Basis of stereoscopic vision.

IT IS NOW generally accepted that stereoscopic depth localization arises solely from the disparity between the images in the two eyes. There is still uncertainty, however, regarding the role played by certain factors associated with the disparate stimuli that are also necessary for the emergence of the stereoscopic experience. The most important of these factors pertain to simultaneity and retinal locations. Must the two stimuli be received by the two eyes at the same time, and must they fall on specific horizontally associated retinal elements? According to the literature, the term "simultaneity" cannot be taken literally. Stereopsis can be experienced from disparate images presented successively first to one eye and then to the other. 1 However, under such conditions we are probably dealing with afterimages, for it has been shown repeatedly that stereopsis can be obtained from disparate after-images induced in each eye separately.* But the stereoscopic experience from such