Effects of stimulus speed on direction discriminations

Difference thresholds for the direction of movement of moving isotropic dot patterns were measured over a wide range of stimulus speeds. These measurements were made in human subjects, in normal cats and in visually deprived cats with abnormalities in neuronal directional response. For all subjects, direction thresholds were inversely related to stimulus speed at low drift rates. Above a "critical speed", direction thresholds were independent of stimulus speed. Between group differences in critical speed parallel differences in visual acuity and absolute thresholds for direction of motion. However, asymptotic direction thresholds appear to be unrelated to acuity and motion detection thresholds. It is argued that the basis for the differences in asymptotic direction thresholds between the groups are differences in properties of directional mechanisms.