Interhemispheric depth judgement.

Interhemispheric depth comparisons were studied by requiring subjects to align in depth two textured plates, one presented to the left hemifield and the other to the right. Callosal agenesis subjects and neurologically-normal control subjects adjusted the plates so that they appeared to be at the same distance. Subjects viewed the plates monocularly or binocularly while keeping their head still, moving it side-to-side or moving it up and down. Subjects fixated a target located between the two plates while performing the task. For all subjects, the results showed that the deviations from veridical settings were significantly smaller for the binocular than for the monocular viewing conditions. Moreover, there were no significant differences among the three binocular viewing conditions (horizontal, vertical or no head movement), indicating that neither vertical nor horizontal motion parallax improves the precision of depth judgement when binocular disparity is available. These results further suggest that the precision of interhemispheric comparison for binocular depth is not affected by the absence of the corpus callosum. Looking at the plates monocularly, the control subjects judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that motion parallax is a useful depth cue when relative motion is extracted from different hemifields. Unlike the control subjects, the callosal agenesis subjects did not judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that interhemispheric comparison of depth using relative motion is not possible without the corpus callosum.