Improving source modeling of auditory steady-state responses with frequency-specific brain maps

Auditory steady-state responses (ASSRs) are evoked brain responses to modulated or repetitive acoustic stimuli. Due to a wide range of clinical and research applications, there is a great (clinical) interest to investigate the underlying neural generators of ASSRs. The cortical sources of ASSRs mostly are located in the auditory cortex (AC), although some studies avoiding prior assumptions regarding the number and location of the sources have also reported activity of sources outside of the AC. However, little is known about the number and location of these sources. In this study, we present a novel extension to minimum-norm imaging (MNI) which facilitates ASSR source reconstruction and provides a comprehensive and consistent picture of sources in response to low- as well as high modulation frequencies, monaurally presented to the left and right ears. Results demonstrate that the proposed MNI approach is successful in reconstructing sources located both within (primary) and outside (non-primary) of the AC. The locations of the non-primary sources are consistent with the literature. Primary sources are detected in every experimental condition, thereby corroborating the robustness of the approach. Moreover, we show that the MNI approach is capable of reconstructing the subcortical activities of ASSRs. In summary, the results indicate that the MNI approach outperforms the previously used method of group-ICA, in terms of detection of sources in the AC, reconstructing the subcortical activities and reducing computational load.