It is well-established that stereoscopic depth is obtained over a large range of retinal disparities, including those that produce diplopia (double images). Under normal viewing conditions, observers make vergence eye movements to minimize large disparities, and it has been suggested that observers judge depth sign for diplopic stimuli by monitoring the vergence signal. Here we ask if vergence eye movements are required to judge depth order (disparity sign) of diplopic stimuli. We created an open-loop stimulus by presenting stereoscopic afterimages, for which eye movements cannot provide feedback about depth sign or magnitude. We produced afterimages of line stereograms consisting of precision-milled slits in aluminum plates that were back-illuminated by a photographic flash. Each half-image consisted of two thin (1x10mm) vertical slits, positioned above and below a small (1mm) fixation LED. The half-images were viewed through a modified mirror stereoscope, so that the fused image formed two narrow bars in the mid-sagittal plane. On each trial, the upper and lower bars were displaced in depth by one of five equal and opposite disparities (two in the range of fusion, one zero and two that were diplopic). After each presentation, observers (n=15) judged which bar was closer to them. Observers reliably judged the sign of disparity for both diplopic and fused images. We conclude that judgments of disparity sign for diplopic stimuli do not depend on extraretinal information, but are recovered directly from the retinal disparity signal.
Meeting abstract presented at VSS 2012