Test of the ballistic hypothesis of movement and the ‘point of no return’

Abstract

How long does it take to stop or modify a movement once it has begun? Where there is an error in producing an action as intended (usually because of control-display incompatibility) subjects may correct it in less than their normal reaction time (RT) by proprioception¹ or efference-copy monitoring^2—4. Where subjects work from a visual (external) error signal, however, they are usually said to take a full RT to stop⁵ or correct^6,7 ongoing movements, so that there is a ‘point of no return’, after which they are committed for about 300 ms to any action decided on⁸. During this time the action is ballistic at the perceptual-motor level^6,9,10. But in a recent tracking experiment¹¹ where subjects wrongly anticipated the target motion, and so began a movement in the wrong direction, the correction time to stop the action (CorT) was often much shorter than the normal RT to initiate movements, down to as little as 120 ms in some cases. This suggests that fast error correction can occur from a visual signal alone, at least to stop movements (as in an emergency). Repeating the experiment we find that movements can indeed be arrested soon after they begin, but only if the beginning is delayed; it always takes one RT from the occurrence of the error signal. We therefore infer that there is a ‘point of no return’ at the decision level, but that movements are not necessarily ballistic at the motor level.