Visual stimuli modulate local field potentials but drive no high-frequency activity in human auditory cortex.

Neuroimaging studies suggest cross-sensory visual influences in human auditory cortices. Whether these influences reflect active visual processing in human auditory cortices (ACs), which drives neuronal firing and concurrent broadband high-frequency activity (BHFA; >70 Hz), or whether they merely modulate sound processing is still debatable. Here, we presented auditory, visual, and audiovisual stimuli to 16 participants (7 women, 9 men) with stereo-EEG depth electrodes implanted near ACs for presurgical monitoring. Anatomically normalized group analyses were facilitated by inverse modeling of intracranial source currents. Analyses of intracranial event-related potentials (iERP) suggested cross-sensory responses to visual stimuli in ACs, which lagged the earliest auditory responses by several tens of milliseconds. Visual stimuli also modulated the phase of intrinsic low-frequency oscillations and triggered 15-30-Hz event-related desynchronization in ACs. However, BHFA, a putative correlate of neuronal firing, was not significantly increased in ACs after visual stimuli, not even when they coincided with auditory stimuli. Intracranial recordings demonstrate cross-sensory modulations, but no indication of active visual processing in human ACs. Significance Statement Visual information has a profound influence on auditory processing, particularly in noisy conditions. These "cross-sensory" influences start already in auditory cortices, the brain area that processes sound signals. It has, however, been unclear whether auditory cortex actively processes visual information or whether visual signals only change the way sounds are processed. We studied this question by neurophysiological recordings from 16 participants with epilepsy who had electrodes implanted in their brains due to medical reasons. Using these intracranial recordings, we show that cross-sensory visual information modulates sound processing but triggers no high-frequency activity -- a correlate of local neuronal firing -- in auditory cortex. This result provides important information on the role of sensory areas in multisensory processing in the human brain.