Corrections to unexpected visual changes in the perceived position of the hand during rapid movements

Subjects tracked a constant velocity target on a computer monitor using a hand held joystick, the position of which was displayed visually by a cursor. At different intervals during their response the cursor was suddenly stepped forward or backward (60% of trials) so that subjects' hands seemed either closer to or further away from the target. The eyes moved directly to the target but wrist movements required correction. This corrective latency was longer if the cursor-step occurred early during the preparation for the first movement than if it occurred later during the actual movement. There was no alteration in the amplitude of initial eye or wrist movement. These results suggest that, when the movement itself is visually perturbed, a 'motor refractory period' constrains the initiation of compensatory hand movements. This is in contrast to rapid movement corrections which are possible when the visual location of the target is altered.