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Abstract

Our different senses provide complementary views of the environment, and integrating information across senses is necessary for disambiguating sensory objects and for reliable interaction with these. Supposedly, multisensory information is integrated only by higher cortical association areas. Contrasting this belief, we demonstrate multisensory integration in areas proximal to primary sensory areas - in the so called auditory belt. Using functional magnetic resonance imaging (fMRI) of macaque monkeys, we quantified the integration of simultaneous audio-visual and audio-tactile stimulation in anaesthetized animals at 4.7Tesla. Technically, integration was assumed if the response to the combined stimulus was stronger than the sum of the responses to individual stimuli. Integration of auditory broad-band noise with tactile stimulation of hands and foot was found at the posterior end and along the lateral side of the auditory belt in six animals. This integration occurred only for temporally coincident stimuli and obeyed the principle of inverse effectiveness: integration was stronger for less effective stimuli. Voxels with significant integration responded to auditory alone stimulation but only few to tactile alone. Combining visual and auditory stimulation in different paradigms we could not find robust multisensory integration in auditory cortex. Further, audio-tactile integration was mostly limited to auditory cortex and much weaker in nearby ‘multimodal’ areas such as the claustrum. Our findings demonstrate that multisensory integration can occur early in the processing hierarchy - one processing stage above primary auditory cortex. Further, this multisensory integration occurred 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.