Dichoptic Presentation Research Articles

Dynamics of dichoptic masking in the primary visual cortex

The inputs from the two eyes interact in a nonlinear fashion. This interaction can be either excitatory or inhibitory: Excitatory interaction (combination) occurs first in the primary visual cortex but little is known about the site of the inhibitory interaction (suppression). To investigate the latter, experimental paradigms typically present distinct inputs to the eyes (dichoptic presentation, one target and one mask are respectively presented to different eyes at the same time). Here we used magnetoencephalography (MEG) source imaging to establish the site of the cortical neural signature of interocular suppression in visual cortex. We presented different noise stimuli to each eye: The target-noise was presented for contrasts ranging between 0 and 32 %. The mask-contrast was presented to the other eye at fixed contrast (32%). We flickered the two noise stimuli (4 and 6 Hz) to elicit a frequency-tagged steady-state visual evoked response (SSVER) that was readily detectable in MEG traces [1]. Four participants passively observed the visual presentation while keeping their gaze fixed on the center of the screen. The Brainstorm application was used to analyze the MEG data [3]. MEG source time series were extracted from cortical regions of interest (ROIs) defined from the visual retinotopic maps of each participant obtained from previous fMRI acquisitions. Using the power of the cortical responses to the frequency-tagged stimuli, we constructed contrast response functions for all the ROIs (Figure 1 A). To investigate dynamics of propagation of the response along visual cortical areas, the instantaneous phase of the signal was identified in each ROI. As expected [2], when the target was presented alone, the power of the responses was found to increase monotonically with contrast (Figure 1 B, solid line). When a mask was added to the other eye, the contrast response was attenuated (Figure 1 B, dotted line). Interestingly, the mask presented at a fixed contrast was also found to be gradually suppressed with increasing target contrast (Figure 1 C). These effects were revealed for responses as early as the primary visual cortex. In the time-domain, we detected a progressive phase shift between the cortical responses along the ventral and dorsal streams. Figure 1 A. Localization of V1 on the activity map B. Normalized power of SSVER response as a function of contrast for target only (solid line), target and mask (dotted line); C. Normalized power in a 32% contrast mask as the contrast in the target is increasing. ...

The Effect of Motion Lines on Apparent-Motion Correspondence under Dichoptic Presentation

Motion lines enhance the impression of motion when viewing static/dynamic images. I investigated the binocularity of the motion-enhancing effect in an ambiguous apparent-motion display. Even in the dichoptic presentation condition, the effect of motion lines on apparent- motion correspondence was as strong as that in the monoptic condition. Additionally, there were no effects of stereo-depth separation. These results suggest that the effect of motion lines arises from a higher-level motion-processing mechanism that occurs after integrating information from both eyes.

Presenting a target and its surrounding flankers in different eyes reduces visual crowding, even though eye of origin difference is imperceptible

Perceptual distinction between a peripheral target and its surrounding flankers is known to relieve the target from visual crowding. This distinction can be caused by color, depth, shape, luminance, size, contrast polarity, spatial frequencies, motion, and contrast. It has also been reported that crowding is not affected by whether the target and flankers are presented in the same or different eyes (Levi 2008). However, in a task to discriminate the orientation of a letter C, whether it is in its normal or 180 degree rotated version, we found that crowding was substantially reduced when the flanking Os were in a different eye from the target, for both 4 and 8 flankers, in all three observers tested. Observers reported that they could not tell whether all items were in the same eye. However, such a relieve from crowding by dichoptic presentation was not apparent when the task was to identify the orientation of a letter T flanked by four other Ts, each randomly oriented in one of four possible orientations (see also Kooi, Toet, Tripathy, and Levi 1994). We suggest that the relieve from crowding was achieved by a more salient target under dichoptic presentation, making the target more easily selected by exogenous attention and thereby decoded in subsequent visual processing. According to the V1 saliency hypothesis (Li 2002), the saliency of any location is determined by the highest V1 neural response to it. An eye of origin singleton target becomes salient when the flanking Os suppress each other more strongly than they do to the target, due to V1’s intra-cortical suppression that is eye of origin selective (DeAngelis, Freeman Ohzawa 1996, Zhaoping 2008). However, collinear facilitation between V1 neurons responding to various bars in the Ts may interfere with intra-cortical suppression to make the saliency mechanism effective. Meeting abstract presented at VSS 2012

The effects of categorical and linguistic adaptation on binocular rivalry initial dominance

Binocular rivalry (BR) is a phenomenon in which visual perception alternates between two different monocular stimuli. There has been a long debate regarding its nature, with a special emphasis on whether low- or high-level mechanisms are involved. Prior adaptation to one of the two monocular stimuli is known to affect initial dominance in the subsequent dichoptic presentation. In the present work, we have used three different types of adaptation in order to investigate how each one affects initial dominance during BR. In the first adaptation type, adapting to a stimulus identical to the one used during rivalry has led to its consequent suppression, verifying previous findings. The binocular presentation which we have used excludes the possibility of eye-adaptation, suggesting that it is the specific stimulus that the brain adapts to. In the second adaptation type, we find suppression effects following adaptation to stimuli belonging to the same category (face or house) but are different from the specific ones used in the following BR presentation. In the final adaptation type, in which the words “face” or “house” are used as adaptors, no statistically significant effect was found. These results suggest that perceptual selection can be directly influenced by the prior presentation of visual stimuli different to the ones used during BR, and thus support a higher-level, cognitive influence on the latter.

Dichoptic presentation of the contrast asynchrony suggests a retinal locus for the contrast response

The contrast asynchrony is a stimulus configuration that illustrates that the visual system has separable responses to color and color contrast (Shapiro et al, 2004; Shapiro 2008). The basic version of the stimulus consists of two disks whose luminance modulates sinusoidally in time at 1 Hz; one disk is surrounded by a dark ring, and the other disk by a bright ring. The luminance of the disks modulates in phase with each other, and the contrast of the disks relative to their surrounding ring modulates in antiphase. When viewing the contrast asynchrony at 1 Hz, observers perceive the disks to modulate in antiphase relative to each other (consistent with the processing of contrast information), but to get light and dark at the same time (consistent with the processing of luminance information). Here we present the results of experiments in which observers viewed the contrast asynchrony dichoptically. We report that no asynchrony is perceived when one eye is presented with modulating disks, and the other eye is presented with the black and white surround rings, nor is an asynchrony perceived in gradient versions of the contrast asynchrony (like Shapiro et al, 2005). We compare these results to dichoptically presented brightness settings. Consistent with what has been reported elsewhere in the literature, black and white surround rings presented to one eye induce brightness changes in disks presented to the other eye. When we present demonstrations that compare the temporal response of the contrast asynchrony to the temporal response of dichoptic brightness induction, the contrast asynchrony occurs at rates much faster than brightness induction (as expected). The results give further evidence that the contrast response can be separated from brightness induction. The results are consistent with a retinal locus of the contrast asynchrony, whereas aspects of brightness induction may occur cortically. Background Shapiro et al (2004, 2005, 2008) presented two physically identical disks, one surrounded by a dark ring and the other by a light ring. The luminance levels of both disks are modulated simultaneously at 1 Hz. The luminance of the disks modulates in phase, while the contrast between the disks and the rings modulates in antiphase. Contrast asynchrony The stimulus leads to the paradoxical perception that the disks modulate in antiphase yet also get light and dark at the same time. The appearance The contrast asynchrony shows separable responses to color and to color contrast (Shapiro, 2008). What are the origins of the separate responses? There are three possible skeletal models: The Question Can the luminance contrast response be perceived dichoptically? Methods and Equipment Simple Contrast Asynchrony Monocular contrast Binocular contrast Contrast Asynchrony against gradient backgrounds

Dichoptic presentation of the contrast asynchrony suggests a retinal locus for the contrast response

The contrast asynchrony is a stimulus configuration that illustrates that the visual system has separable responses to color and color contrast (Shapiro et al., 2004; Shapiro, 2008). The basic version of the stimulus consists of two disks whose luminance modulates sinusoidally in time at 1 Hz; one disk is surrounded by a dark ring, and the other disk by a bright ring. The luminance of the disks modulates in phase with each other, and the contrast of the disks relative to their surrounding ring modulates in antiphase. When viewing the contrast asynchrony at 1 Hz, observers perceive the disks to modulate in antiphase relative to each other (consistent with the processing of contrast information), but to get light and dark at the same time (consistent with the processing of luminance information). Here we present the results of experiments in which observers viewed the contrast asynchrony dichoptically. We report that no asynchrony is perceived when one eye is presented with modulating disks, and the other eye is presented with the black and white surround rings, nor is an asynchrony perceived in gradient versions of the contrast asynchrony (like Shapiro et al., 2005). We compare these results to dichoptically presented brightness settings. Consistent with what has been reported elsewhere in the literature, black and white surround rings presented to one eye induce brightness changes in disks presented to the other eye. When we present demonstrations that compare the temporal response of the contrast asynchrony to the temporal response of dichoptic brightness induction, the contrast asynchrony occurs at rates much faster than brightness induction (as expected). The results give further evidence that the contrast response can be separated from brightness induction. The results are consistent with a retinal locus of the contrast asynchrony, whereas aspects of brightness induction may occur cortically.

Direction discrimination thresholds in binocular, monocular, and dichoptic viewing: Motion opponency and contrast gain control

Purpose: The present study investigated the binocular organization of motion opponency and its relation to contrast gain control. Method: We measured luminance contrast thresholds for discriminating direction of motion for drifting Gabor patterns (target) presented on counter-phase flickering Gabor patterns (pedestal; equivalent to the superposition of two Gabors drifting in opposite directions). There were four presentation conditions. (i) Binocular: all stimuli were presented to both eyes, (ii) Monocular: all stimuli were presented to the same one eye, (iii) Dichoptic: the target was presented to one eye while the pedestal was presented to the other eye, (iv) Half-binocular: the target was presented to one eye while pedestals were presented to both eyes. In addition, we tested an increment-and-decrement condition, in which the target increased contrast for one direction of movement, but decreased it by the same amount for the opposite moving component of the pedestal. The decrement was either in the same eye as the increment, or in the other eye. Results and Discussion: Threshold-versus-pedestal-contrast (TvC) functions showed a dipper shape: thresholds decreased and then increased with pedestal contrast. At low pedestal contrasts, there was binocular summation: binocular thresholds were lower than monocular. But at high pedestal contrasts there was little difference between them. The ‘dip’ was smaller for dichoptic presentation than for other presentations. Thresholds were similar for monocular and half-binocular presentations at low pedestal contrasts, but half-binocular thresholds became higher and closer to dichoptic thresholds as pedestal contrast increased. The added decremental target lowered thresholds by about a factor of 2, compared with half-binocular, when the decrement was in the same eye as the increment, or the opposite eye. Several models fit to the data strongly suggest that motion opponency and contrast gain control operate at a binocular level of processing.

Dichoptic reduction of the direction illusion is not due to binocular rivalry

Simultaneous direction repulsion (the direction illusion) occurs in bidirectional motion displays, typically transparent motion random dot kinematograms. Several laboratories have reported a greatly reduced illusion with dichoptic presentation of the two coherently translating stimuli as compared to monocular or binocular presentation. Some researchers have argued that those results might be due to a confounding factor, namely binocular rivalry occurring between test and inducing stimuli in the dichoptic condition, and so have attributed decisive weight to the results reported by Kim and Wilson (1997, Vision Research, 37, 991–1005) who used centre–surround grating stimuli and found large monocular as well as large dichoptic effects. Here we use centre–surround dot stimuli – with which no binocular rivalry occurs – to confirm a strong monocular contribution to the direction illusion. In addition, we fail to find evidence of a direction illusion with centre–surround grating stimuli, even when seeking to replicate the methods of Kim and Wilson (1997). In light of other evidence that a global motion-sensitive mechanism can determine the magnitude of the direction illusion, we propose that simultaneous direction repulsion can result from activity at multiple stages of the motion processing hierarchy.

The effects of flankers on contrast detection and discrimination in binocular, monocular, and dichoptic presentations

We investigated how two co-aligned adjacent stimuli (flankers) influence threshold versus pedestal contrast (TvC) functions in binocular, monocular, and dichoptic presentations. Targets were presented to the two eyes or to only one eye. Pedestals and flankers were presented to the same eye to which the target was presented (binocular or monocular presentations) or to the other eye (dichoptic presentation). In the binocular presentation of targets and pedestals, the binocular flankers lowered thresholds at low pedestal contrasts. The monocular flankers had a similar effect to the binocular flanker, although the threshold reduction was smaller. In the dichoptic presentation of a target and a pedestal, flankers lowered thresholds when flankers were presented to the eye where targets were presented. In contrast, dichoptic flankers elevated thresholds at intermediate pedestal contrasts when a pedestal was also dichoptically presented. We fitted binocular contrast gain control models to the data. It follows from the fitting results that flankers modulate outputs from spatial filters in the monocular processing stage of contrast gain control.

DLP™-based dichoptic vision test system

It can be useful to present a different image to each of the two eyes while they cooperatively view the world. Such dichoptic presentation can occur in investigations of stereoscopic and binocular vision (e.g., strabismus, amblyopia) and vision rehabilitation in clinical and research settings. Various techniques have been used to construct dichoptic displays. The most common and most flexible modern technique uses liquid-crystal (LC) shutters. When used in combination with cathode ray tube (CRT) displays, there is often leakage of light from the image intended for one eye into the view of the other eye. Such interocular crosstalk is 14% even in our state of the art CRT-based dichoptic system. While such crosstalk may have minimal impact on stereo movie or video game experiences, it can defeat clinical and research investigations. We use micromirror digital light processing (DLP) technology to create a novel dichoptic visual display system with substantially lower interocular crosstalk (0.3%; remaining crosstalk comes from the LC shutters). The DLP system normally uses a color wheel to display color images. Our approach is to disable the color wheel, synchronize the display directly to the computer's sync signal, allocate each of the three (former) color presentations to one or both eyes, and open and close the LC shutters in synchrony with those color events.

Illusory Motion Produced by Dichoptic Stimuli during Saccades

A new motion illusion is reported in which saccadic eye movements can produce a perceived jump of a static stimulus presented dichoptically. In three experiments, observers made saccades while viewing a stationary stimulus consisting of a disk and random dots presented separately to the two eyes. In experiments 1 and 2, by measuring the strength of the perceived motion and the velocity of binocular eye movements, we found that (a) motion ratings were high for the stimulus that contained a large interocular difference in luminance, and (b) the saccadic strategy of the observer was virtually identical across different stimulus conditions. In experiment 3, by measuring the detectability of a short temporal gap introduced into the stimulus around saccades, we found that saccadic suppression was normal in the dichoptic presentation. We discuss possible mechanisms underlying the illusory motion.

Cross orientation masking in color vision: Cortical processing assessed with dichoptic presentation

Cross orientation masking in color vision: Cortical processing assessed with dichoptic presentation

A New Interocular Suppression Technique for Measuring Sensory Eye Dominance

Recently devised tests have implemented forms of interocular suppression (e.g., binocular rivalry) to assess eye dominance. In an effort to combine the strengths of these tests, the authors introduce a new technique for quantifying the magnitude of interocular suppression by using an easily administered psychophysical test. Eighty-eight observers participated in the interocular suppression test, which involved dichoptic presentation of dynamic noise to one eye and a target stimulus to the other. Observers made a form-discrimination judgment once the target emerged from suppression. The authors reasoned that the dominant eye is less susceptible to interocular suppression and as a result, perception and thus, form discrimination would be faster when the target is presented to the dominant eye as opposed to the nondominant eye. Observers' sighting dominance, acuity, contrast sensitivity, and test-retest reliability were also assessed. There were significant interocular differences in mean reaction times within and across observers. Of the observers, 68% and 32% observers were categorized as right eye dominant and left eye dominant, respectively, according to the test. Moreover, 38% of observers showed strong eye dominance. Observers' discrimination accuracy (98%) and test-retest reliability (r = 0.52-0.67) were high. Consistent with results in previous studies, statistical correlations were weak between the sighting dominance test, acuity scores, contrast sensitivity measures, and the interocular suppression test. This interocular suppression technique offers an efficient, reliable, quantitative method of evaluating eye dominance and may be useful in making decisions about differential refractive correction of the two eyes.

Importance of phase alignment for interocular suppression

We measured contrast thresholds for Gabor targets in the presence of maskers which had higher or lower spatial frequencies than the targets. A high-pass fractal masker elevated target contrast thresholds at low and intermediate pedestal contrasts in both monocular and dichoptic modes of presentation, suggesting that the masking occurs after a monocular processing stage. Moreover we found that a high-pass checkerboard masker elevated thresholds at the low and intermediate pedestal contrasts and that most of this threshold elevation disappeared when the phase of the masker’s spatial components were scrambled. This masking was effective only in the dichoptic presentation, not in the monocular presentation. These results indicate that phase alignment of the high spatial frequency components plays a crucial role for interocular suppression. We speculate that phase alignments signal the existence of a luminance contour in the monocular image and that this signal suppresses processing of information in the other eye when there is no corresponding signal in that eye.

Inter-ocular contrast normalization in human visual cortex

The brain combines visual information from the two eyes and forms a coherent percept, even when inputs to the eyes are different. However, it is not clear how inputs from the two eyes are combined in visual cortex. We measured fMRI responses to single gratings presented monocularly, or pairs of gratings presented monocularly or dichoptically with several combinations of contrasts. Gratings had either the same orientation or orthogonal orientations (i.e., plaids). Observers performed a demanding task at fixation to minimize top-down modulation of the stimulus-evoked responses. Dichoptic presentation of compatible gratings (same orientation) evoked greater activity than monocular presentation of a single grating only when contrast was low (<10%). A model that assumes linear summation of activity from each eye failed to explain binocular responses at 10% contrast or higher. However, a model with binocular contrast normalization, such that activity from each eye reduced the gain for the other eye, fitted the results very well. Dichoptic presentation of orthogonal gratings evoked greater activity than monocular presentation of a single grating for all contrasts. However, activity evoked by dichoptic plaids was equal to that evoked by monocular plaids. Introducing an onset asynchrony (stimulating one eye 500 ms before the other, which under attentive vision results in flash suppression) had no impact on the results; the responses to dichoptic and monocular plaids were again equal. We conclude that when attention is diverted, inter-ocular suppression in V1 can be explained by a normalization model in which the mutual suppression between orthogonal orientations does not depend on the eye of origin, nor on the onset times, and cross-orientation suppression is weaker than inter-ocular (same orientation) suppression.

Measuring fixation disparity with infrared eye-trackers

Fixation disparity, that is, misalignment of the gaze direction of both eyes, may be observed in static conditions (through standard optometric evaluation) and dynamic conditions (through eye movement recording). A computation method is presented to determine vergence angles and fixation disparity from gaze positions as commonly recorded by infrared eye-trackers when a participant looks at a personal computer (PC) screen. Eye-tracking devices provide gaze position in coordinates relative to the bidimensional screen surface. From these data, vergence angles can be calculated by trigonometric triangulations; fixation disparity is then calculated from the vergence angles. The application of the procedure to the recordings of one participant is described. To control for the effective alignment of the two eyes on the target during binocular calibration, a procedure based on the dichoptic presentation of nonius lines was used. The recordings confirm that computation and the dichoptic calibration procedures ensure reliable measures of vergence and fixation disparity. The usefulness of this approach with infrared recording of eye position is discussed.

A dichoptic projection system for visual psychophysics in fMRI scanners

Here we describe a projection system based on the combination of linear polarizing filters, designed to allow dichoptic presentation of visual stimuli during fMRI experiments. Currently available MRI compatible dichoptic presentation systems are either highly expensive, require degradation of the projected stimulus such as the removal of all but one color or are difficult to deploy in a range of scanner environments. The system described here is relatively low in cost, requires no change in stimulus properties and could be used in many types of scanner facilities. We provide a verification of the system demonstrating minimal cross-talk between the images presented to the two eyes.

Spatio-Temporal Interpolation Is Accomplished by Binocular Form and Motion Mechanisms

Spatio-temporal interpolation describes the ability of the visual system to perceive shapes as whole figures (Gestalts), even if they are moving behind narrow apertures, so that only thin slices of them meet the eye at any given point in time. The interpolation process requires registration of the form slices, as well as perception of the shape's global motion, in order to reassemble the slices in the correct order. The commonly proposed mechanism is a spatio-temporal motion detector with a receptive field, for which spatial distance and temporal delays are interchangeable, and which has generally been regarded as monocular. Here we investigate separately the nature of the motion and the form detection involved in spatio-temporal interpolation, using dichoptic masking and interocular presentation tasks. The results clearly demonstrate that the associated mechanisms for both motion and form are binocular rather than monocular. Hence, we question the traditional view according to which spatio-temporal interpolation is achieved by monocular first-order motion-energy detectors in favour of models featuring binocular motion and form detection.

Binocular Coordination of the Eyes during Reading

Saccadic eye movements and fixations are the behavioral means by which we visually sample text during reading. Human oculomotor control is governed by a complex neurophysiological system involving the brain stem, superior colliculus, and several cortical areas. A very widely held belief among researchers investigating primate vision is that the oculomotor system serves to orient the visual axes of both eyes to fixate the same target point in space. It is argued that such precise positioning of the eyes is necessary to place images on corresponding retinal locations, such that on each fixation a single, nondiplopic, visual representation is perceived. Vision works actively through a continual sampling process involving saccades and fixations. Here we report that during normal reading, the eyes do not always fixate the same letter within a word. We also demonstrate that saccadic targeting is yoked and based on a unified cyclopean percept of a whole word since it is unaffected if different word parts are delivered exclusively to each eye via a dichoptic presentation technique. These two findings together suggest that the visual signal from each eye is fused at a very early stage in the visual pathway, even when the fixation disparity is greater than one character (0.29 deg), and that saccade metrics for each eye are computed on the basis of that fused signal.

The Sightless View: Neural Correlates of Occluded Objects

The relationship between neural activity and object perception has received considerable attention using stimulus manipulations such as masking or dichoptic presentation. Here we investigate the same problem by occluding objects with an opaque screen that acts to dissociate the direct perception of the object from the awareness of its presence. We used functional magnetic resonance imaging to measure brain activity when subjects viewed objects (faces and houses) that underwent occlusion and found that the response of the majority of the fusiform face area (FFA) and lateral occipital cortex is the same whether the object is visible or occluded. This suggests that when objects are directly viewed, activity within object-selective regions may reflect the awareness of presence, not the direct perception, of the object. Additionally, we identify a region of premotor cortex that is selectively activated by occlusion of either object type, suggesting its generic involvement with processing occluded objects.