Influence of mechanical load on sequential effects

Abstract

Almost two decades ago, sequential effects of human grasping behaviour were described for the first time: In a sequential task, participants persisted in using the previous grasp type. According to the plan-modification hypothesis, such sequential effects reduce the movement planning costs and occur within a limited range of indifference. In the current study, we asked whether the anticipated mechanical costs of a movement would compete with the movement planning costs and, thus, reduce the magnitude of the sequential effect. To this end, participants were tested in a sequential, perceptual-motor task (opening a column of drawers), which offered a continuous range of posture solutions for each trial. In a pre-/post-test design, the magnitude of the sequential effect was measured before and after a manipulation phase with increased mechanical costs. Participants displayed a sequential effect for the majority of drawers in the pre-test, which was significantly reduced in the post-test. This finding indicates that each executed movement is a weighted function of both its cognitive and mechanical costs. The result also implies that sequential effects do not result solely from dynamical properties of the motor system, but instead reflect computational features of the movement selection process.