The effect of interstimulus intervals and between-block rests on the auditory evoked potential and magnetic field: is the auditory P50 in humans an overlapping potential?

Objective: The human auditory P50 may consist of overlapping potentials. To test this hypothesis, we manipulated interstimulus intervals (ISIs) and be tween-block rests, and recorded the P50, P50m, N100 and the N100m simultane ously. Methods: Subjects were 12 right-handed healthy adults. Four conditions incl uded: (1) 1.5 s/rest, (2) 1.5 s/successive, (3) 0.5 s/rest, and (4) 0.5 s/s uccessive. Auditory stimuli were presented a total of 880 times for each co ndition. Auditory evoked potentials and magnetic fields were recorded. We e xamined the P50, N100, P50m, N100m and dipoles of the P50m and the N100m. Results: There was no significant effect of the ISI on the P50 amplitudes, but the P50m amplitudes in the 0.5 s IST conditions were significantly smal ler than those in the 1.5 s ISI conditions. The N100 and the N100m amplitud es in the 0.5 s ISI conditions were significantly smaller than those in the 1.5 s ISI conditions. The N100 and the N100m amplitudes in the resting con ditions were significantly larger than those in the successive conditions. The P50m dipoles were slightly deeper and more anterior than those of the N 100m in primary auditory cortex. Conclusions: Central structures other than supratemporal cortex contribute to the P50 and that the P50 in humans represents overlapping potentials. (C ) 2000 Elsevier Science Ireland Ltd. All rights reserved.