Fixation disparity in vertical vergence.

Generally, the vertical vergence response is smaller than the disparity that evokes it, even within the diplopia threshold. However, subjective and objective fixation disparity values obtained with Ogle's device agreed fairly well. The reference lines in themselves appeared to be a stimulus to align the eyes. This finding limits the value of the subjective fixation disparity method when studying the vertical fusional response. Vertical fixation disparity is dependent on time of exposure. Responses of increasing amplitude take longer to reach full amplitude from the position of rest than in the reverse direction.