THE KNOWLEDGE USED IN VISION AND WHERE IT COMES FROM

Knowledge is often thought to be something brought from outside to act upon the visual messages received from the eye in a 'top-down' fashio n, but this is a misleadingly narrow view First, the visual system is a multilevel heterarchy with connections acting in all directions so i t has no 'top'; and second, knowledge is provided through innately det ermined structure and by analysis of the redundancy in sensory message s themselves, as well as from outside. This paper gives evidence about mechanisms analysing sensory redundancy in biological vision. Automat ic gain controls for luminance and contrast depend upon feedback from the input, and there are strong indications that the autocorrelation f unction, and other associations between input variables, affect the co ntrast sensitivity function and our subjective experience of the world . The associative structure of sensory message can provide much knowle dge about the world we live in, and neural mechanisms that discount es tablished associative structure in the input messages by recoding them can improve survival by making new structure more easily detectable. These mechanisms may be responsible for illusions, such as those produ ced by a concave face-mask, that are classically attributed to top-dow n influences.