The visual perception of motion

Sensing the movements of the world and the objects within it appears to be a fundamental job for our visual system. In rare cases of brain damage, we find that individuals lacking motion perception live in a very different world of frozen images, where simple tasks like filling a kettle or crossing the road take on alarming difficulties. That tasks such as driving a fast car down the freeway require a good sense of the movements of yourself and other objects are obvious, but motion information is used in many less obvious ways. For example, it may seem a trivial task to us to follow a moving object with our eyes, but without motion perception these smooth pursuit eye movements are not possible. One way to show this is to have people attempt to move their eyes smoothly along a line etched on a wall. At the same time we place a very bright light just under this line. The bright light burns an afterimage into the retina which can then be examined at leisure. In attempting the eye movement we find that the afterimage is not a smooth line but a series of ‘dots’. This is because you find it difficult to smoothly move yours eyes along the line – even more so if it is vertical – instead flicking your eyes in a series of small, fast jumps known as saccades. Each dot represents the alighting point of a saccade and each gap the distance moved by the saccade. Now, if the line is replaced with a moving dot and we try to track this, the resulting afterimage is a smooth line. This shows that our eyes moved at a constant rate so that the very centre of our vision, where it is best at seeing fine detail, remained focused on the target. Not surprisingly, damage to areas of the brain involved in analysing the moving image destroys this ability