Impact of various stimulus properties on fusional vergence range and heterophoria

Abstract

Binocular fusion is our ability to combine the two separate retinal images into a single fused image, even if the images are separated by a certain amount of disparity. We hypothesized that lower spatial frequencies sampled over a large field of view (FOV) contribute to extending the fusional vergence range (FVR), which is the extent to which binocular single vision (BSV) can be held together with increasing image disparities. Testing these properties allows for the illustration of the contribution of the monocular periphery to binocular fusion. Two separate experiments were run to test this hypothesis; one which varied the spatial frequency of stimuli and one which varied the FOV. Both experiments involved the use of a head-mounted display (HMD) to haploscopically present stimuli to participants. The procedure involved participants manually moving the stimuli apart from each other until they perceptually separated and then back together until they fused, with the resulting disparity at these points being used to calculate the FVR, along with the hysteresis and any potential heterophoria present. The first experiment found the opposite effect of spatial frequency than what was reported in the literature, with lower spatial frequencies resulting in a lower FVR. The second experiment did not find any effect of FOV. These differences compared to the literature could be the result of using an HMD methodology, or the use of complex images as stimuli (which was found to result in a larger FVR than the literature). In future research, the use of different methodologies with complex images would provide a useful comparison to the current results obtained with an HMD, as well as a test of the interactions between spatial frequency and FOV.