Stimulus Preceding Negativity: Arguments in Favour of Non Motoric Slow Waves

Abstract

The answer to the question "under what circumstances is stimulus anticipation accompanied by a slow wave?" can help us to understand the processes underlying anticipatory behaviour. The classic task used in the electrophysiology research of anticipation is the forewarned reaction time (RT) task in which a warning stimulus (WS) announces the future presentation of an imperative stimulus (RS) to which a response has to be made. The late wave of the Contingent Negative Variation (CNV) that is recorded under these circumstances reflects at least two different processes: anticipation of the RS and preparation for the movement. Both are irrevocably confounded in this task and their contribution to the negativity recorded prior to RS is not equal. Rohrbaugh and Gaillard (1983) have presented evidence for the similarity of the late wave and the readiness potential (RP) which can be recorded preceding unsignalled movements. The largest part of the late wave is certainly an RP, related to motor preparation. This statement should not make us forget that the RP itself reflects, apart from strictly motoric aspects (Barrett et al., 1986; Kutas and Donchin, 1977), cognitive processes which are related to the uncertainty about the force needed (Hink et al., 1983), to the timing of the movement (Libet et al., 1982), its motivational consequences (McAdam and Seales, 1969) and the information that becomes available following the movement (Freude et al., 1988; Hink et al., 1982). It is reasonable to suppose that the anticipation of the RS is also accompanied by a negative wave although a crucial experiment to demonstrate its existence is difficult to perform.