Binocular fusion, suppression and diplopia: scale invariance and the influence of relative contrast between the eyes

With dierent images in the two eyes, one may experience fusion, suppression of one eye’s view, ordiplopia. To understand better the underlying spatial mechanisms and interocular interactions, we studiedthe influence of binocular disparity, spatial scale and relative contrast of edges shown to each eye. Single,Gaussian-blurred, horizontal edges (blur B=2 to 32 minarc) were presented at various vertical disparitiesand contrast ratios. Observers indicated ‘1 central edge’, ‘1 edge above’, ‘1 edge below’ or ‘2 edges’.We defined the subjectively ‘balanced’ contrast ratio as that yielding the greatest proportion of ‘2 edge’responses. Next, we used disparities of 0 to 8B, and several contrast ratios (relative to ‘balanced’). Atbalanced contrasts, there was little or no interocular suppression at any disparity. As disparity increased,the proportion of fusion (or diplopia) responses fell (or rose) monotonically, and the fusional disparityrange was nearly proportional to edge blur (about 2.5B, implying scale invariance). However, withunbalanced contrasts, the (relatively) lower contrast edge tended to be suppressed at larger disparities(≥5B). Fusion responses were little aected by contrast imbalance. Thus, a contrast imbalance betweenthe eyes increases interocular competition (suppression), and so reduces diplopia, but leaves Panum’sfusional range largely unaltered.