Slow potential shifts of human brain during forewarned reaction.

The slow brain potentials observed during the 4 sec interstimulus interval (ISI) of a CNV paradigm were investigated by a factorial manipulation of the parameters that have been identified as affecting the CNV at ISIs of 1 or 2 sec. Subjects performed choice and simple reaction time (RT) tasks under all possible combinations of auditory and visual warning (S1) and imperative (S2) stimuli. The choice task involved easy or difficult intensity discriminations at S2, which were forewarned by S1s that cued the subject as to the level of difficulty for that trial. Principal components analysis (PCA) of the data revealed two negative afterwave factors and a late positive component (LPC) related to warning stimulus variables, and a late negative shift preceding S2 that was related to motor preparation. The negative afterwave components were enhanced in the choice RT task, and the earlier of the two components showed a modality-specific distribution. The LPC factor was also enhanced in the choice task and was further enhanced by S1s signaling difficult discriminations. Additionally, the LPC was larger for visual S1s. The S1-related components are interpreted in terms of orienting, while the later shift is identified with the readiness potential and motor preparation. The results support the notion that the conventionally recorded CNV consists of S1 evoked and response-related potentials rather than non-specific and modality-specific anticipatory potentials.