increases in spike rate. We investigated whether humans display a similar asymmetry by measuring thresholds for detecting changes in the motion coherence of dynamic random dot stimuli. The middle temporal visual area (MT) has been shown to play an important role in discriminating random dot stimuli, and the responses of its individual neurons to dynamic random dots are well characterized. Although both increments and decrements in motion coherence have heterogeneous effects on MT responses, increments cause on average more increases in firing rates. Consistent with this, we found that subjects are more sensitive to increments of random dot motion coherence than to decrements of coherence. The magnitude of the difference in detectability was consistent with the expected difference in neuronal signal-to-noise associated with MT spike rate increases driven by coherence increments and decrements. The results add strength to the notion that the circuit mechanisms that read out cortical signals are relatively insensitive to decrements in cortical spiking.

Perceptual Learning Induced by Short-Term Training with Visual Stimuli: Effects of Retinal Position and Stimulus Type

We investigated perceptual learning in 85 healthy adults with stereoscopic information contained in dynamic random dot stimuli or with vernier targets. Stimuli were flashed simultaneously at 8 locations at an eccentricity of 1.15° or 2.3°, and subjects had to detect a target in an “8 Alternative Forced Choice” task. For training at a given eccentricity stimuli at the other eccentricity served as a “no training” control. Viewing of visual targets for about 20 minutes resulted in a significant increase of discrimination performance only for the trained stimuli (significant interaction between training and time). Thus, learning is position specific: improved performance can be demonstrated only when test and training stimuli are presented to the same retinal areas. In combination with our earlier electrophysiological results, this study illustrates how perceptual training induces stimulus and visual field specific neural plasticity in adults.

Latent Stereopsis for Motion in Depth in Strabismic Amblyopia

To investigate the residual stereo function of a group of 15 patients with strabismic amblyopia, by using motion-in-depth stimuli that allow discrimination of contributions from local disparity as opposed to those from local velocity mechanisms as a function of the rate of depth change. The stereo performance (percentage correct) was measured as a function of the rate of depth change for dynamic random dot stimuli that were either temporally correlated or uncorrelated. Residual stereoscopic function was demonstrated for motion in depth based on local disparity information in 2 of the 15 observers with strabismic amblyopia. The use of a neutral-density (ND) filter in front of the fixing eye enhanced motion-in-depth performance in four subjects randomly selected from the group that originally displayed only chance performance. This finding was true across temporal rate and for correlated and uncorrelated stimuli, suggesting that it was disparity based. The opposite occurred in a group of normal subjects. In a separate experiment, the hypothesis was that the beneficial effect of the ND filter is due to its contrast and/or mean luminance-reducing effects rather than any interocular time delay that it may introduce and that it is specific to motion-in-depth performance, as similar improvements were not found for static stereopsis. A small proportion of observers with strabismic amblyopia exhibit residual performance for motion in depth, and it is disparity based. Furthermore, some observers with strabismic amblyopia who do not display any significant stereo performance for motion in depth under normal binocular viewing may display above-chance stereo performance if the degree of interocular suppression is reduced. The authors term this phenomenon latent stereopsis.

The Effects of Interocular Correlation and Contrast on Stereoscopic Depth Magnitude Estimation

Decreasing the interocular correlation in random dot stereograms elevates disparity detection thresholds. Whether decorrelation also affects perceived depth from suprathreshold disparity magnitudes is unknown. The present study investigated the effects of interocular correlation and contrast on the magnitude of perceived depth in suprathreshold random dot stereograms. Stereoscopic depth magnitude estimation as a function of percent interocular correlation of dynamic random dot stimuli was measured for five human subjects with clinically normal binocular vision. Each trial's stimulus was randomly assigned one of two magnitudes of either crossed or uncrossed relative disparity. Subjects verbally reported the direction and magnitude of perceived relative depth for each trial using a modulus-free scale. Normalized depth magnitude estimations as a function of the percent interocular correlation demonstrated the relationship between perceived depth, interocular correlation and contrast within subjects. Inter-subject variability was examined with comparisons of data across subjects. The depth magnitude perceived for a given magnitude of disparity declined as the percent of correlation of elements between the eyes decreased for both crossed and uncrossed directions. The effect generally was greater for uncrossed disparities and lower contrast. Some subjects demonstrated asymmetries in perceived depth for crossed vs. uncrossed disparities of the same magnitude. Magnitude estimation of suprathreshold stimuli provided a method of studying performance characteristics of stereoscopic depth perception across the range of functional disparities. Differences found in depth magnitude estimation as a function of the sign of disparity suggest that the neural mechanisms underlying depth perception from uncrossed disparity are more sensitive to image decorrelation, particularly at low contrast, than the mechanisms underlying depth estimation from crossed disparity. These results could occur from differences in near and far disparity-sensitive neurons, from the geometrical relationship between disparity and physical distance in normal viewing, or from the response measure independent of perception.

Eye dominance and response latency in area V1 of the monkey

We measured the latency of 35 cells from V1 in two rhesus monkeys, to dynamic random dot stimuli monocular and binocularly presented. Mean latencies after non-dominant eye stimulation (97.9 ms) were longer than those for dominant eye (78.2 ms) and binocular (70.7 ms) stimulation. Differences between latencies for dominant eye and binocular stimulation were not statistically significant. For dominant eye, there was a significant statistical correlation between dominance strength and latency (R = -0.36; p = 0.03). We failed to find significant statistical differences between latencies for cells with temporal and nasal dominant receptive-field. We conclude that, in V1, the response latency is largely determined by the dominant eye, whereas interocular interactions do not seem to play a relevant role regarding response latency.

Segregation from direction differences in dynamic random-dot stimuli

Previous research has shown that a field of random dots in which each dot alternates between a slow and a fast speed, can give rise to the percept of two superimposed sheets of moving dots when the alternations are out of phase or asynchronous with each other [Vis. Res. 35 (1995) 1691]. Under those conditions, observers can discriminate changes in the slow speed independent of changes in the fast speed. The present study investigated whether such motion-based segregation could result when dots alternated between two different directions. Three observers viewed a variety of displays containing two directions of motion, one upward and one oblique, with the task of discriminating small trial-to-trial changes in the direction of the upward component. The oblique direction component also changed direction from trial-to-trial. The field of dots either alternated synchronously (all dots moved in the same direction and switched to the other direction simultaneously) or asynchronously. Results showed that when the dots alternated synchronously between the directions, observers’ direction discrimination performance was generally poor. However, when dots switched directions asynchronously, direction discrimination was only slightly elevated in comparison to that produced by a field of dots all moving in a single direction. Additional experiments demonstrated that this performance was not due to judging the global direction of the random-dot display. Thus the visual system had to segregate the stimulus into its component directions before integrating to arrive at the motion signal to be discriminated. It is concluded that for displays comprising elements that alternate between different directions, local direction signals can be used by the human visual system to effectively segregate a display so long as both direction signals are present simultaneously.

Surface Interpolation in Three-Dimensional Structure-from-Motion Perception

Although it is appreciated that humans can use a number of visual cues to perceive the three-dimensional (3-D) shape of an object, for example, luminance, orientation, binocular disparity, and motion, the exact mechanisms employed are not known (De Yoe and Van Essen 1988). An important approach to understanding the computations performed by the visual system is to develop algorithms (Marr 1982) or neural network models (Lehky and Sejnowski 1988; Siegel 1987) that are capable of computing shape from specific cues in the visual image. In this study we investigated the ability of observers to see the 3-D shape of an object using motion cues, so called structure-from-motion (SFM). We measured human performance in a two-alternative forced choice task using novel dynamic random-dot stimuli with limited point lifetimes. We show that the human visual system integrates motion information spatially and temporally (across several point lifetimes) as part of the process for computing SFM. We conclude that SFM algorithms must include surface interpolation to account for human performance. Our experiments also provide evidence that local velocity information, and not position information derived from discrete views of the image (as proposed by some algorithms), is used to solve the SFM problem by the human visual system.

Effects of movement in the background field on long-range apparent motion.

Two experiments are reported using time-till-breakdown as a measure of apparent motion (AM) in untrained subjects. In Experiment 1 centrally viewed dynamic random dot stereograms and non-disparate stimuli in dynamic random dot fields yielded higher optimal frequencies than did a standard binocular condition. In Experiment 2 a higher optimal frequency was observed for disparate dynamic random dot stimuli compared with both standard binocular and non disparate stimuli presented on a static random dot field. Interaction between short- and long-range AM processes is considered in the interpretation of the findings.

Evoked potentials to dynamic random dot stimuli with varying dot density ratios of disparity to background.

A systematic study was made of visual evoked responses to dynamic random dot stimuli containing controllable, monocularly visible contrast cues. Ratios of dot densities for the centrally presented, disparate figure and the background were varied in steps of 1/8 maximum density between 0/8 and 8/8. The figure was either a square or an equivalent area of nebulous shape. A 30-arc min disparity was compared with binocular nondisparate and monocular conditions. Evoked responses (scalp sites 02, 01, T6, T5) were averaged for each of 24 disparity-contrast-shape conditions. At all contrast levels, response amplitudes and latency over the left hemisphere was significantly greater than over the right hemisphere. For 30 arc min disparity, amplitudes in the 8/8 condition were significantly smaller than in conditions where stimulus/background contrast could afford monocular depth cues. Hemisphere amplitude differences diminished as contrast decreased. Factor analyses isolated two overlapping components in the response to disparate stimuli. The earlier, at 236 ms latency, may index the stereoscopic stimulus features. The later, at 295 ms, peaking at maximum contrast and present in all suprathreshold nondisparity conditions, may index contrast features of the stimulus. The results indicate the importance of controlling dot density ratios in electrophysiological studies of the stereoscopic response to random dot stimuli.

Stereoscopic stimuli activate different cortical neurones in man: electrophysiological evidence

Dynamic random dot stimuli offer the possibility to study cortical neuronal mechanisms related to depth perception in man because these stimuli operationally skip all stages prior to the activation of cortical binocular disparity neurones. In the present paper, the scalp topography of brain potential components evoked by stereoscopic stimuli are compared with those evoked by similar binocular contrast stimuli, and our results present evidence that different neuronal populations of the visual cortex are activated in these two stimulus conditions.

Some recent findings on the development of human binocularity: A review

Evidence on the development of binocular function in infancy is reviewed. (1) Visual evoked potentials (VEP) may be recorded from infants in response to dynamic random dot stimuli which alternate between positive and negative binocular correlation. Such responses can only arise in neurones receiving binocular input. (2) Infants' looking behaviour may be shown to depend on the presence of binocular disparity in the stimulus (either random-dot or line stereograms). Results of these techniques agree that binocular function normally develops initially between 2 and 4 months of age. Our own data using VEP show a median age of first binocular response of 13 weeks but with marked individual variations. Binocular development involves the interplay of sensory interaction and oculomotor coordination, but it is unlikely that alignment of the two eyes is the dominant constraint determining the onset of binocular vision. It is possible, but not yet established, that the detection of binocular correlation may precede the ability to discriminate stereoscopic disparities. Infants in the first 3 months of life show an asymmetry of monocular optokinetic nystagmus (MOKN). The response to temporalwards field motion which they lack is driven in cat by a pathway via binocular cortex: thus the development of this response in human infants might depend on development of binocularity. However, the correlation across individual infants between the age of onset of binocularity and the age at which symmetrical MOKN is attained is relatively weak. It is possible that the neuroanatomical basis of MOKN control differs between cat and human.

Sustained cortical potentials evoked in humans by binocularly correlated, uncorrelated and disparate dynamic random-dot stimuli

On-line computer generated random-dot targets (‘Julesz patterns’) were alternated every 720 msec between binocular correlation (percept: flat) and uncorrelation (percept: wooly) or partial disparity (percept: depth). Cortical evoked potentials recorded between anterior and posterior electrodes showed sustained late responses (peaks 200–400 msec), anteriorly negative for correlation and positive for uncorrelation or disparity, in all 13 subjects used. Shift of attention from one to the other stimulus condition and random stimulus intervals did not change the results.