Two visual systems: Separate pathways for perception and action in the human cerebral cortex

Our appreciation of the world beyond our bodies owes more to vision than to any other sense. The perception of objects and events "out there" is so compelling that it is sometimes difficult to comprehend that this experience arises entirely from the activity of neurons inside our head. But the phenomenal intensity of visual consciousness makes us forget that this particular function of the visual system is a relative newcomer on the evolutionary landscape. Vision began, not as a system for "representing" the world, but as a system for the distal sensory control of movement. Primitive organisms, whose activity is modulated by light, do not "see" the world. Indeed, even when explaining the visually guided behavior of more complex organisms, there is no need to invoke representational vision; one can talk entirely in mechanistic terms without appealing to perception or experience. What is perhaps more difficult to accept is the idea that many of our own visually guided actions can be accounted for in exactly the same way. But to develop this idea will require a more detailed discussion of the origins of vision, first as a system for the control of movement, and then later, as a system for representing the world.