Long‐Latency Evoked Potentials and Reaction Time

ABSTRACT Tone pips separated by a constant 1 sec interval were delivered to 12 subjects. The pips were of three pitches given in a random sequence. The 400 and 1600 Hz pips each had a probability of 0.1 and the 800 Hz pips had a probability of 0.8. For each subject, either the 400 or 1600 Hz pips were designated as targets in a reaction time (RT) task. Stimulus‐synchronized averages (SSAs) to infrequent tones contained two prominent waves: a P3 and a slow wave (SW). SW and P3 amplitudes were larger, and P3 latency was 15 msec shorter, to targets than to non‐targets. SSAs were computed for RT quartiles (Q1–Q4) of each subject. Between Q1 and Q4, P3 amplitude decreased, while SW amplitude increased, demonstrating behavioral and dissociation of the two waves. Between Q1 and Q4, RTs increased from 366 to 540 msec, while P3 latency at C z increased from 328 to 359 msec. Response‐synchronized averages computed for each RT quartile showed no response‐related deflections that could explain the effects in the SSAs.