THERE is no general answer to the question of the possible existence and function of a centrifugal pathway to the retina. Evidence for the presence of such a system in mammalian species is controversial, but Holden1 has pointed out that in the case of the pigeon there is substantial anatomical and physiological evidence for a centrifugal system. He goes on to suggest possible functions for such a pathway in mammals mediating known visual phenomena although Brindley2 has argued that there is little behavioural evidence for the necessity of such a pathway.