LOCALIZATION OF THE GROUPING EFFECT IN OV ERLAPPING TASKS

In certain kinds of overlapping tasks subjects have to respond to two stimuli (S1 and S2), presented in close succession with variable onset asynchrony (SOA), carrying out two different responses (R1 and R2). W hen responding to S1 is given priority over S2 processing, reaction ti me (RT) to the first stimulus is unaffected by SOA. However, sometimes subjects adopt, either spontaneously or because of instruction, a dif ferent processing strategy called grouping. In this case R1 is postpon ed until R2 has been determined too. The present study addresses the l ocalization of the postponed processing stage by means of the laterali zed readiness potential (LRP). One group of subjects was instructed to group responses and a second group was to give priority to S1 process ing. The adherence to instructions was verified by subjects' reaction times. In case of grouping the interval between LRP onset and overt re action increased proportionally to the postponement caused by SOA. In contrast, the interval between stimulus and LRP onset was unaffected b y SOA. These findings speak against assumptions that grouping causes r esponse activation postponement; rather, the corresponding reaction is already selected and activated prior to postponement.