Assessing visual modulation along the human subcortical auditory pathway.

Experience of the world is inherently multisensory. It has been suggested that audiovisual modulation occurs as early as subcortical auditory stages. However, this was based on the frequency-following response, a measure recently found to be significantly generated from cortical sources. It therefore remains unclear whether subcortical auditory processing can indeed be modulated by visual information. We aimed to trace visual modulation along the auditory pathway by comparing auditory brainstem response (ABR) and middle-latency response (MLR) between unimodal auditory and multimodal audiovisual conditions. EEG activity was recorded while participants attended auditory clicks and visual flashes, either synchronous or asynchronous. No differences between auditory and audiovisual responses were observed at ABR or MLR levels. It suggested that ascending auditory processing does not seem to be modulated by visual cues at subcortical levels, at least for rudimentary stimuli. Multimodal modulation in the auditory brainstem observed in previous studies might therefore originate from cortical sources and top-down processes. More studies are needed to further disentangle subcortical and cortical influences on audiovisual modulation along the auditory pathway.