STEREOSCOPIC VISION I. EFFECT OF BINOCULAR TEMPORAL SUMMATION*

-UNTIL 1926 it was believed that stereoscopic vision required the simultaneous function of both eyes. Two simultaneous disparate retinal images Were thought necessary to produce a synchronous discharge in the optic nerves before a sense of depth could be achieved. Langlands (1929), in his study of binocular depth acuity, showed that both eyes need not observe the scene simultaneously, provided that they viewed it alternately. With a sufficiently high rate of alternation of the "viewings" by the left and the right eyes a sense of depth could still be realized. Langlands did not study the time relationships between these alternate viewings in any detail as he was more interested in a quantitative analysis of the limits of stereoscopic visual acuity. The phenomenon of stereoscopy with alternate " viewings" has been studied by Pi Suiner (1947). In the course of the present experiment, which was primarily designed to demonstrate a "scanning mechanism" within the occipital cortex, Langland's studies were repeated and greatly extended. The results of these experiments, though not confirming the existence of a scanning mechanism, did reveal a form of temporal summation within the visual system which -had not been previously studied. The present experiment was designed to study "stereoscopic vision" but not "visual depth perception". The difference between these two terms is -profound. Visual perception of depth is determined by a large number of sensory and psychological factors. The least significant of these, from a practical point of view, is binocular stereoscopy. The other factors which are chiefly responsible for the sense of depth have been studied in great detail by Riddoch (1917), Neff (1936), Pi Su-ner (1947), and Vernon (1952), and do not concern us here, except in so far as they have been specifically eliminated from the experimental setting. They are based in large part on the psychological assumption that the object casting the largest retinal image will appear to be the closest to the subject. This instinctive assumption is rapidly and unconsciously modified if there is any previous information concerning the absolute size, shape, colour, or surface features of the object.