Binocular fusion limits are independent of contrast, luminance gradient and component phases

Panum's binocular fusion limit has been shown to increase with the size of graded contrast targets (Schor, Wood & Ogawa, 1984). This suggests the hypothesis that the fusion limit may be controlled by the maximum luminance gradients present in the stimuli. The luminance gradient is reciprocally related to image contrast, so the hypothesis predicts that the fusion limits should also decrease with increasing contrast. To investigate this luminance gradient hypothesis we designed stimuli in which the contrast and phase of the spatial frequency components could be varied independently of the luminance gradients. Disparity limits for fusion were unaffected by variations of as much as a log unit in contrast, luminance gradient or phase of the frequency components, disconfirming the luminance gradient hypothesis. Instead, fusion limits for various compound frequency targets were well predicted by the smallest fusion range for any spatial frequency component in the image that was above its contrast detection threshold.