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Abstract:
Recent results from human imaging and electrophysiological studies promote the view that processing within auditory cortex can be influenced by cross-modal stimulation of other sensory modalities. Here we scrutinize the neuronal basis of these sensory interactions by probing regions in the monkey auditory pathway for multisensory influences using combinations of functional imaging (fMRI) and electrophysiology.
Using functional imaging, we previously found that caudal fields of the auditory cortex show enhanced fMRI-BOLD responses when auditory stimuli were complemented by simultaneous visual or touch stimulation [see Kayser et al. Neuron 48, 2005 and J. Neurosci. 27(8), 2007]. This sensory interaction was much enhanced in the superior temporal regions but was less evident in anterior auditory fields and the insula. To validate these results at the level of individual neurons, we now record field potentials and single unit activity from these regions. We find that within caudal auditory cortex, only 12 of the neurons show cross-modal influences, such as response enhancement or suppression. This visual modulation occurs only for a narrow time window of stimulus onset asynchronies and is independent of the particular kind of stimulus used. In the acoustically responsive region of the insula a similar proportion of neurons show such kind of audio-visual interaction, while in the superior temporal region visual, auditory and multisensory neurons are spatially intermingled and occur in equal proportions.
Our findings reveal how the presence of visual input increases along the auditory processing stream and demonstrate that already early auditory cortices are susceptible to cross-modal influences. As a consequence we conclude that the processing at these stages not only reflects acoustical stimuli but is also dependent on their visual context.
