Event-related potentials in a self-paced novelty oddball task

The event-related potentials (ERPs) to the stimuli triggered by a participant's self-paced key press were compared with the ERPs to the same stimuli presented at an external pace. Fourteen participants responded to 2000 Hz target tones interspersed among 1000 Hz standard tones and various unique novel sounds. The amplitudes of the P3s elicited by target and novel stimuli were larger in the self-triggered condition than in the machine-triggered condition in which the stimuli were presented automatically with the same intervals as in the self-triggered condition. The effect was prominent at fronto-central scalp sites. When the timing of stimulus is unequivocal, a neural activity related to stimulus evaluation may increase and manifest itself as a large late positive ERP component.