Binocular combination under asynchronous viewing conditions

In the standard model, two main processes underlie binocular combination: binocular summation, whereby the input from each eye is added together, and interocular suppression, whereby each eye suppresses the other’s input. To function properly, both processes require cooperation between the eyes, both in terms of the spatial information they contribute, and presumably, the temporal synchronization of information processing. Previous research indicates that de-synchronization of the ocular inputs generally impairs binocular vision. However, the effect of varying degrees of de-synchronization, and whether summation and suppression are affected to the same degree, is not known. Our goal was to compare the effect of precisely de-synchronizing the ocular inputs on psychophysical measures of summation and suppression. To measure interocular suppression, we employed a dichoptic masking paradigm using a two-alternative-force-choice task. A Gabor target and noise mask were displayed on a passive 3D screen at the same spatial location but with a variable temporal interval between them. To achieve a high degree of temporal precision, stimulus presentations were brief, lasting 8.3ms. Target detection thresholds were measured for each temporal interval between the target and the mask using a 2-up 1-down staircase. For binocular summation, two targets with identical adjustable contrasts were presented dichoptically at different inter-stimulus intervals. The detection threshold of the 2 targets was measured using a 2-up 1-down staircase. Dichoptically, threshold elevation tended to increase with increasing asynchronicity, particularly when the mask followed the target. Summation index tended to decrease with increasing asynchronicity. At a low spatial frequency, effects were larger and more consistent across tasks. Our findings demonstrate that desynchronization of the ocular inputs impacts both main processes underlying binocular combination and provides insight into the binocular deficits observed in conditions associated with natural interocular asynchronicity, such as amblyopia.