Authors: Galia Issashar Leibovitzh¹, Graham E. Trope¹, Yvonne M. Buys¹, Luminita Tarita-Nistor².
¹Toronto Western Hospital, ²Krembil Research Institute.

Author Disclosure Block:G. Issashar Leibovitzh: None. G.E. Trope: None. Y.M. Buys: None. L. Tarita-Nistor: None.

Abstract Body:

Purpose: Behavioural studies on binocular rivalry point to a dysregulation in inter-hemispheric transfer in patients with mild glaucoma. During binocular rivalry, spatially separated stimuli with common features tend to group together; the grouping is mediated by lateral connections of the cortical hypercolumns. In this study we tested perceptual grouping during intra- and inter-hemispheric binocular rivalry to probe the strength of neural connectivity of the visual cortex involved in early visual processing in patients with mild glaucoma.
Study Design: Observational, case-control study.
Methods: Eight patients with glaucoma with MD better than -2dB and 9 age-matched healthy controls participated. The 2 groups were equivalent in visual acuity and stereo-acuity. Rivalry stimuli were 1.8 deg-diameter discs, containing orthogonal sine wave gratings (spatial frequency of 4 cpd), viewed dichoptically. In a control condition, the stimuli were presented centrally. To test grouping, 2 spatially separated adjacent rivalry stimuli were presented eccentrically to the same or different eyes and to the same or different hemifields. The outcome measures were time of exclusive dominance of the percept with synchronized orientations (i.e., both horizontal or both vertical), rivalry rate, and epochs of exclusive dominance.
Results: Mixed factorial ANOVAs showed that for both groups, synchronized dominance was longer when identical stimuli were presented to the same eye (i.e., both horizontal to one eye and both vertical to the other eye) than to different eyes irrespective of the hemifield (p < .001, partial η² = 0.85). Rivalry rates were significantly lower in the glaucoma group than in the control group across all conditions (p < .001, partial η² = 0.27). For the control group, rivalry rates for the central, same eye/same hemifield, and same eye/different hemifields conditions were identically high, but for the glaucoma group, the highest rivalry rate was observed for the same eye/same hemifield condition where no inter-hemishperic transfer was involved. The epochs of exclusive synchronized dominance in the same eye/same hemifield condition were a median of 374ms longer for the control group but 187ms shorter for the glaucoma group when compared to those of the same eye/different hemifield condition.
Conclusions: In addition to an inter-hemispheric transfer dysfunction, the results show impairment in perceptual grouping during rivalry in patients with mild glaucoma, suggesting that the strength of the lateral connectivity of the hypercolumns in the primary visual cortex is diminished. These deficits may have implications for higher levels of visual processing such as object recognition and scene segmentation.