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Abstract

Traditionally it is assumed that information from different senses is integrated only in higher association cortices.
Contrasting this belief, we demonstrate multisensory integration in areas proximal to primary sensory areas - in
the so called auditory belt.
Using a combination of high-resolution functional magnetic resonance imaging (fMRI) in electrophysiological
recordings in macaque monkeys, we quantify the integration of audio-visual and audio-tactile stimulation.
Integration of auditory noise with tactile stimulation of the hand is reliably found in anaesthetized animals at the
posterior end and along the lateral side of the auditory belt. This integration occurs only for temporally coincident
stimuli and obeys the principle of inverse effectiveness: integration is stronger for less effective stimuli. Locations
with significant integration responded to auditory alone stimulation but only few to tactile alone. Combining visual
and auditory stimulation, robust multisensory integration in auditory cortex was found in alert animals, but only
weaker in anaesthetized animals. Similar to audio-tactile integration, the audio-visual interaction was found in
areas of the belt. Together our results suggest that touch and vision related activity in auditory cortex arise from
a different set of projections. Touch related information arrives as feed-forward input, vision related input arrives
in a top-down fashion.
Our findings demonstrate that multisensory integration can occur early in the processing hierarchy - one
processing stage above primary auditory cortex. Further, this multisensory integration occurs pre-attentive, as
demonstrated in anaesthetized animals. Such early integration might be necessary for quick and consistent
interpretation of our world and might explain multisensory illusions where a stimulus perceived by one modality is
altered by a stimulus in another modality.