Brain mapping of auditory steady‐state responses: A broad view of cortical and subcortical sources

Abstract

Auditory steady‐state responses (ASSRs) are evoked brain responses to modulated or repetitive acoustic stimuli. Investigating the underlying neural generators of ASSRs is important to gain in‐depth insight into the mechanisms of auditory temporal processing. The aim of this study is to reconstruct an extensive range of neural generators, that is, cortical and subcortical, as well as primary and non‐primary ones. This extensive overview of neural generators provides an appropriate basis for studying functional connectivity. To this end, a minimum‐norm imaging (MNI) technique is employed. We also present a novel extension to MNI which facilitates source analysis by quantifying the ASSR for each dipole. Results demonstrate that the proposed MNI approach is successful in reconstructing sources located both within (primary) and outside (non‐primary) of the auditory cortex (AC). Primary sources are detected in different stimulation conditions (four modulation frequencies and two sides of stimulation), thereby demonstrating the robustness of the approach. This study is one of the first investigations to identify non‐primary sources. Moreover, we show that the MNI approach is also capable of reconstructing the subcortical activities of ASSRs. Finally, the results obtained using the MNI approach outperform the group‐independent component analysis method on the same data, in terms of detection of sources in the AC, reconstructing the subcortical activities and reducing computational load.

Neural Generators Underlying Temporal Envelope Processing Show Altered Responses and Hemispheric Asymmetry Across Age

“…The brain sources of ASSRs were reconstructed using a variety of MNI, which was adapted for ASSR source analysis (Farahani et al, 2020). An overview of this approach is given below [for more details, see Farahani et al (2020)].…”

“…For each EEG channel, the ASSR strength (amplitude at the modulation frequency) was obtained from the fast Fourier transform (FFT). The maximum response amplitude across channels was assigned to the signal of interest (Farahani et al, 2020). The background noise of each channel was estimated based on the average of 30 neighboring frequency bins on the left and the right side of the response frequency bin.…”

“…The background noise of each channel was estimated based on the average of 30 neighboring frequency bins on the left and the right side of the response frequency bin. The median of the background noise across channels was assigned to the noise level (Farahani et al, 2020).…”

“…In response to low modulation frequencies, more activity in the cortical sources [auditory cortex (AC)] than subcortical sources is observed, while for high modulation frequencies, the subcortical sources are more active (Giraud et al, 2000;Liégeois-Chauvel et al, 2004). At the cortical level, not only primary sources in the AC are involved in the generation of ASSRs but also sources outside of the AC, designated as non-primary ones (Farahani et al, 2017(Farahani et al, , 2019(Farahani et al, , 2020. However, due to the volume conduction of the brain tissue, a weighted average of activity of different neural generators is recorded at sensor-level data.…”

“…Primary and non-primary neural sources of ASSRs are reconstructed using a minimum-norm imaging (MNI) approach (Farahani et al, 2020). For each neural source, the ASSR amplitude and phase coherence (inter-trial phase coherence) are calculated to investigate the response strength and the phaselocking to the stimulus, respectively.…”