Two mechanisms for the detection of slow motion.

The lowest velocity at which motion can be detected, the lower threshold of motion (LTM), has been investigated as a function of eccentricity. Results show that temporal properties of the LTM change with eccentricity, suggesting a dichotomy in the temporal properties of the neural mechanisms subserving this aspect of central and peripheral vision. Suprathreshold drifting sinusoidal gratings were used with a method of constant stimuli (a two-alternative forced-choice procedure) to obtain the LTM for different stimulus durations at different eccentricities. Results for central vision support a displacement-based model of movement detection similar to that of Reichardt [in Sensory Communication, W.A. Rosenblith, ed. (Wiley, New York, 1961), p. 303]. The LTM decreases as the stimulus duration increases, thus maintaining a constant minimum displacement. However, the results for the far periphery (35 deg of eccentricity) suggest a different mechanism, in which detection depends not on a constant displacement but on a constant velocity; as the stimulus duration increases, the LTM remains constant. These results for gratings can also be generalized to other stimuli, for example, random-dot patterns presented in apparent motion.