INTERHEMISPHERIC DEPTH JUDGMENT

Interhemispheric depth comparisons were studied by requiring subjects to align in depth two textured plates, one presented to the left hemif ield and the other to the right. Callosal agenesis subjects and neurol ogically-normal control subjects adjusted the plates so that they appe ared to be at the same distance. Subjects viewed the plates monocularl y or binocularly while keeping their head still, moving it side-to-sid e or moving it Up and down. Subjects fixated a target located between the two plates while performing the task. For all subjects, the result s showed that the deviations from veridical settings were significantl y smaller for the binocular than or the monocular viewing conditions. Moreover, there were no significant differences among the three binocu lar viewing, conditions (horizontal, vertical or no head movement), in dicating that neither vertical nor horizontal motion parallax improves the precision of depth judgement when binocular disparity is availabl e. These results further suggest that the precision of interhemispheri c comparison for binocular depth is not affected by the absence of the corpus callosum. Looking at the plates monocularly, the control subje cts judge the relative depth between the plates more precisely when th ey moved their head than when they kept it still. These results show t hat motion parallax is a useful depth cue when relative motion is extr acted from different hemifields. Unlike the control subjects, the call osal agenesis subjects did not judge the relative depth between the pl ates more precisely when they moved their head than when they kept it still. These results show that interhemispheric comparison of depth us ing relative motion is not possible without the corpus callosum.