Relationships between attention effects and intensity effects on the cognitive N140 and P300 components of somatosensory ERPs

Objectives: This study attempts to elucidate the relative contributions of exogenous and endogenous components to the N140 and P300 potentials elicited by somatosensory stimulation. Methods: Somatosensory event-related potentials (ERPs) were evoked using an odd-ball paradigm with the frequent (80%) stimuli delivered to the left index finger and the infrequent (20%) stimuli delivered to the right index finger. Both types of stimuli had the same intensity within each experiment. The experiment was repeated using 6 different stimulus intensities ranging from the sensory threshold to 3 times the threshold. Each experiment was done under two conditions. In one, the subjects were asked to count and respond to the infrequent stimuli. In the other, the subjects were instructed to ignore the stimuli whether frequent or infrequent. In addition, the compound sensory potential of the right median nerve was separately recorded from electrodes at the wrist using the same range of stimulus intensities applied to the right index finger. Results: Amplitudes of the N140 and P300 elicited by both attended and unattended infrequent stimuli increased in a parallel fashion as a function of stimulus intensity, so that the amplitude difference between attended and unattended responses was independent of the stimulus intensity. The amplitude of the compound sensory nerve potential at the wrist exhibited a similar slope to those of the N140 and P300. Conclusions: Thus, it is concluded that the scalp N140 and P300 consist of two components: an endogenous component, which is independent of the stimulus intensity, and an exogenous component, which increases as a function of stimulus intensity. The relative contribution of these components to the N140 and P300 amplitudes is also discussed.