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 steps over a period of 63 seconds, the sensitivity of the joystick/display 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 quickly back on target. On the step following the change, all subjects initially overshot (if sensitivity increased) or undershot (if sensitivity decreased) the target by a factor of about three. But within 2–4 reaction time intervals (i.e., before the next target step), all subjects nevertheless reached the target. On the subsequent step all subjects again initially overshot or undershot the target, but this time by a smaller amount than previously, indicating that partial adaptation to the changed sensitivity had occurred on the first step. In the subsequent 2–3 reaction time intervals, they again reached the target. This pattern 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 reached the target regardless of the extent of sensory-motor adaptation demonstrates that an immediate ‘compensatory’ strategy began to operate within the first reaction time of detecting the changed sensory-motor relation. A theoretical mechanism for this strategy is proposed.