Effects of warning-signal duration on the early and late components of the contingent negative variation.

Slow EEG potentials were recorded from three sites on the scalp (Fz, Cz and Pz) during a simple reaction time task in which the duration of the warning signal was either 0.5 or 2 sec. The duration of the foreperiod was held constant, and order of conditions was varied according to a latin square design. As predicted, the longer warning signal evoked increased amplitude of the early component of the contingent negative variation (CNV). These results confirm the interpretation of that wave as an orienting response. In contrast, the duration of the warning signal did not affect the second CNV component or reaction time latency. Additional dissociation between the two CNV components was evident in their distribution on the scalp. The early component was smallest at Pz, whereas the late component attained its lowest amplitude at Fz. Concurrently recorded palmar skin potentials exhibited different polarity and latency from the CNV.