Conscious and unconscious visual processing in the human brain

Abstract This chapter discusses two distinct and quasi-separate brain systems, the dorsal and ventral visual streams. First, it uses the metaphor of two people singing from the same hymn sheet, and argues that the two visual streams necessarily sing from different, though in large part mutually consistent ones. It then considers evidence derived from experimental strategies to inquire into the nature of the hymn sheets from which each visual processing stream is singing. Clearly the hymn sheets have to be different. In order to provide direct control of one's movements, the dorsal stream has to see the world egocentrically, that is to code visual information in direct relation to the observer's body coordinates. In contrast, the ventral stream specifically needs to encode the world in a way that will be useful not only in the short but also in the long term, for which purpose egocentric coding would be useless.