Probability and interstimulus interval effects on the N140 and the P300 components of somatosensory ERPs

Event-related potentials (ERPs) evoked by somatosensory stimuli were studied in two experiments with manipulation of the interstimulus interval (ISI) in the range from 1 to 4 seconds and the stimulus probability from 10 to 50% in fine steps. All the stimuli were presented randomly on both index fingers as odd-ball paradigm, and the stimulus intensities on both sides were equally set at 2.5 times the subjective sensory threshold at which the stimuli could easily be ignored when delivered as nontarget stimuli. During the course of two experiments, the N140 was evoked by the target and nontarget stimuli, otherwise the P300 was evoked by the target stimuli alone. In the experiment on the effects of ISIs change, the N140 amplitude showed a tendency to increase with increasing ISI and the P300 amplitude increased with increasing ISI. In the other experiment on the effects of change in stimulus probability, the amplitudes of both the N140 and the P300 decreased with increasing stimulus probability. From the results across two experiments, it is argued that the effects of both probability and ISI changes on the amplitudes of both the N140 and the P300 do not directly reflect the recovery time of psychological functions but may also involve the complicated effects derived from the habituation of the individual generators and the difficulty for performing the task. Moreover, it is indicated that the stimulus intensity used is one of major factors to be considered for comparing results to discuss the effects of ISI and probability on the ERPs.