Neuromagnetic evidence of an amplitopic organization of the human auditory cortex

It is well known that the location of the source of cortical auditory evoked responses, which can be determined neuromagnetically in humans using the concept of an equivalent current dipole (ECD), shifts with changing stimulus frequency ('tonotopic organization'). Not investigated so far, however, is the question of whether there exists also an 'amplitopic organization' of the human auditory cortex, i.e., a spatial distribution of neurons maximally responsive to respective 'best stimulus intensities.' We measured, in the study presented here, in 3 normally hearing subjects the auditory evoked magnetic field (AEF) in response to tone-burst stimulation with a carrier frequency of 1000 Hz at 6 different intensities (30-80 dB HL in 10 dB steps). The influence of stimulus intensity was quantified in terms of changes in the ECD parameters (amplitude, direction and spatial coordinates) which were determined such that a maximum correspondence between observed and calculated field distributions was obtained. The results of the neuromagnetic measurements presented here prove that the ECD location also shifts with changing stimulus intensity. The depth of wave M100 (latency of about 100 msec) decreases monotonically with increasing stimulus intensity while the horizontal ECD position is slightly shifted in the anterior direction. The results imply that, while topical mechanisms of frequency coding are similar at cortex and at the cochlear level, topical mechanisms of intensity coding are different at these levels.