Have you ever stopped to consider how amazing the human vision system is? It allows you to see in very bright and dull environments; it allows you to detect minute variations in colour and it also allows you to detect rapid movement, be that a darting animal or a sudden flash of lightning. However, the eye is not perfect. We are limited to a narrow spectrum of colours (between ultra-violet and infra-red). Some objects move too quickly (or too slowly) to see any movement.

These aspects, however, are governed by the physical composition of the eye. What we are interested in is how the brain processes the information which the eye does manage to capture. If we are to create usable interfaces, then we need to examine how the brain processes the information presented to it.

There are a number of theories which have been proposed to explain how we process what we see. Broadly speaking, they fall in to two main categories: constructivist and ecological. Constructivist theories state that our visual perception of the world is constructed by applying our knowledge of the world to what our eyes are currently seeing. Ecological theories, however, state that there is no high level processing, we extract information directly from the light captured by the eye. Neither approach is exclusively correct and there is much evidence supporting both points of view.