Representations of the temporal envelope of sounds in human auditory cortex: can the results from invasive intracortical "depth" electrode recordings be replicated using non-invasive MEG "virtual electrodes"?

Magnetoencephalography (MEG) beamformer analyses use spatial filters to estimate neuronal activity underlying the magnetic fields measured by the MEG sensors. MEG "virtual electrodes" are the outputs of beamformer spatial filters. The present study aimed to test the hypothesis that MEG virtual electrodes can replicate the findings from intracortical "depth" electrode studies relevant to the processing of the temporal envelopes of sounds [e.g. Nourski et al. (2009) "Temporal envelope of time-compressed speech represented in the human auditory cortex," J. Neurosci. 29:15564-15574]. Specifically we aimed to determine whether it is possible to use non-invasive MEG virtual electrodes to characterise the representation of temporal envelopes of 6-Hz sinusoidal amplitude modulation (SAM) and speech using both auditory evoked fields (AEFs) and patterns of power changes in high-frequency (>70 Hz) bands. MEG signals were analysed using a location of interest (LOI) approach by seeding virtual electrodes in the left and right posteromedial Heschl's gyri. AEFs showed phase-locking to the temporal envelope of SAM and speech stimuli. Time-frequency analyses revealed no clear differences in high gamma power between the pre-stimulus baseline and the post-stimulus presentation periods. Nevertheless the patterns of changes in high gamma power were significantly correlated with the temporal envelopes of 6-Hz SAM and speech in the majority of participants. The present study reveals difficulties in replicating clear augmentations in high gamma power changes using MEG virtual electrodes cf. intracortical "depth" electrode studies (Nourski et al., 2009).