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R Blake

Publications and source records attributed to R Blake.

At least 91 records · Page 5Linked to original sources
PubMed1991

Spatial frequency tuning of human stereopsis.

A masking paradigm was employed to measure the spatial frequency selectivity of channels underlying human stereopsis. Observers viewed spatially filtered (0.4 octave bandwidth) random-dot stereograms in which a disparate bar appeared in either the top or bottom half of the display; superimposed on one RDS half-image was a noise target whose spatial frequency content was varied relative to that of the RDS. A staircase procedure was used to measure the monocular noise energy (and hence the signal-to-noise ratio) at which observers could judge the location of the disparate bar on 71% of trials. Statistical analyses showed that the resulting stereoscopic masking functions could be grouped into two sets, one with peak sensitivity at 3 c/deg and the other with peak sensitivity at 5 c/deg. These two channels were observed for both crossed and uncrossed disparities ranging from coarse to fine. Essentially the same results were obtained with binocular noise and with stereo displays flashed too briefly to be affected by eye movements. Our results are inconsistent with models of stereopsis in which the disparity range to which a channel is sensitive varies with that channel's peak spatial frequency. These data imply that the spatial frequency selectivity of stereopsis differs from the tuning of spatial channels underlying the detection and discrimination of form.

Depth Perception↗
PubMed1991

On the coexistence of stereopsis and binocular rivalry.

Dichoptically viewed complex texture stereograms with correlated spatial frequency information can yield stable depth perception, implying cooperative interaction between the two eyes. Dichoptically viewed dissimilar texture pairs may yield competition in the form of binocular rivalry. To study whether stereopsis and rivalry can spatially coexist when stimulus conditions for both are present, we had observers dichoptically view spatial frequency filtered random-dot patterns. The left eye viewed one half-image of an RDS; the right eye viewed the superimposition of the other RDS half-image (which when paired alone with the left-eye RDS yielded stereoscopic depth) and a noise target (which on its own engaged in rivalry with the right eye target). Observers judged the quality of depth and the rate of rivalry for these stereo-pairs. When the contrast of the noise component was low, observers experienced stereopsis and stable single vision that included the noise. At intermediate noise contrasts, local regions were seen either in rivalry or in stereoscopic depth, but rivalry and depth were not experienced at the same spatial location simultaneously. At high noise contrasts, the right eye target dominated almost exclusively, with little hint of stereopsis. Essentially the same pattern of results was obtained in forced-choice experiments in which observers judged the direction of stereoscopic tilt from vertical cosine gratings differing slightly in spatial frequency. Considered together, these results are inconsistent with theories positing that rivalry and stereopsis coexist at the same spatial location because they occur within independent, parallel pathways.

Depth Perception↗
PubMed1991

Apparent motion can survive binocular rivalry suppression.

For short-range motion, observers dichoptically viewed a random-dot cinematogram and a rival target. Upon keypress, the first frame of the cinematogram was replaced by the second frame. Observers judged the direction of motion, which was governed by the initial position of the central region. Performance was well above chance during both dominance and suppression. For long-range motion, observers rated the motion produced by sequentially flashing two small spots, with the first spot contained within a rivalrous region. Suppression reduced but did not prevent perception of this motion. Presenting the second motion frame to both eyes weakened both forms of motion.

Female↗
PubMed1991

Neural models of stereoscopic vision.

Human stereopsis remains an enigma: how does the brain match features between the left and right eye images and compute disparity between these matched features? Developments in computational neuroscience and machine vision have led to several models of human stereopsis that provide insight into possible mechanisms underlying this phenomenon. These models, reviewed in this paper, adopt one of three general strategies. One class of models employs cooperative interactions, whereby a unique solution to the matching problem emerges from excitatory and inhibitory interactions among binocular neural elements. A second class of models implements matching and disparity computation serially over multiple spatial scales. A third class relies on local, non-interacting computations performed in parallel to overcome speed limitations inherent in the other models. Considered together, these theoretical developments offer fresh insights concerning the actual neural concomitants of binocular stereopsis.

Animals↗
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↗
PubMed1991

Coarse spatial scales constrain the range of binocular fusion on fine scales.

To examine interactions among spatial scales in disparity processing, we have measured the upper disparity limit for binocular single vision (the diplopia threshold) for high-spatial-frequency test stimuli in the presence of cosine gratings of lower spatial frequency that defined a surface in depth. When the frequency of this grating surface was 2.0 octaves below that of the test, the test fusion range was reduced by a factor of 3-4 relative to the condition in which no grating surface was present. However, gratings 4.0 octaves below the test frequency had no effect, and the test and grating were seen transparently at different depths. Further experiments indicate that the effect is orientation specific and that high-frequency gratings do not affect low-frequency tests. Finally, experiments using grating surfaces tilted in depth indicate that fusion at high spatial frequencies is constrained to a range centered on the local disparity of the surface defined by the lower frequency. These results are important for computational models for stereopsis that are based on coarse-to-fine matching strategies.

Depth Perception↗
PubMed1991

The interplay between stereopsis and structure from motion.

In a series of psychophysical experiments, an adaptation paradigm was employed to study the influence of stereopsis on perception of rotation in an ambiguous kinetic depth (KD) display. Without prior adaptation or stereopsis, a rotating globe undergoes spontaneous reversals in perceived direction of rotation, with successive durations of perceived rotation being random variables. Following 90 sec of viewing a stereoscopic globe undergoing unambiguous rotation, the KD globe appeared to rotate in a direction opposite that experienced during the stereoscopic adaptation period. This adaptation aftereffect was short-lived, and it occurred only when the adaptation and test figures stimulated the same retinal areas, and only when the adaptation and test figures rotated about the same axis. The aftereffect was just as strong when the test and adaptation figures had different shapes, as long as the adaptation figure contained multiple directions of motion imaged at different retinal disparities. Nonstereoscopic adaptation figures had no effect on the perceived direction of rotation of the ambiguous KD figure. These results imply that stereopsis and motion strongly interact in the specification of structure from motion, a result that complements earlier work on this problem.

Adaptation, Ocular↗
PubMed1991

Discriminating binocular fusion from false fusion.

In a series of psychophysical experiments, observers discriminated between briefly flashed stimuli (cosine gratings, cosine plaids) that were either identical to the two eyes (dioptic) or differed between the two eyes (dichoptic). Although dioptic and dichoptic binocular stimuli were perceptually similar, they were distinguishable well above chance at exposure durations too brief for the onset of binocular rivalry. Random variations in display contrast did not alter this pattern of results. These results show that the neural events that signal false fusion of dissimilar monocular stimuli are not equivalent to those that underlie binocular fusion of matched monocular views.

Humans↗
PubMed1990

Preattentive vision and perceptual groups.

Recent evidence suggests that preattentive processing may not be limited to the analysis of simple stimulus features as previously suggested. To explore this idea a visual search task was used to test whether the shapes of several perceptual groups can be processed in parallel. Textured displays that give rise to strong perceptual grouping were used to create figures on a background. Search times for a target figure distinguished by a unique shape were found to be independent of the number of distractor figures in the display. This result indicates that perceptual groups may be processed in parallel and suggests an expanded role for preattentive processing in vision.

Adult↗
PubMed1990

Detection and discrimination of coherent motion.

When viewing a pair of bars defined by the difference of spatial Gaussian functions (DOGs), human observers can discriminate accurately the relative movements of the bars, even when they differ in spatial frequency. On each trial, observers viewed two brief presentation intervals in which a pair of vertically oriented DOGs moved randomly back and forth within a restricted range. During one interval, both bars moved in the same horizontal direction and by the same magnitude (correlated movements); in the other interval, their movements were uncorrelated. When discrimination accuracy is related to the simultaneous detection of two independent movements, it was found that, if observers can detect the movements of spatially separated bars, they can tell whether their relative movements are correlated. Performance remained remarkably accurate even when the two bars differed in spatial frequency by more than two octaves or were presented separately to the two eyes. Apparently, the accurate discrimination of coherent motion involves an efficient spatial integration of optical motion information over multiple spatial locations and multiple spatial scales.

Adult↗
PubMed1990

Temporal perturbations of binocular rivalry.

Successive durations of binocular rivalry are sequentially independent, random variables. To explore the underlying control process, we perturbed the cycle during a 30-sec viewing period by immediately forcing an eye to return to dominance whenever it became suppressed. During this period of forced dominance, that eye's individual dominance durations were unusually brief, but immediately following the period of forced dominance that eye's suppression durations were unusually long. However, no long-term change in the sequential pattern of rivalry occurred, and the stochastic independence of successive durations was maintained during and following the period of forced dominance. The same pattern of results was obtained with even longer periods of forced dominance. These results are consistent with the existence of a short-term adaptation, or fatigue, process responsible for transitions from dominance to suppression.

Adult↗
PubMed1990

The neural site of binocular rivalry relative to the analysis of motion in the human visual system.

Neural processing is disrupted during suppression phases of binocular rivalry, as evidenced by the temporary invisibility of an otherwise complex, high-contrast visual stimulus. This paper investigates the locus of this disruption relative to the processing of information about image motion. In one experiment, observers tracked binocular rivalry between a stationary textured field and a plaid composed of 2 drifting cosine gratings, with the angle between components varied to produce different pattern speeds. (Plaid speed is given by the ratio of the component speed to the cosine of the angle between the 2 directions of motion.) Predominance of the moving plaid increased with pattern speed, even though the speed of the individual components remained constant. Control measures verified that this influence of plaid speed was not attributable to specific component orientations. Information about coherent motion influences the rivalry process, implying that the site of coherent motion analysis, presumably the middle temporal area (MT), received input during dominance phases of rivalry. A second experiment investigated the effect of suppression on the processing of complex, nonlinear motion. Observers tracked rivalry phases for a rotating spiral, then indicated the duration of the subsequently perceived spiral aftereffect (SAE) for both rivalry and nonrivalry conditions. The SAE was reduced when adaptation occurred under the rivalry condition, with aftereffect duration proportional to the total duration of spiral visibility during adaptation. Earlier work places rivalry after the site of the linear motion aftereffect, and the present results show that rivalry suppression occurs prior to the site of spiral motion processing.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Ocular↗
PubMed1989

Neural integration of information specifying structure from stereopsis and motion.

When one views a two-dimensional parallel projection of dots on the surface of a rotating globe, the direction of rotation is ambiguous, and the perceived direction of rotation of the two-dimensional figure is unstable over time. Stability can be temporarily induced by adaptation to a three-dimensional globe with a direction of rotation unambiguously specified by stereo disparity; adaptation causes the two-dimensional figure to appear to rotate in the direction opposite that experienced during stereoscopic adaptation. This adaptation effect is selective for axis of rotation but is not shape-specific. It does depend on simultaneous stimulation by multiple depth planes defined by elements moving in different directions. Evidently information about stereopsis and information about structure from motion are integrated within a common neural site in the brain.

Adaptation, Ocular↗
PubMed1989

A fresh look at the temporal dynamics of binocular rivalry.

Human observers viewed dichoptic orthogonal sine-wave gratings and indicated when exclusive visibility occurred in either eye. Contrast was held constant in one eye and was increased or decreased in the other eye for a number of alternation cycles (continuous presentation) or for only the duration of a single period of exclusive visibility (synchronous presentation). The synchronous presentation condition allowed us to identify the differing effects of contrast during the suppressed and during the dominant periods. Mixed phases were recorded as distinct from suppressed and dominant phases, and new classifications of compound-dominant and compound-suppressed phases are defined. The results indicate that binocular rivalry responds to stimulus contrast in two ways. 1) The duty-cycle of dominance and suppression is determined by the relative image contrast between the two eyes, with dominance of the higher contrast image being favored, and 2) the overall rate of alternation is driven by monocular image contrast during the suppressed phase (increased monocular contrast increases the alternation rate) and to a lesser extent by monocular contrast during the dominant phase (increased monocular contrast decreases the rate). A model is developed to reflect these ideas. These results support a reciprocal inhibition oscillator as the underlying mechanism of binocular rivalry.

Functional Laterality↗
PubMed1989

A neural theory of binocular rivalry.

When the two eyes view discrepant monocular stimuli, stable single vision gives way to alternating periods of monocular dominance; this is the well-known but little understood phenomenon of binocular rivalry. This article develops a neural theory of binocular rivalry that treats the phenomenon as the default outcome when binocular correspondence cannot be established. The theory posits the existence of monocular and binocular neurons arrayed within a functional processing module, with monocular neurons playing a crucial role in signaling the stimulus conditions instigating rivalry and generating inhibitory signals to implement suppression. Suppression is conceived as a local process happening in parallel over the entire cortical representation of the binocular visual field. The strength of inhibition causing suppression is related to the size of the pool of monocular neurons innervated by the suppressed eye, and the duration of a suppression phase is attributed to the strength of excitation generated by the suppressed stimulus. The theory is compared with three other contemporary theories of binocular rivalry. The article closes with a discussion of some of the unresolved problems related to the theory.

Attention↗
PubMed1988

Cats see subjective contours.

Behavioural techniques were used to determine whether cats are able to see subjective contours. Through several stages of testing with increasingly complex displays, cats continued to respond to a figure defined by subjective contours. This result provides the first direct evidence that a nonhuman perceives subjective contours.

Animals↗
PubMed1988

Binocular reaction times to contrast increments.

Binocular and monocular reaction times were measured in response to an abrupt increment in the standing contrast of a grating. For near threshold contrast increments the advantage of binocular over monocular viewing was substantial when the standing contrast was low, but this advantage was reduced at higher standing contrasts. With high contrast increments the advantage of binocular over monocular viewing was uninfluenced by standing contrast and exceeded the level expected from probability summation.

Form Perception↗