A combined functional in vivo measure for primary and secondary auditory cortices

Auditory evoked magnetic fields are reliable physiological in vivo markers of activity generated in auditory cortices. In recent years, several compon ents of auditory evoked fields have been demonstrated with specific topogra phies within the auditory cortex in man. Their differential elicitation and analyses has rendered the discrimination of neural activities in primary v s. secondary auditory cortical fields possible. This in vivo measure may be of interest in a number of (neuro)psychiatric and neuropsychological disor ders with central auditory deficits, in which in vivo anatomical measures d o not allow a clear distinction of primary vs. secondary auditory cortex in volvement. To help better understand the pathophysiology of such disorders, we developed and introduce a combined measure of steady-state field (SSR) and the N1 component of the transient evoked field. The acoustic stimulus f or this paradigm consists of a 500-ms tone burst with 39-Hz amplitude modul ation of the carrier frequency. This combined stimulation allows assessment of both auditory cortex components in one brief examination to be well tol erated by patients. We examined the source locations of SSR and N1 componen t with separate classical stimulation and combined stimulation within-sessi on in healthy volunteer subjects. We demonstrate here that the distinct sou rces of steady-state (primary auditory cortex) and N1 (secondary auditory c ortex) responses can be reliably measured without significant spatial disto rtion with this combined stimulation paradigm. (C) 2000 Elsevier Science B. V. All rights reserved.