Motion detection in the presence of local orientation changes.

To investigate the orientation selectivity of motion detectors, we measured the perception of horizontal apparent motion of dense patterns of line elements with randomized orientation. Horizontal motion of these displays became invisible when the simultaneous rotation of the line elements exceeded a critical rate (pc). The value of pc increased for higher horizontal velocities, approximately according to a square-root relation. In a direct test of orientation selectivity, the discrimination of horizontal motion direction disappeared when the orientation change per horizontal jump exceeded 30 deg. Thus, for the perception of the global flow the orientation change should not exceed a critical angle during the traverse of a critical distance. The critical distance increases according to a square-root relation as a function of horizontal velocity. These results strongly suggest that bilocal motion detectors are involved in horizontal motion detection and that these detectors are selective for orientation. The properties of these detectors, such as the orientation sensitivity that is reported in this paper, seem highly relevant to the perception of coherent motion.