Background: When two acoustic sinusoids of slightly different frequencies are presented in the same ear, the listener perceives a beating sound with a rate the difference frequency between the two sinusoids. In the dichotic presentation of two acoustic sinusoids with slightly different frequencies, separately at the two ears, the listener perceives beats at the difference frequency and these beats are called Binaural Beats. As opposed to the Monaural Beats the Binaural Beats require the combined action of the two ears in order to be perceived. Monaural and Binaural Beats can be assessed with EEG recordings of the Auditory Steady-Steady Response. While the Monaural Beats are already created at the level of the cochlea, the Binaural Beats are caused by bilateral inputs at higher levels of the auditory processing chain. This suggest that possible differences between the generators of the ASSRs elicited by Monaural and by Binaural Beats can help in the understanding of the binaural processing of sound. Objective: The objective of this study is to localize the generators of ASSRs evoked by MBs and by BBs, with the goal to detect possible spatial differences between their sources.
Methods: EEG recordings from 11 participants in response to multiple sinusoidal acoustic stimuli were used. Monaural beats, binaural beats, as well as other higher order monaural and binaural interactions between the input acoustic sinusoids were detected as ASSRs at the respective frequency of the beat or at the frequency of the other interactions. All the significant ASSRs were identified and then a spatial filtering technique was applied for the localization of their generators in the brain. The Dynamic Imaging of Coherent Sources (DICS) beamformer with left/right symmetric scanning dipoles was used to scan the whole brain and estimate the sources of the ASSRs. Results: The sources of the ASSRs evoked by monaural beats of 33Hz and 39Hz were identified in the auditory cortex. Maximum activation of the brain was found in the auditory cortex contralateral to the side of the monaural beat stimulus. ASSRs of 4 Hz Binaural Beat and 6Hz binaural interaction were localized in the right auditory cortex, slightly lower than the source of the Monaural Beats. Finally, sources of ASSRs evoked by other binaural beats and other monaural and binaural interactions were found in the following brain areas: temporal middle gyrus, parietal lobule, frontal gyrus, and precentral gyrus. Based on comparisons with results from other studies, the localization accuracy of subject-level source estimates was estimated 24.7mm. Conclusion: Generators of ASSRs evoked by monaural and binaural beats were estimated in the auditory cortex. The overall source results of the study suggest right-asymmetric contribution to the ASSR from the auditory cortex. Areas not belonging to the auditory system were also localized as sources of ASSRs. This suggests that brain areas outside the auditory system might be involved in the processing of the sound and sound perception. Finally, the results of this study were not enough to answer the question whether there are differences between the generators of ASSRs evoked by MBs and by BBs.