Abstract
The auditory evoked N1m-P2m response complex presents a challenging case for MEG source-modelling, because symmetrical, phase-locked activity occurs in the hemispheres both contralateral and ipsilateral to stimulation. Beamformer methods, in particular, can be susceptible to localisation bias and spurious sources under these conditions. This study explored the accuracy and efficiency of event-related beamformer source models for auditory MEG data under typical experimental conditions: monaural and diotic stimulation; and whole-head beamformer analysis compared to a half-head analysis using only sensors from the hemisphere contralateral to stimulation. Event-related beamformer localisations were also compared with more traditional single-dipole models. At the group level, the event-related beamformer performed equally well as the single-dipole models in terms of accuracy for both the N1m and the P2m, and in terms of efficiency (number of successful source models) for the N1m. The results yielded by the half-head analysis did not differ significantly from those produced by the traditional whole-head analysis. Any localisation bias caused by the presence of correlated sources is minimal in the context of the inter-individual variability in source localisations. In conclusion, event-related beamformers provide a useful alternative to equivalent-current dipole models in localisation of auditory evoked responses.
Highlights
There is evidence to suggest that beamformer methods are not appropriate for the localisation of auditory evoked responses, because they depend on the assumption of no linearly correlated sources within the brain for the duration of the analysis time-window[11]
The correlation between source time-series has been shown to cause some localisation bias e.g.18, and may introduce spurious sources[19,20]. This inaccuracy is contrasted with inter-individual variability in source localisations when comparing data from monaural and diotic stimulation, conditions which differ in the extent of correlated activity across hemispheres
By comparing results from both monaural and diotic stimulation, this study shows that the whole and half-head ER-beamformer can reliably localise both components of the auditory evoked response to auditory cortex, with similar accuracy at the group level as traditional dipole-modelling which can be considered the ‘gold standard’ (Fig. 2)
Summary
There is evidence to suggest that beamformer methods are not appropriate for the localisation of auditory evoked responses, because they depend on the assumption of no linearly correlated sources within the brain for the duration of the analysis time-window[11]. Special beamformer implementations have been developed that reduce the impact of correlated sources on the accuracy of source reconstruction[18,20,21,22,23] Another approach to combatting the effects of bilateral correlated sources is to use standard (e.g., manufacturer-supplied) beamformer software packages in a region-of-interest approach whereby data from the channels over the hemisphere contralateral to each ear of stimulation are analysed separately, excluding as much as possible the correlated ipsilateral activity[17,24]. We compare event-related beamformer analysis of the auditory N1m and P2m responses with traditional dipole-modelling, and explore the effects of different modes of beamformer analysis (whole-head and half-head) of data recorded during either monaural or diotic auditory stimulation, a contrast designed to manipulate the degree of correlation between sources in opposite hemispheres
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