We describe a series of psychophysical observations on the detection and discrimination of motion signals presented in a field of dynamic visual noise. The target (see below) was designed so that judgements would have to be based on motion cues, in the hope that performance would depend on the function of a visual motion system like the one that has been identified in the primate visual cortex. Using this stimulus, Newsome and Pare (1988, Journal of Neuroscience) showed that lesions of visual area MT selectively elevated thresholds for detecting motion. Britten, Newsome and Movshon (1989, this meeting), also using the same target, have shown that behavioral thresholds for motion detection are similar to thresholds for reliable activation of neurons in MT. We sought to establish whether parametric variations in the density, area, duration, velocity and eccentricity of our motion target might provide further information about the processing of these signals.