Auditory cortical generators of the Frequency Following Response are modulated by intermodal attention

Abstract

The vast efferent connectivity of the auditory system suggests that subcortical (thalamic and brainstem) auditory regions should also be sensitive to top-down processes such as selective attention. In electrophysiology, the Frequency Following Response (FFR) to simple speech stimuli has been used extensively to study these subcortical areas. Despite being seemingly straight-forward in addressing the issue of attentional modulations of subcortical regions by means of the FFR, the existing results are highly inconsistent. Moreover, the notion that the FFR exclusively represents subcortical generators has been recently challenged. By applying these techniques to data recorded from 102 magnetoencephalography (MEG) magnetometers in 34 participants during a cross-modal attention task, we aimed to gain a more differentiated perspective on how the generators of the FFR are modulated by either attending to the visual or auditory input. In a first step our results confirm the strong contribution of also cortical regions to the FFR. Interestingly, of all regions exhibiting a measurable FFR response, only the right primary auditory cortex was significantly affected by intermodal attention. By showing a clear cortical contribution to the attentional FFR effect, our work significantly extends previous reports that focus on surface level recordings only. It underlines the importance of making a greater effort to disentangle the different contributing sources of the FFR and serves as a clear precaution of simplistically interpreting the FFR as brainstem response.