Mapping the "forbidden zone" near and away from the fixation point

Burt and Julesz modified the classical view of Panum's fusional area by incorporating the concept of the disparity gradient. They found that an area of double vision, described as the forbidden zone for single vision, is present when the angular separation between two objects is small. However, Krol and van de Grind argued that Burt and Julesz's finding could be explained by the classical theories (specifically Ogle, 1932 JOSA 22 664-728) in which the probability for fusion is a function of peripheral angle and disparity magnitude. We examined whether a forbidden zone can be specified within Ogle's experimental space. We found forbidden zones (a) when fixation was placed on one of the two objects and (b) when fixation was placed away from both objects. Krol and van de Grind's arguments do not easily explain the results in (b). The size of the forbidden zones was less than the predicted disparity gradient of unity that Burt and Julesz found in their experiment.