Deconvolution of 40 Hz steady-state fields reveals two overlapping source activities of the human auditory cortex

Steady-state auditory evoked fields were recorded from 15 subjects using a whole head MEG system. Stimuli were 800 ms trains of binaural clicks with c onstant stimulus onset asynchrony (SOA). Seven different SOA settings (19, 21, 23, 25, 27, 29 and 31 ms) were used to give click rates near 40 Hz. Transient responses to each click were reconstructed using a new algorithm that deconvoluted the averaged responses to the different trains. Spatio-te mporal multiple dipole modelling in relation to 3D MRI scans revealed two o verlapping source components in both the left and right auditory cortex. Th e primary sources in the medial part of Heschl's gyrus exhibited a N19-P30- N40 m pattern. The secondary, weaker sources at more lateral sites on Hesch l's gyrus showed a N24-P36-N46 m pattern. When applied to transient middle latency auditory evoked fields (MAEFs) recorded at SOAs of 95-135 ms, the p rimary sources imaged activities similar to the deconvoluted steady-state r esponses, but the secondary source activities were inconsistent. Linear summation of the deconvoluted source waveforms accounted for more th an 96% of the steady-state variance. This indicates that the primary activi ty of the auditory cortex remains constant up to high stimulation rates and is not specifically enhanced around 40 Hz. (C) 1999 Elsevier Science Irela nd Ltd. All rights reserved.