Abstract : Human detection of a moving target is a function of (1) target size and contrast and (2) target velocity. Recent work in human psychophysics and animal electrophysiology has shown that visual information is processed in many 'channels', each specialized to detect a particular aspect of of the visual stimulus. For example, the human visual system appears to contain channels sensitive to spatial frequency (or target size). It has also been shown that different spatial frequencies are processed at different temporal rates (Tyman and Sekuler, 1974; Breitmeyer, 1975), suggesting that detection may depend upon an interaction of target size and velocity.