Positive and negative shifts of the readiness potential: preparatory effects

Freude and Ullsperger (1989) reported that positive potentials occurred on some trials in a single trial analysis of the Readiness Potential (RP). The present study investigated the effect of split attention on the occurrence of RPs of different polarities as well as comparing the post-movement potential and auditory-evoked potential after RPs of different polarities. In part 1 of the study, subjects (n = 16) pressed a key in a self-paced manner, concentrating on the movement. In part 2 they were asked to discriminate between two randomly-ordered tones, triggered by the key press (with a delay of 560 ms), and to announce the result to the experimenter verbally. The results of part 1 matched previous findings, in that in 41% of all trials RP was positive. In part 2 there was a significant increase in the relative frequency of positive RPs to 54%. This resulted in the magnitude of the averaged RP decreasing from part 1 to part 2. The early component of the averaged RP, which had begun about 1300 ms before the movement in part 1 of the study, disappeared in part 2, where the averaged RP began only 350 ms before the movement. The post-movement potential was significantly larger when the preceding RP was negative. In part 2 of the study we found an N100 in response to the tone; this was larger after positive RPs, but the difference was not significant. The implications of these novel findings for interpretation of the averaged RP are discussed.