GABAergic inhibition in the human visual cortex relates to eye dominance

Binocular vision is created by fusing the separate inputs arriving from the left and right eyes. ‘Eye dominance’ provides a measure of the perceptual dominance of one eye over the other. Theoretical models suggest that eye dominance is related to reciprocal inhibition between monocular units in the primary visual cortex, the first location where the binocular input is combined. As the specific inhibitory interactions in the binocular visual system critically depend on the presence of visual input, we sought to test the role of inhibition by measuring the inhibitory neurotransmitter GABA during monocular visual stimulation of the dominant and the non-dominant eye. GABA levels were measured in a single volume of interest in the early visual cortex, including V1 from both hemispheres, using a combined functional magnetic resonance imaging and magnetic resonance spectroscopy (combined fMRI-MRS) sequence on a 7-Tesla MRI scanner. Individuals with stronger eye dominance had a greater difference in GABAergic inhibition between the eyes. This relationship was present only when the visual system was actively processing sensory input and was not present at rest. We provide the first evidence that imbalances in GABA levels during ongoing sensory processing are related to eye dominance in the human visual cortex. Our finding supports the view that intracortical inhibition underlies normal eye dominance.

A binocular test of sensory eye dominance based on the Pulfrich effect

Many studies have shown inconsistent results among traditional measures of sensory ocular dominance (SED), seriously questioning the very concept of SED as a unitary aspect of the visual system (e.g., Laby & Kirschen, 2011; Mapp et al., 2003; Walls, 1951).The test outcomes may also change even if the same measure is repeated under different conditions, for example by varying the distance and eccentricity of the target (e.g., Rice et al., 2008; Kahn & Crawford, 2001).On the other hand, some authors suggest that such inconsistencies may result from uncontrolled variables. A candidate to produce confusing variables is the frequent format of these tests, often dichotomous and introspective.In this paper, we propose a possibility of measuring SED on a continuous scale and in a comparative way using a stimulus that induces a Pulfrich effect. Here a dichoptic motion stimulus, borrowed from a previous study (Reynaud & Hess, 2017), was used, which produced different degrees of 3D illusory perception strength through the variation of retinal disparity. We observed that the responses of the subjects varied according to their classical SED test outcomes and we estimated the differences in terms of the time delay of the information coming from the two eyes. In our sample, it appears that information from the dominant eye was processed 8.2±5.8 ms faster than that of the fellow eye. People with a left dominant eye showed more marked differences in processing time (6.8±2.0 ms) than people with a right dominant eye (1.8±0.9 ms). Eyes without a clear dominance did not show significant differences in processing time (1.2±1.7 ms). These results are consistent with the previous literature and could lead to the development of a new continuous-scaled SED test.

Stimulus dependence of interocular suppression

Interocular suppression is the phenomenon in which the signal from one eye inhibits the other eye in the presence of dissimilar images. Various clinical and laboratory-based tests have been used to assess suppression, which vary in color, contrast, and stimulus size. These stimulus variations may yield different spatial extents of suppression, which makes it difficult to compare the outcomes. To evaluate the role of stimulus characteristics, we measured the suppression zone using a binocular rivalry paradigm in normally-sighted observers by systematically varying the stimulus parameters. The stimuli consist of a constantly visible horizontal reference seen by one eye while two vertical suppressors were presented to the other eye. With a keypress, the suppressors appeared for 1 s, to induce a transient suppression zone in the middle part of the reference. Subjects adjusted the width between the suppressors to determine the zone. The zone decreased significantly with increasing spatial frequency and lower contrast. The width was 1.4 times larger than the height. The zone was smaller with negative compared to positive contrast polarity but independent of eye dominance, luminance, and colored filters. A departure from scale invariance was captured with a model suggesting a stimulus-dependent and a small fixed non-stimulus-dependent portion.

A Comprehensive Approach to Asian Upper Eyelid Ptosis Correction: The Levator Musculo-Aponeurotic Junction Formula

Background The unique anatomy of the Asian upper eyelid requires specific adaptation to the levator advancement technique for ptosis correction to achieve predictable and reproducible outcomes. Objectives The levator musculo-aponeurotic junction was used as our key landmark. With a formula that we developed, the location of fixation relative to this landmark can be predicted preoperatively. Our clinical experience and outcomes with this technique are presented in this study. Methods Our inclusion criteria were Asian patients with mild to severe ptosis with at least fair levator function. Patients with acquired or congenital ptosis, primary and revisional cases were all included. The location for the placement of the advancement sutures was measured from the musculo-aponeurotic junction of the upper eyelid levator. This distance was determined by a formula that takes into consideration 1) the amount of elevation of the upper eyelid margin needed, 2) the degree of compensatory brow elevation that is present and 3) eye dominance. Results One hundred and fifty-six Asian patients were included in this prospective study. Of these 148 were bilateral and 8 were unilateral corrections. The technique was predictable with resolution of symptoms of eyelid ptosis post-surgery and good long-term symmetry of the palpebral aperture and crisp upper eyelid creases. The formula for estimating the fixation point on the levator was accurate to within +/- 1mm in the majority of patients. Our aperture revision rate was 2%. Conclusions This novel technique provides a predictable and reliable approach for upper eyelid ptosis correction in Asian patients.

Stimulus Dependence of Interocular Suppression

Suppression is assessed using a variety of methods with different stimuli that vary in color, contrast, size, and luminance. We hypothesized that stimulus variation may yield different spatial extents of suppression. Here, to evaluate the role of stimulus characteristics, we measured the suppression zone using a binocular rivalry paradigm in normal observers by systematically varying the parameters of dichoptic Difference of Gaussian stimuli. The stimuli consist of a constantly visible horizontal reference seen by one eye while two vertical suppressors were presented to the other eye. With a keypress, the suppressors appeared for 1 second, to induce a robust transient suppression zone in the middle part of the reference. Subjects adjusted the width between the suppressors to determine the zone. The zone decreased significantly with increasing spatial frequency and lower contrast. The horizontal zone was larger than the vertical zone by a factor of 1.4. The zone was smaller with negative contrast stimuli compared to positive contrast polarity but independent of eye dominance, luminance and colored filters. We then fit a model to determine the optimal parametric definition of the suppression zone and found that the zone consists of two parts: a stimulus-dependent and a fixed non-stimulus dependent zone.