Monocular Pathways Research Articles

The role of eye-specific attention in ocular dominance plasticity.

It is well known how selective attention biases information processing in real time, but few work investigates the aftereffects of prolonged attention, letalone the underlying neural mechanisms. To examine perceptual aftereffect after prolonged attention to a monocular pathway, movie images played normally were presented to normal adult's one eye (attended eye), while movie images of the same episode but played backwards were presented to the opposite eye (unattended eye). One hour of watching this dichoptic movie caused a shift of perceptual ocular dominance towards the unattended eye. Interestingly, the aftereffect positively correlated with the advantage of neural activity for the attended-eye over unattended-eye signals at the frontal electrodes measured with steady-state visual evoked potentials. Moreover, the aftereffect disappeared when interocular competition was minimized during adaptation. These results suggest that top-down eye-specific attention can induce ocular dominance plasticity through binocular rivalry mechanisms. The present study opens the route to explain at least part of short-term ocular dominance plasticity with the ocular-opponency-neuron model, which may be an interesting complement to the homeostatic compensation theory.

A binocular synaptic network supports interocular response alignment in visual cortical neurons

In visual cortex, signals from the two eyes merge to form a coherent binocular representation. Here we investigate the synaptic interactions underlying the binocular representation of stimulus orientation in ferret visual cortex with invivo calcium imaging of layer 2/3 neurons and their dendritic spines. Individual neurons with aligned somatic responses received a mixture of monocular and binocular synaptic inputs. Surprisingly, monocular pathways alone could not account for somatic alignment because ipsilateral monocular inputs poorly matched somatic preference. Binocular inputs exhibited different degrees of interocular alignment, and those with a high degree of alignment (congruent) had greater selectivity and somatic specificity. While congruent inputs were similar to others in measures of strength, simulations show that the number of active congruent inputs predicts aligned somatic output. Our study suggests that coherent binocular responses derive from connectivity biases that support functional amplification of aligned signals within a heterogeneous binocular intracortical network.

Size-contrast illusion induced by unconscious context.

The present study examined whether the Ebbinghaus illusion can be induced by surrounding contexts that are suppressed from conscious perception. We employed continuous flash suppression to render the contextual stimuli invisible. The surrounding contexts were presented to one eye and the masking stimulus was presented to the other eye, followed by the presentation of the two center test stimuli to both eyes. The illusory size effect was observed even when the inducers were rendered invisible, although the effect was weak-approximately one third the strength of that induced by visible contexts. The effects induced by both visible and invisible contexts decayed with equal speed as the interstimulus interval between the inducer and test stimuli increased. Moreover, interocular transfer of illusory size effect did not occur when the inducers were suppressed from awareness. These results suggest that the size-contrast effect in the Ebbinghaus illusion is mediated in V1, and the monocular pathway in V1 is involved in the unconscious effect.

Contrast-dependent red-green balance shifts depend on S-cone activity.

Previous research from our lab has established that red-green-balanced yellow targets become greenish-brown as surround luminance increases, while red-green-balanced brown targets become reddish-yellow as surround luminance decreases. To help assess the generality and underlying processes of this contrast-dependent red-green hue shift, we investigated red-green hue shifts for target stimuli that appeared achromatic or blue as well as yellow/brown. Results confirmed that the red-green hue shift was largest for yellow/brown targets and was progressively reduced for achromatic and blue targets as target excitation of S cones increased. The magnitude of the hue shift could be predicted by the S/(L+M) excitation of the target when bright white surrounds are used. The hue shift also requires that the target and surround are presented to the same eye, consistent with processing in monocular pathways. Increased S-cone excitation by the surround was associated with red-green hue shifts for all targets equally. Thus, S-cone signals from bright white surrounds might play a role in the contrast-dependent red-green hue shift, but the source of the variation of the magnitude of the hue shift with variations in target S-cone excitation when presented on those surrounds is unknown.

Monocular deprivation of Fourier phase information boosts the deprived eye’s dominance during interocular competition but not interocular phase combination

Ocular dominance has been extensively studied, often with the goal to understand neuroplasticity, which is a key characteristic within the critical period. Recent work on monocular deprivation, however, demonstrates residual neuroplasticity in the adult visual cortex. After deprivation of patterned inputs by monocular patching, the patched eye becomes more dominant. Since patching blocks both the Fourier amplitude and phase information of the input image, it remains unclear whether deprivation of the Fourier phase information alone is able to reshape eye dominance. Here, for the first time, we show that removing of the phase regularity without changing the amplitude spectra of the input image induced a shift of eye dominance toward the deprived eye, but only if the eye dominance was measured with a binocular rivalry task rather than an interocular phase combination task. These different results indicate that the two measurements are supported by different mechanisms. Phase integration requires the fusion of monocular images. The fused percept highly relies on the weights of the phase-sensitive monocular neurons that respond to the two monocular images. However, binocular rivalry reflects the result of direct interocular competition that strongly weights the contour information transmitted along each monocular pathway. Monocular phase deprivation may not change the weights in the integration (fusion) mechanism much, but alters the balance in the rivalry (competition) mechanism. Our work suggests that ocular dominance plasticity may occur at different stages of visual processing, and that homeostatic compensation also occurs for the lack of phase regularity in natural scenes.

Individual differences in sensory eye dominance reflected in the dynamics of binocular rivalry

Normal binocular vision emerges from the combination of neural signals arising within separate monocular pathways. It is natural to wonder whether both eyes contribute equally to the unified cyclopean impression we ordinarily experience. Binocular rivalry, which occurs when the inputs to the two eyes are markedly different, affords a useful means for quantifying the balance of influence exerted by the eyes (called sensory eye dominance, SED) and for relating that degree of balance to other aspects of binocular visual function. However, the precise ways in which binocular rivalry dynamics change when the eyes are unbalanced remain uncharted. Relying on widespread individual variability in the relative predominance of the two eyes as demonstrated in previous studies, we found that an observer’s overall tendency to see one eye more than the other was driven both by differences in the relative duration and frequency of instances of that eye’s perceptual dominance. Specifically, larger imbalances between the eyes were associated with longer and more frequent periods of exclusive dominance for the stronger eye. Increases in occurrences of dominant eye percepts were mediated in part by a tendency to experience “return transitions” to the predominant eye – that is, observers often experienced sequential exclusive percepts of the dominant eye’s image with an intervening mixed percept. Together, these results indicate that the often-observed imbalances between the eyes during binocular rivalry reflect true differences in sensory processing, a finding that has implications for our understanding of the mechanisms underlying binocular vision in general.

Flicker adaptation and neural transmission speed in the human MC pathway

INTRODUCTION: Flicker-induced adaptation of the human magnocelluar (MC) pathway increases contrast thresholds (Zhuang, Pokorny & Cao, IOVS 2015). This study tests whether flicker adaptation also reduces neural transmission speed for suprathreshold stimuli by measuring the response time to a suprathreshold disc. The working hypotheses are that (#1) flicker presented to a given eye can adapt the monocular pathway and also a subsequent binocular mechanism and (#2) flicker presented to both eyes causes greater adaptation at the binocular level than flicker to only one eye. METHODS: Adaptation was induced with a flickering 16x13 deg rectangular field modulated at 9.4 Hz with 100% Michelson contrast. Adaptation could be in neither eye (baseline using a steady adapting field), in only one eye or in both eyes. The test stimulus for response time, a foveal 0.5 deg diameter disc, could be presented to either eye alone or both eyes. Observers responded to the test stimulus as quickly as possible. RESULTS AND DISCUSSION: Response time was the slowest when a monocular test stimulus was presented to the same monocularly adapted eye. Surprisingly, with a monocular test the response time was significantly faster with binocular adaptation than with same-eye monocular adaptation. This suggests that when both eyes are adapted to flicker, the overall adapting effect for a monocular test is inhibited by flicker adaptation in the fellow eye. Overall, the results confirm that flicker adaptation reduces neural transmission speed for suprathreshold stimuli (consistent with working hypothesis #1), but also show that transmission speed for a stimulus presented to only one eye can be speeded up by adding adaptation in the fellow eye (contrary to #2). The latter finding corroborates a similar conclusion from measurements of monocular and binocular contrast thresholds (Zhuang & Shevell, Vision Research 2015). Meeting abstract presented at VSS 2016

Dichoptic perception of brown.

Two experiments assessed mechanisms underlying brown induction by presenting a foveal target disk and concentric annular surround stimuli that varied in contrast relative to larger backgrounds. Stimuli were presented under monocular, binocular, and dichoptic viewing conditions. Observers adjusted the luminance of the target disk to a criterion brown level. We found evidence for at least two separate mechanisms for brown induction: one mechanism that is dependent on physically contiguous contrast and operates in monocular pathways and another mechanism that responds to high luminance contrast anywhere in the visual field and can operate after convergence of signals from the two eyes.

Motion-induced position shift in stereoscopic and dichoptic viewing.

The static envelope of a Gabor patch with a moving sinusoidal carrier appears shifted in the direction of the carrier motion (De Valois & De Valois, 1991). This phenomenon is called motion-induced position shift. Although several motion-processing stages, ranging from low- to high-level processes, may contribute to position estimation, it is unknown whether a binocular matching stage or an even earlier stage exerts an influence. To elucidate this matter, we investigated the disparity tuning of this illusion by manipulating the binocular disparities of the carrier and the envelope. If the mechanisms underlying the illusion have disparity selectivity, the illusory shift should disappear when the carrier and envelope have sufficiently different disparities. We conducted an experiment in which a sinusoidal grating inside a Gaussian envelope had a crossed or uncrossed disparity and the background was filled with static random noise; each subject correctly judged whether the grating was in front of or behind the fixation plane. Position shift occurred even when the moving carrier had a vastly different disparity from that of the envelope, suggesting that one of the mechanisms responsible for the phenomenon exists at a monocular visual stage. To confirm this, in the next experiment we examined whether depth perception can be produced by an illusory disparity due to illusory position shifts in opposite directions between eyes. Two Gabor-like patches moving in opposite directions were presented at the same retinal position dichoptically. We found that when each monocular patch had a soft edge in its contrast envelope, the depth perception of such a patch was biased toward the depth consistent with the illusory crossed or uncrossed disparity, whereas depth perception of a stimulus with a hard edge was less biased. We suggest that the underlying mechanisms of motion-induced position shift are present at an early stage of monocular visual processing, and that the altered positions are represented in the left-eye and right-eye monocular pathways in a way that allows them to function as tokens of binocular matching.

Perception of stationary plaids: The role of spatial filters in edge analysis

Orientation-tuned spatial filters in visual cortex are widely held to act as "orientation detectors", but our experiments on the perception of stationary two-dimensional (2-D) plaids require a new view. When two sinusoidal gratings at different orientations (say 1 c/deg, +/- 45 deg from vertical) are superimposed to form a standard plaid they do not, in general, look like two sets of oblique contours (diamonds) but more like a blurred checkerboard (squares) with vertical and horizontal edges, although the Fourier components are oblique. The pattern of edges seen in this plaid and others corresponds to the zero-crossings (ZCs) in the output of a circular filter, but adaptation and masking experiments suggest that oriented filters are being summed to emulate circular filtering, before ZC analysis. At low contrasts or after adaptation to an intermediate orientation, the combining of filters can fail or be "broken", and the diamond structure of the components is seen instead. Adding a low contrast third harmonic to one component in square-wave phase also changed the plaid's appearance from squares to diamonds, but adapting to the third harmonic enhanced the square appearance. Filters can evidently switch from combining across orientation to combining across spatial frequency. The combination stage of edge detection may involve variably weighted summing of oriented filters in monocular pathways, followed by a process that makes explicit the locations and orientations of features.

Color from motion: dichoptic activation and a possible role in breaking camouflage.

'Color from motion' describes the perception of a spread of subjective color over achromatic regions seen as moving. The effect can be produced in a display of multiple frames shown in quick succession, each frame consisting of a fixed, random placement of colored dots on a high-luminance white background with color assignments of some dots, but not dot locations, changing from frame to frame. Evidence is presented that the perception of apparent motion and the spread of subjective color can be activated by binocular combination of disjoint signals to each eye. The dichoptic presentation of every odd-numbered frame of the full stimulus sequence presented to one eye and, out of phase, every even-numbered frame to the other eye produces a compelling perception of color from motion equal to that seen with the full sequence presented to each eye alone. This is consistent with the idea that color from motion is regulated in sites at or beyond the convergence of monocular pathways. When the background field in the stimulus display is of low luminance, an amodally complete object, fully colored and matching the dots defining the moving region in hue and saturation, is seen to move behind a partially occluding screen. Observers do not perceive such an object in still view. Hence, color from motion can be used by the visual system to produce amodal completion, which suggests that it may play a role in enhancing the visibility of camouflaged objects.

Texture discrimination at the cyclopean retina.

One of the open questions within the study of texture discrimination is whether the underlying neural mechanisms are located within the two monocular pathways, or whether they are located at more central areas that process binocular information. This question is considered here in psychophysical experiments of texture discrimination involving stereograms. The results show that texture discrimination for differences in form and size occur after binocular fusion. Moreover, the suitability of random-dot stereograms versus line-figure stereograms for this research has been studied. It was found that discrimination for differences in form was better with line-figure stereograms whereas discrimination for differences in size was better with random-dot stereograms.

Monocular contribution to the peak time of the binocular pattern visual evoked potential.

The contribution of each monocular pathway to the timing of the binocular pattern visual evoked potential was assessed in situations where a significant interocular timing discrepancy was observed. Monocular and binocular pattern visual evoked potentials to 0.5 degree checks were recorded from normal subjects, normal subjects in whom one eye was blurred, patients with monocular amblyopia, and patients with resolved unilateral optic neuritis. Normal subjects showed facilitation, while suppression was evidenced in subjects with monocular blurring. In patients with amblyopia, the affected pathway had no effect on binocular pattern visual evoked potential latency, suggesting that the amblyopic eye was suppressed. In contrast, all patients with optic neuritis showed binocular averaging. Our results show that different forms of binocular interaction are evidenced in normal subjects, in amblyopia and in optic neuritis, and suggest that a comparative analysis of monocular and binocular pattern visual evoked potential peak times brings valuable information to the clinical evaluation that could be used to distinguish disease processes further.

Rod-cone interaction in monocular but not binocular pathways

Photopic background stimulation elevates scotopic increment thresholds (rod-cone interaction) at moderate background levels when both test and concentric disk-background stimuli enter the same eye (monocular condition) but not when they enter different eyes (dichoptic condition). Only when background levels are made extremely high is there any measurable dichoptic interaction, and this interaction does not resemble that observed monocularly. Rod-cone interaction, as usually studied, is a property of monocular pathways in human vision.

Grating induction: A new type of aftereffect

After adaptation to a spatiotemporal counterphase flickering grating, a fine grating of the same orientation may be perceived in a uniform blank field. The spatial frequency of the induced grating remains approximately constant at about 18 c/deg regardless of the spatial frequency of the adapting spatiotemporal grating. It does not exhibit interocular transfer. The induced grating is not produced by contingent aftereffects, spatial frequency doubling, retinal afterimages, spatial frequency inhibition. Fourier harmonics or any other known type of aftereffect. It therefore represents a new class of aftereffect in the human visual system, suggesting unsuspected neuronal interactions in monocular pathways.