Binocular fusion, suppression and diplopia: effects of disparity, contrast polarity and contrast imbalance

With different images to each eye, one may experience fusion, suppression of one eye’s view, or diplopia. To understand better the underlying binocular processes, we studied perception of binocular edges as a function of binocular disparity. Single, Gaussian-blurred, horizontal edges (blur B = 8 min of arc) were shown to each eye at various vertical disparities (0 to 8B), with the same or opposite contrast polarity. Observers could indicate the position and polarity of a single perceived edge, or report 2 edges. Diplopia increased with disparity, but when contrasts were unequal the lower-contrast edge was often not seen, particularly at disparities 3 to 5B. We developed a simple descriptive model to interpret the behavioural responses as arising from (a) the probability of fusion (assumed to fall with increasing disparity), (b) the probability of suppression occurring when fusion fails, and (c) the role of positional noise and criterion in judgments of edge position. From this modelling, we conclude that fusion extends to disparities of about 2.5B for all observers, but is absent for opposite polarities. Probability of suppression also declined monotonically with increasing disparity, increased with contrast imbalance, and tended to be lower for opposite polarities, but was highly variable across observers.