Psychophysics: catching the old codger's eye

Some visual stimuli are bistable, with perception alternating irregularly between two alternatives. Recent work suggests that the neural processing of these alternations must occur at low levels of the visual system. In 1760, Étienne François Dutour 1. and 2. reported that when different images are shown to each eye, only one of them is seen at any time, with the visible image alternating irregularly every second or so. This has come to be known as binocular rivalry. It is perhaps the earliest, and certainly the most dramatic example of a bistable visual stimulus, in which perception alternates irregularly between two alternatives. Other such stimuli include the Necker cube, the Rubin face–vase figure (Figure 1) and the kinetic depth effect (see demonstrations at [3]). These stimuli are interesting because perception, or in the case of binocular rivalry, visual consciousness, changes without any change in the images falling onto the retina. Such stimuli offer a key to understanding the neural bases of perception [4].