THE CONTRIBUTION OF RETINAL AND EXTRARETINAL SIGNALS TO MANUAL TRACKING MOVEMENTS

We have assessed the contribution made by retinal and extraretinal signals when subjects used their hand to track targets moving at constant velocities. Comparisons were made between responses produced under the following conditions: (1) with full vision of the hand and unrestricted movement of the eyes, (2) without vision of the hand or (3) while visually fixating a stationary LED. Target velocity was varied in a pseudo-random order across trials. In each condition response latency decreased as target velocity was increased. There was a similar to 24 ms increase in latency when vision of the hand was removed or eye movements were restricted. Under normal conditions, subjects were able to accurately catch up to and match target velocity with their hand. When vision of the hand was removed, subjects lagged behind the target but were able to match target velocity. This deficit was eliminated when vision of the hand was made available for the beginning of the response. When subjects were required to visually fixate they could catch up to the target with their hand, but subsequently produced a steady state hand velocity that was greater than target velocity. When the LED was positioned such that the target started in the peripheral visual field, the overestimation of target velocity was evident from the beginning of the response: subjects produced initial accelerations with their hand that were significantly greater than in normal conditions. Finally, normal responses were produced when subjects were required to visually pursue a second target that moved at the same speed and in the same direction as the main target. When the velocities of these two targets differed, subjects produced hand movements that were initially more appropriate for the target being visually pursued. Together these results suggest that vision of the hand and how it is initially positioned relative to the target is necessary to catch up to the target; whereas the extraretinal signal concerned with eye velocity is required to produce an accurate steady state hand velocity.