The silent period between sounds has a stronger effect than the interstimulus interval on auditory evoked magnetic fields

Auditory evoked cortical responses, electric N1 and magnetic N1m, increase when the interstimulus interval (ISI) increases. We assumed that the response to a tone is mainly affected by the immediately preceding ISI, by the immediately preceding pause between stimuli (PBS) and by the previous stimulus duration (PSD). These 3 values are connected by the following expression: ISI = PBS + PSD. We examined the dependence of the auditory evoked brain magnetic responses on the ISI with the constant PSD (conventional paradigm), on the PBS with the constant ISI, and on the ISI with the constant PBS. Peak latencies and peak amplitudes of the 3 components, P1m, N1m and P2m, are recorded in one block using all possible combinations of 5 PSDs (0.05, 0.5, 1.0, 1.5 and 2.0 s) and 5 ISIs (0.5, 1.0, 1.5, 2.0 and 2.5 s). Peak latencies of these 3 components do not show any significant dependence either on the PBS or on the ISI. Neither the PBS nor the ISI brings a statistically significant effect on the P1m peak amplitude. On the other hand, the N1m peak amplitude increases as either the PBS (constant ISI) or the ISI (constant PSD) increases. The regression coefficient to the PBS is more than a double of that to the ISI. Moreover, the ISI does not show any significant effect on the N1m peak amplitude when the PBS is constant. This stronger PBS effect means that the N1m peak amplitude dependence on the ISI, which has been reported in several papers using the constant PSDs, includes more dependence on the PBS. The P2m peak amplitude shows the same tendency as the N1m because of the strong correlation in peak amplitude between them. (C) 1997 Elsevier Science Ireland Ltd.