Do people only adjust ongoing movements vigorously when it is advantageous to do so?

Abstract

We previously found that arm movements towards a jittering target are constantly guided by the latest target position: the responses to target displacements became more vigorous as the movement proceeded, as required for the movement to reach the latest position smoothly within the remaining time. Here we examine whether this behaviour was a consequence of how that experiment was designed. We compared the vigour of adjustments in blocks of trials in which targets followed a random walk, as in our previous studies, with the vigour of adjustments in blocks of trials in which the target position varied at random with respect to a fixed position. For the random walk, the latest position is the best estimate of the final position, so neglecting earlier information can be useful. For random variability around a fixed position, the target's position at any instant is equally informative about the final position, so making vigorous adjustments in response to the latest information is pointless. In that case, the best estimate of the final position is the average of all the encountered positions. Some participants responded less vigorously in the latter case, but most did not. We discuss why tuning the adjustments to be complete within the remaining time may be a good strategy, even when the target does not follow a random walk.