CORRECTIONS TO UNEXPECTED VISUAL CHANGES IN THE PERCEIVED POSITION OFTHE HAND DURING RAPID MOVEMENTS

Subjects tracked a constant velocity target on a computer monitor usin g a hand held joystick, the position of which was displayed visually b y a cursor. At different intervals during their response the cursor wa s suddenly stepped forward or backward (60% of trials) so that subject s' hands seemed either closer to or further away from the target. The eyes moved directly to the target but wrist movements required correct ion. This corrective latency was longer if the cursor-step occurred ea rly during the preparation for the first movement than if it occurred later during the actual movement. There was no alteration in the ampli tude of initial eye or wrist movement. These results suggest that, whe n the movement itself is visually perturbed, a 'motor refractory perio d' constrains the initiation of compensatory hand movements. This is i n contrast to rapid movement corrections which are possible when the v isual location of the target is altered.