Streams and consciousness: visual awareness and the brain

Vision is our most powerful sense and, arguably, it gives us our most vivid sensory and imaginal experiences. It is also one of the best understood systems in contemporary neuroscience. Yet, contrary to both traditional assumptions and our phenomenological intuition, recent research has shown that vision is not a monolithic system that creates a single general-purpose representation in the brain. For example, selective brain damage can compromise visuomotor control while leaving perception intact, and damage elsewhere can compromise visual perception while leaving visuomotor control intact. Thus, it is becoming apparent that we have two (largely) separate visual systems. One of them is dedicated to the rapid and accurate guidance of our movements: it is a complex and powerful system, and yet it lies outside the realm of our conscious visual awareness. The other seems to provide our perceptual phenomenology, although its primary purpose is probably to provide suitably coded visual inputs for storage in and retrieval from memory. According to this conceptualization, both systems can be seen as serving our behaviour, but each does so on a different time scale. Recent studies suggest that neuropsychological research in humans can play a central role in bridging the gap between neurobiological studies of the monkey's visual system and the search to narrow down the brain mechanisms that mediate our visual awareness.