STRATEGIC AND ADAPTIVE RESPONSES TO CHANGES IN A SENSORY-MOTOR RELATION

Twenty subjects used a joystick control to track a visual target which stepped a constant distance alternately up and down a display screen at random intervals between 1 and 6 seconds. After 9 up and 9 down ste ps over a period of 63 seconds, the sensitivity of the joystick/displa y relation was covertly increased or decreased threefold. The size of this change, being greater than twofold, meant that simply correcting the tracking error would not be sufficient to bring the subjects quick ly back on target. On the step following the change, all subjects init ially overshot (if sensitivity increased) or undershot (if sensitivity decreased) the target by a factor of about three. But within 2-4 reac tion time intervals (i.e., before the next target step), all subjects nevertheless reached the target. On the subsequent step all subjects a gain initially overshot or undershot the target, but this time by a sm aller amount than previously, indicating that partial adaptation to th e changed sensitivity had occurred on the first step. In the subsequen t 2-3 reaction time intervals, they again reached the target. This pat tern was repeated over the subsequent 14 steps such that the overshoot or undershoot decreased in a quasi-exponential fashion as adaptation to the changed sensitivity occurred. The fact that the subjects reache d the target regardless of the extent of sensory-motor adaptation demo nstrates that an immediate 'compensatory' strategy began to operate wi thin the first reaction time of detecting the changed sensory-motor re lation. A theoretical mechanism for this strategy is proposed.