INTENSITY DEPENDENCE OF AUDITORY-EVOKED DIPOLE SOURCE ACTIVITY

The spatio-temporal approach of dipole source analysis represents a cr ucial methodological progress in research on individual differences in the dependence of auditory evoked potential amplitude on stimulus int ensity (augmenting/ reducing) because overlapping subcomponents of the N1/P2-component can be separated and can be related to their generati ng cortical structures. Basic aspects of the intensity dependence of a uditory evoked dipole source activity were analysed in 40 healthy subj ects. The evoked responses to binaural 1000-Hz tones at five levels of intensity (60, 70, 80, 90, 100 dB sound pressure level) were recorded at 33 sites across the scalp. The dipole source analysis of the grand average data confirms the reports in the literature that the N1/P2 po tentials at the scalp can be explained by two dipoles per hemisphere: a tangential dipole, representing activity of the superior temporal co rtex (including primary auditory cortex), and a radial dipole, represe nting activity of the lateral temporal cortex (secondary auditory area s). The intensity dependence of the tangential dipole activity was sig nificantly more pronounced than that of the radial dipoles, supporting the assumption that radial and tangential dipoles represent different physiological processes. A high reliability of the intensity dependen ce of the tangential dipole(Pearson correlation: r = 0.88) was found w hen retesting the subjects after three weeks. Age was negatively corre lated with the intensity dependence of the tangential dipole. Dipole s ource analysis proved to be a reliable method which allows, at least i n part, to study separately the intensity dependence of the evoked res ponses from primary and secondary auditory cortices. This is of import ance with regard to the hypothesis that the central serotonergic syste m modulates the intensity dependence of the evoked N1/P2-response of p rimary auditory cortex.