Interactions between perception and action in a reaction task with overlapping S-R assignments

We have used a novel task to study relationships between perception and act ion. Four experiments studied stimulus-response (S-R) relationships under c onditions in which stimuli and responses were functionally -unrelated (i.e. , not assigned to each other by instruction) and merely overlapped in time. On each trial, participants carried out movements on a graphic tablet whil e observing motions displayed on a computer screen. The movement on trial n was specified by the motion observed on the previous trial n - 1, whereas the motion observed on trial n specified the movement to be performed on tr ial n + 1. Results showed that stimulus motion had a contrast-like impact o n response movement. Watching a small motion while performing a medium-size d movement increased movement size, whereas watching a large motion led to a decrease (Experiment 1). Further experiments showed that the contrast pat tern was not affected by the mode of motion presentation (Experiment 2), or by the interval between motion and movement execution (Experiment 3). Cont rast was also observed in the reverse direction, i.e., from action to perce ption (Experiment 4). We propose that the contrast effect is due to a mecha nism for selective code modification. This mechanism acts to increase the d istinctiveness of simultaneously activated perception and action codes in a common representational domain.