The role of pattern and flicker mechanisms in determining the spatiotemporal limits of velocity perception. 1. Upper movement thresholds.

For rapidly moving gratings, the percept of smooth drift is limited by the "coherent drift' threshold. At more rapid rates of movement, the spatial and temporal structure of the stimulus may be individually perceived, but the global spatiotemporal percept of smooth drift breaks down and the stimulus appears to be made up of discrete patches of flickering grating. This coherent drift threshold is strongly dependent upon both spatial frequency and contrast and roughly parallels the "pattern', rather than the "flicker' threshold. The relationships between the three types of threshold suggest that the global percept of smooth drift depends upon mechanisms which are the same as those which underpin the separate percepts of spatial and temporal structure.