Dichoptic Stimuli Research Articles

Depth from Binocular Rivalry without Spatial Disparity

Some new stereoscopic effects are reported that arise from dichoptic stimuli containing no binocular disparity. In one effect, identical arrays of small black discs are presented to the two eyes and slightly smaller white discs are superimposed on one of each pair of black discs. This creates the impression of a surface with holes in it, through which is seen a surface with fluctuating black and white areas. This is referred to as the 'sieve effect'. The white discs must subtend less than about 1 deg of visual angle. With larger discs the black and white areas no longer exhibit alternating rivalry but combine to produce binocular lustre. This destroys the sieve effect. The sieve effect is weak or nonexistent when the black and white discs are the same size, showing that well-defined binocular rims are required for the effect. When the monocular white discs are reduced to dots, the impression of a surface seen through holes gives way to the impression of an array of dots behind or standing out from the background. In this case the monocular dots permanently dominate the homogeneous backgrounds in the other eye and the impression of depth can be explained in terms of apparent parallax or of disparity due to the instability of vergence.

Spatial frequency channels for stereoscopic depth perception

We investigated spatial frequency tuning of the mechanisms for stereoscopic depth perception, using a masking technique, and compared these tunings with those for luminance pattern detection. Observers discriminated depth direction in random-dot stereograms with various contrasts of a masking pattern, which was dichoptic stimulus of uncorrelated random dots. The strength of masking effect as a function of spatial frequency (i.e., masking function) was measured: masking function approximates the spatial frequency tuning of the mechanism that detects the depth in the test pattern. The masking functions for stereoscopic depth were found to be similar to either of the three of the six spatial frequency channels proposed by Wilson and Gelb (1984) for the detection of luminance patterns.

Effects of stimulus distance on measurements of dark convergence

Dark convergence is often measured psychophysically by aligning two dichoptic stimuli, which are briefly flashed in a dark surround. In the present study, the effect of different stimulus parameters on dark convergence measurements was investigated. Chief among these was the effect of viewing distance (50, 100 and 500 cm) of the stimulus. Measured dark convergence was biased towards the actual viewing distance of the stimuli; this effect was stronger with bar stimuli than with point-and-line stimuli. Measured dark convergence depended on the viewing distance to a lesser extent if bar stimuli were observed through pin-holes (open accommodative feedback loop). The subject's knowledge of the viewing distance had a small but significant effect; the size of the nonius bars had none. To reduce artefacts due to the stimulus distance on psychophysical measurement of dark convergence, point-and-line stimuli should be used at a viewing distance of about 1 m. In this case, the effect of stimulus distance can be about 0.25 meter angle (ma), averaged over subjects, but it may be as large as 0.5 ma in some individuals.

Effects of stimulus distance on measurements of dark convergence

Dark convergence is often measured psychophysically by aligning two dichoptic stimuli, which are briefly flashed in a dark surround. In the present study, the effect of different stimulus parameters on dark convergence measurements was investigated. Chief among these was the effect of viewing distance (50, 100 and 500 cm) of the stimulus. Measured dark convergence was biased towards the actual viewing distance of the stimuli; this effect was stronger with bar stimuli than with point-and-line stimuli. Measured dark convergence depended on the viewing distance to a lesser extent if bar stimuli were observed through pin-holes (open accommodative feedback loop). The subject's knowledge of the viewing distance had a small but significant effect; the size of the nonius bars had none. To reduce artefacts due to the stimulus distance on psychophysical measurement of dark convergence, point-and-line stimuli should be used at a viewing distance of about 1 m. In this case, the effect of stimulus distance can be about 0.25 meter angle (ma), averaged over subjects, but it may be as large as 0.5 ms in some individuals.

Two-pulse monocular and binocular interactions at the differential luminance threshold

Interaction between two pulses at the differential luminance threshold was studied for stimuli pairs presented to the same eye or to opposite eyes with an interocular delay. With monocular stimuli, the results replicated the earlier observations by Ikeda (1965) and Rashbass (1970) indicating linear interaction followed by rectification occurring at about 50–60 msec into the integration epoch. Binocular results were different, in accord with observations made in the contrast domain by Green and Blake (1981). Binocular stimuli of opposite polarity showed no cancellation. Binocular facilitation at threshold was found when either the stimuli of the same sign (+ + or − −) occurred with little interocular delay (stimulus onset asynchrony, SOA < 15msec), or the stimuli of the opposite sign (+ − or − +) were presented with an interocular delay between 15 and 100 msec SOA; the latter effect was at maximum with flashes 50 msec in duration presented with 50 msec interocular SOA. These results imply that binocular interaction takes place between rectified internal effects of luminance pulses. From the two-channel binocular model of Cogan (1987), binocular facilitation is attributed to the “fused” response derived from multiplicative excitation between same-sign (half-wave rectified), internal pulse responses. The absence of cancellation between simultaneous opposite-sign dichoptic stimuli is attributed to the “either-eye” binocular process dealing with full-wave rectified internal pulse responses to transient stimuli.

Nonius alignment apparatus for measuring vergence.

The present paper describes an apparatus designed for the assessment of vergence. Based on the Nonius alignment system of viewing dichoptic stimuli, this apparatus provides a compact, portable, easily constructed, and easily maintained piece of equipment. It can be used for the assessment of vergence under a wide variety of viewing conditions and laboratory demands. Also included are an example of one way in which the apparatus may be set up, the equations for finding vergence angle using that set-up, and data concerning the validity and reliability of measurements made with the apparatus.

Binocular interaction in induced line rotation.

Binocular interaction in induced movement was studied using the apparent rotation of a line superimposed on an expanding or contracting grating. The symmetrical transformation reduced the probability of differently directed tracking eye movements* influencing the outcome with dichoptic stimuli. Presentation of the line and background to separate eyes eliminated the apparent rotation, but this was likely to have been due to binocular rivalry. When the inducing field was itself split into dichoptically presented components, an induced rotation was obtained of around half the monocular value. However, the dichoptic rotation could be accounted for by the effects of the two monocular half-fields, when presented in isolation. If backgrounds of differing spatial properties were seen by each eye, the combined effects were predictable from their independently measured monocular effects. It is concluded that the results provide little support for the occurrence of dichoptic induced line rotation.

Orientation sensitivity of the cat assessed from evoked potentials: central and peripheral contributions.

To estimate contour-orientation sensitivity of the cat and the degree to which precortical processing contributes to such estimates, the amplitude of visually evoked potentials (VEP) recorded from the visual cortex of cats in response to a visual stimulus (S2) presented at various intervals after presentation of another visual stimulus (S1) was measured under several conditions. Recordings were made when both stimuli were presented through one eye (monoptic condition) or when S1 was presented to one eye and S2 to the other (dichoptic condition). In some experiments, simultaneous recordings were made from the optic tract and visual cortex. The stimuli were pairs of sinusoidal gratings with a spatial frequency of 0.5 cycles/deg and of various orientations. Each stimulus was presented by stepping the grating contrast from 0.0 (adapting field) to 0.5 for 50 ms. The intervals between the presentation of the two test stimuli (S1 and S2) was varied from 0 to 1,550 ms, and on different trials the orientation of the S2 grating relative to that of S1 was varied from 0 to 90 degrees. Results showed that under monoptic conditions, the VEP to the second stimulus (S2) was reduced by presentation of the first stimulus (S1) when the interstimulus interval was less than 200 ms, whereas under dichoptic conditions, the response to S2 was reduced with interstimulus intervals less than 75 ms. The response reduction was always in a forward direction (e.g., reduced S2 response), increased in magnitude with decreases in the interstimulus interval, and was larger under monoptic conditions than under dichoptic conditions. The response reduction produced monoptically was orientation selective in that it was greatest when the orientation of S1 and S2 was the same, and it recovered by half when the orientation differed by 6 to 15 degrees (orientation half-band pass). In some cortical recordings, the orientation-selective response reduction was superimposed on a response reduction that was not selective for S2 orientation. Stimultaneous recording in the optic tract also showed a response reduction of S2 response that was not orientation selective, suggesting that precortical neural elements contribute to the cortical VEP. With dichoptic stimulus presentation an orientation-nonspecific response reduction was obtained. We hypothesized that binocular inhibitory effects, resulting from disparate retinal input, produced this surprising finding. The results demonstrate that the VEP recorded at the cortex can be used to estimate orientation sensitivity, but that response interactions in peripheral (precortical) neural elements can contribute to such estimates.

Frequency of Fusion and of Loss of Fusion, and Binocular Depth Perception with Alternating Stimulus Presentation

Stereoscopic vision was investigated with an experimental design allowing dichoptic stimulus presentation at different frequencies of image alternation. For twenty subjects the frequency of binocular fusion and the frequency of loss of fusion to one stereoscopic image was measured as a function of the convergence angle. In thirteen subjects no dependence of the fusion frequency was found, while seven subjects showed a marked increase of the fusion frequency with increasing angle of convergence. In all cases the frequency of fusion was higher than the frequency of loss of fusion. Both frequencies, however, are lower than the flicker fusion frequency. Under conditions where no monocular cues and no references for stereoptic depth comparisons were presented, the apparent distance of the image from the observer could not be assessed, but perception of relative motion in depth was possible. All subjects assessed the direction of motion accurately down to changes of the convergence angle of 0.2 deg s-1.

Binocular rivalry occurs without simultaneous presentation of rival stimuli.

It has always been assumed that for binocular rivalry to occur, the dichoptic stimuli must be exposed simultaneously. However, we demonstrate binocular rivalry between rival stimuli that are never simultaneously displayed. This successive binocular rivalry is indistinguishable from normal rivalry with equal-duration alternate presentation of the rival stimuli at rates down to about 20 Hz. Rivalry is evident despite obvious flicker for presentation rates as low as 3 Hz. Rivalry also survives a dark interval of more than 100 msec, introduced between alternate brief presentations (5 msec) of the dichoptic stimuli. Simultaneous brief exposure of the rival stimuli produces an equivalent amount of rivalry, which persists for more than 300 msec. This implies that rivalry persistence involves a peripheral, monocular component and a central, binocular component, as has been shown for stereopsis.

Depth from dichoptic edges depends on vergence tuning.

Small but reproducible fixation disparities occur in normal subjects when they view certain types of dichoptic stimuli. During dichoptic as well as stereoptic stimulation the motor fusion process determines first the average vergence state of the eyes. The subsequent fine tuning of vergence is shown to depend on the spatial distribution of contrast edges both of the same contrast sign ('stereoptic edges') and of opposite contrast sign ('dichoptic edges'). Stereoptic edges tend to induce superposition attempts of the vergence control system and dichoptic edges tend to antagonise this process. If a single low-contrast dichoptic edge is presented with zero disparity and within a stereoptic reference frame, a fixation disparity of several minutes of arc results. This influences depth vision since dichoptic edges are perceived (as monocular edges) at the actual rather than at the intended fixation distance. The findings explain previous paradoxical results of eg Kaufman and Pitblado who reported seeing depth in opposite-contrast stereograms. Their results seemed to contradict the well-established 'same-sign rule' (SSR) which states that the stereoptic system only detects disparities of edges with the same contrast sign. It is concluded that (i) the SSR holds; (ii) dichoptic (and monocular) edges are seen at the horopter; (iii) the vergence fine tuning prevents superposition of dichoptic edges even if this causes a fixation disparity.

Fusional vergence response to local peripheral stimulation

Horizontal, convergent disparities were introduced between dichoptic stimuli confined to small regions of the peripheral retina. Stimuli were presented at 32 locations: radial positions varied in 5-deg increments up to 20 deg, while angular position varied in 45-deg steps. The stimulus size and disparity were scaled in accordance with the cortical magnification factor. Eye movements were objectively measured, and the relative contributions of the motor and nonmotor components to the fusional response were evaluated as a function of stimulus eccentricity and angular position. Vergence responses elicited by peripheral disparities had longer latencies and durations and were more asymmetric than the movements elicited by foveal disparities. The composition of the fusional response changed with the position of the stimulus. The largest percentage of motor compensation was observed for stimuli located either near the line of sight or directly above it. The variations in the size of the motor response with increasing eccentricity could not be explained by the cortical magnification factor.

Temporal order detection for foveal and peripheral visual stimuli

A measure of the temporal resolution of the visual system is the detection of temporal order of the onset of adjacent visual stimuli. The threshold is a few milliseconds and its variation with stimulus separation suggests the existence of a limited processing zone. Measurements were obtained of this distance function in the fovea and retinal periphery of two observers. Temporal order detection threshold does not vary greatly with retinal position out to 20 deg eccentricity, nor does the distance function change much in width, except near the fovea. An independent estimate of the zone for this kind of processing was obtained by testing the range within which signals from oppositely-directed stimulus pairs cancel. Thresholds for dichoptic stimuli are much higher than for monocular or binocular ones.

Superposition of edges and gratings: Interaction between central and peripheral regions

The test pattern was a bipartite field with a sharp edge (step) masked by suprathreshold sinusoidal gratings of variable spatial frequency and phase. With grating contrast increasing an increase of the step threshold contrast was found for cosine phase gratings. In addition to the above effect sine phase gratings lead to a decrease or increase of the step threshold according to the polarity of the grating. It was possible to show that the inhibitory component can be attributed to more peripheral regions of the grating, while the excitatory or inhibitory component is due to a region close to the onset of the edge. Experiments with spatial frequency adaptation showed that the inhibitory influence is selectively impaired. Dichoptic presentation of step and gratings showed an interocular transfer of the inhibitory component, while an interocular transfer of the excitatory or inhibitory component is only found for identical dichoptic stimuli. The results are discussed from the point of view of masking by structure and its interpretation by interchannel inhibition.

Measurement of human vertical fusional response

Objective measurement of human vertical fusional response was carried out using a binocular eye movement monitoring device. The dichoptic stimulus consisted of a single horizontal line subtending 8.5°. The maximum fusible vertical disparity that could be introduced in a single step was 53.2′. The results indicate the presence of a predominant motor component and a small but significant central component in vertical fusional response. Approximately 8 sec were required for the completion of the motor response to a step disparity. Thus, the observed vertical disjunctive movements were roughly 8 times slower than horizontal disjunctive movements. Furthermore, while horizontal disjunctive movements were found to be symmetrical and in accordance with Hering's Law of Equal Innervation, vertical disjunctive movements were found to be unsymmetrical, both in their time course and in their final contributions to the overall motor response.

Binocular single vision achieved by fusion and suppression

The occurrences of fusion and suppression were determined from stereograms which produced two retinal images located at equal distances but in opposite directions from the fovea. Subjects reported whether the dichoptic stimulus appeared single or not, and if single whether it appeared in the center of the visual field. The report of centrality is predicted by the fusion theory of single vision and that of noncentrality by the suppression theory. Experiment 1, with eight subjects, showed that for small disparities perceived singleness was the percept predicted by the fusion theory; for larger disparities, the percepts could sometimes be predicted by the fusion theory and other times by the suppression theory. Experiment 2, with 16 subjects, showed that with larger stimuli the percept predicted by the fusion theory is more likely to occur. Experiment 3, with four subjects, showed that the centrality was reported when the stimuli were presented for 100 msec. This result provided support for our interpretation that the centrality reports in Experiments 1 and 2 were not due to fixation error and suppression.

Visual stimuli for strabismic suppression.

The effects of orientation and spatial frequency of grating stimuli upon suppression were examined with a binocular rivalry paradigm in a group of ten strabismic patients and in a control normal group. Duration, frequency, and period of rivalry were examined as functions of differences in orientation and spatial frequency of dichoptic achromatic sinusoidal gratings. Records were made of responses by the sighting and by the nonsighting eye as well as responses during periods of combined binocular vision. Strabismic subjects reported normal binocular rivalry when presented with gratings of dissimilar orientation. Suppression of the deviating eye in strabismic subjects occurred with stimuli of similar orientation and was unaffected by spatial-frequency differences between dichoptic stimuli. Suppression was most intense under conditions that normally stimulate stereopsis and sensory fusion.

Pattern evoked average EEG potentials and dichoptic visual percepts.

Monocular xenon flashes were used to evoke average EEG potentials while blank, dot or grid targets were shown simmultaneously in ten dichoptic stimulus conditions, either continuously illuminated (added to the flashes, or contraocularly), or presented only by the flashes. Mean evoked-EEG-potential waveforms over repeated conditions were computed from average evoked potentials (EPs) for each stimulus condition and each hemisphere. Correlation coefficients were computed between mena-EP waveforms of all stimulus condition for each subject. Mean correlation coefficients over subjects were treated in a cluster analysis to diagnose conditions associated with similar EP waveforms. Data from the two hemispheres yielded identical cluster results. Presence or absence of continuous target illumination for the flashed eye was the most powerful descriptor. Next important was continuously visible structure (regardless whether seen by the flashed or the nonflashed eye) in conditions with continuous illumination seen by the flashed eye, and flashed structure in conditions without continuous illumination for the flashed eye. It was concluded that a visual percept which enters the processor as non-EP-evoking information nevertheless is reflected by the EP waveform. However, the waveforms are different if the flashes carry the perceptual information exclusively. More powerful than visual structure is the mode of flash presentation (with or withoug continuous target illumination) for the EP waveform.

Peripheral and central contributions to psychophysical spatial interactions

Increment thresholds for a steady light disc and amplitude-modulation thresholds for a time-modulated disc centered at 5 deg from the fovea have been measured in the presence of an annulus of variable diameter concentric to the disc and viewed either by the same eye as the disc or by the other eye. Facilitatory and inhibitory interactions between steady stimuli are found to occur only for homolateral presentation of the stimuli, whereas spatial interactions between time-modulated stimuli occur also with dichoptic stimulus presentation.

Averaged visual evoked potentials in humans: Mechanism of dichoptic interaction studied in a subject with a split chiasma

EEG evoked potentials were averaged from a subject with a sagittal split of the chiasma. Light flashes of constant parameters were shown to one eye while the other eye saw continuously illuminated targets containing different amounts of visual structure. Contrary to earlier results obtained with normal subjects, the pathological subject exhibited no change of the monocularly evoked potentials with increasing amounts of contraocular target structure. This suggests that the change of evoked potentials in dichoptic stimulus conditions in normal subjects reflects the interaction in the visual cortex between afferents from corresponding retinal areas, and does not reflect general changes of retinal or cortical excitability as a function of input to non-corresponding retinal areas.