INPUT AND CENTRAL PROCESSING EXPRESSED IN ERP AND HEART-RATE CHANGES TO RARE TARGET AND RARE NONTARGET STIMULI

Whether late positive components of event-related potentials (ERPs) pa rallel changes in heart rate (HR) indicative of attention/orienting to rare stimuli has been debated. In the present study, a three-stimulus design was used, with rare target, rare nontarget, and frequent stand ard stimuli delivered under identical conditions except that instructi ons to subjects described the targets to which subjects should respond but did not describe the nontargets. In Experiment 1, stimuli varied among modalities; in Experiment 2, auditory stimuli were employed. Bot h ERPs and HR were consistent with automatic processing preceding two stages of controlled processing. Rare stimuli evoked larger parietal P 300 and initial HR deceleration than standards. Presumably because of load-reducing effects of long interstimulus intervals, targets and non targets were not distinguished before a late slow wave and a late phas e of HR acceleration. Neither rare stimulus elicited a recognizable fr ontal P3a.