Direction selective neurons respond to short-range and long-range apparent motion stimuli in macaque visual area MT

To examine the involvement of macaque visual area MT in the short-range process and the long-range process of visual motion (Braddick 1974), direction selective MT neurons responding to alternating slits and alternating random dots were analyzed. Experiments were performed in awake macaque monkeys trained to fixate a pair of small stationary lines. Visual stimuli were presented on a CRT display screen. About 82% of direction selective MT neurons showed directional responses to alternating slits and/or alternating random dots. The maximum spatial separation for eliciting directional responses (Dmax) was determined for each neuron. In most of cases, Dmax for the alternating random dots was smaller than Dmax for the alternating slits. The Dmax for both alternating random dots and alternating slits increased when retinal eccentricity increased. It is suggested that MT neurons might be involved in the long-range process rather than the short-range process of the apparent motion.