Covert signs of expectancy in serial reaction time tasks revealed by event-related potentials

Choice reaction time is strongly determined by the sequence of preceding st imuli. With long response-stimulus intervals (RSIs), a cost-benefit pattern is observed, which has been related to expectancy, whereas with short RSIs a benefit-only pattern emerges, possibly because of automatic facilitation . In the present study, event-related potentials were recorded while subjec ts performed serial choice responses to visual and auditory stimuli at long and short RSIs. As expected, reaction times displayed cost-benefit and ben efit-only patterns at long and short RSIs, respectively. In contrast, seque ntial effects in event-related potential amplitudes displayed a cost-benefi t pattern, unaffected by the RSI. The results demonstrate that an expectanc y-like mechanism is always active in serial tasks but appears to influence performance only when the RSI is long.