Effect of task and stimulus probability on evoked potentials.

Some experimental data demonstrate the existence of at least two different P300s distinguished by latency, topography and functional significance. These positive waves are preceded by a negative component which has been labelled N200. The purpose of this study was to analyze the modifications of these negative and positive components according to probability and task (GO vs. NOGO) in order to better understand the significance of each of them in terms of stages of information processing. Six normal subjects were tested twice on separate days, in a GO-NOGO paradigm. Auditory stimuli - high pitch (GO) and low pitch (NOGO) - were presented in three different occurrence probability combinations: 0.2/0.8; 0.5/0.5; 0.8/0.2, in successive blocks. In both GO and NOGO responses, a vertex N200 component was followed by two positive waves - the first peaking at 300 msec at Cz (Cz : P300), the other one Ca 400 msec at Pz (Pz : P400). An inverse relation between amplitude and probability was seen for the three components but it was twice as large for P400 as for P300. A task effect was only seen for N200, the amplitude and latency of which increased in the GO condition. From the first session to the second, the amplitude of N200 decreased whereas that of P300 increased.