Expectancy-related cerebral potentials associated with voluntary time estimation and omitted stimulus

Expectancy-related and nonexpectancy-related cerebral potentials associated with stimuli and omitted stimuli were recorded in 7 normal subjects. The stimuli were constantly delivered to the right median nerve and the interstimulus interval was set at 7 seconds. When the subject counted to estimate the interstimulus interval correctly, a slow negative deflection appeared about one second prior to both the stimuli and the omitted stimuli. In the case of the omitted stimulus, this expectancy-related negative potential (ENP) returned to the base line after several hundred msec. When the stimuli were delivered, the amplitude of the P300 was much higher when the subject was paying attention to the stimuli than when he was not. The scalp distribution of the ENP was rather anterior to the P300. No ENP appeared when the subject was not paying attention to the stimuli or the omitted stimuli, or when the stimuli were delivered at a random rate.