MOTOR COACTIVATION REVEALED BY RESPONSE FORCE IN DIVIDED AND FOCUSED ATTENTION

Four experiments examined effects of bimodal stimulation on response f orce (RF) in addition to reaction time (RT). In a divided-attention ta sk (Experiments 1 to 3), subjects were asked for a speeded response to either a visual or an auditory signal. In unimodal signal trials, eit her a visual or an auditory signal was presented alone, and in redunda nt-signals trials, both signals were presented simultaneously. The sam e stimulus arrangement was used in a focused-attention task (Experimen t 4), but subjects had to withhold their response when an auditory sig nal was presented alone. In all experiments, the fastest RTs were atta ined in redundant-signals trials. In addition, RF was largest in redun dant-signals trials, especially in the divided-attention task, suggest ing a motor coactivation hypothesis. The results indicate that the typ e of stimulation influences not only when a response is initiated but also how the response is executed. This finding challenges the view, c ommonly held in mental chronometry, that late motoric processes remain untouched by experimental manipulations. A detailed analysis of the r elationship between RT and RF revealed that these variables are not in herently redundant measures, and, therefore, RF recording may suppleme nt the traditional RT measurement in mental chronometry.