Slow shifts in brain potential (commonly called the contingent negative variation), obtained during a warned reaction-time task with a foreperiod of 1 second, were compared with waveforms synthesized by the addition of separately obtained potentials associated with individual (nonpaired) sensory stimuli and self-initiated motor movements. The synthesized waveforms match closely the actual contingent negative variation, suggesting that it is constituted largely of separate, noncontingent elements related to sensory and motor processes.