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Integration of information across the senses is critical for perception. This activity is thought to arise primarily from connections made in the brain's sensory cortical areas. A new paper from the King Group uncovers evidence for the first time on the little understood role of subcortical circuits in shaping the multisensory properties of primary cortical neurons.

Our capacity to combine and integrate messages provided by our different senses can have a profound influence on perception, as illustrated when we watch a movie with a poorly dubbed soundtrack. It is now well established that neurons throughout the neocortex are modulated, and sometimes driven, by different sensory stimuli. Previous work has helped to reinforce the prevailing view that multisensory responses in early sensory cortical areas, such as the primary auditory cortex, arise via inputs from other cortical areas.

However, research in this area has mostly ignored the alternative possibility that cortical multisensory response properties are actually inherited from the thalamus. This is despite the role of subcortical structures, such as the thalamus, in mediating the flow of information between different cortical areas having recently become a key question in neuroscience.

Professor Andrew King’s group investigated the hypothesis that inputs from the thalamus are responsible for the multisensory properties of cortical neurons. They chose to address this question in the context of somatosensory influences on auditory cortical processing because a number of studies in species ranging from mice to primates have shown that touch and sound are integrated in auditory cortex.

This research involved a powerful combination of in vivo optogenetic circuit dissection and recording methods and was carried out by Dr Michael Lohse, Dr Johannes Dahmen, Associate Professor Victoria Bajo Lorenzana and Professor Andrew King. They first showed that whisker stimulation suppresses the responses of auditory cortical neurons to sound in a sound frequency-specific manner, highlighting the dominance of whisker inputs from nearby objects over auditory inputs, and that this multisensory interaction occurs independently of brain state. They next demonstrated that this whisker-induced suppression originates from the primary somatosensory cortex. But rather than arising from direct connections between these cortical areas, as might have been expected, the somatosensory influences are inherited from the core regions of thalamus that provide most of the ascending auditory input to the primary auditory cortex. Most surprisingly, the whisker-driven suppression of auditory cortical responses is mediated by descending projections from the primary somatosensory cortex to the inferior colliculus in the midbrain, which then inhibits auditory thalamocortical neurons. The brain circuits responsible for the integration of touch and sound therefore involve crossmodal loops linking somatosensory and auditory cortical areas via the auditory midbrain and thalamus.

Professor King said: "This study provides compelling new evidence into how different sensory cortical areas communicate with each other and for the role of descending projections in mediating multisensory integration."

Dr Lohse, former graduate student at DPAG, said: "Not only does this study reveal a new way our senses can influence each other, but it also unveils a fundamentally new type of pathway for communication between cortical areas."

The full paper, "Subcortical circuits mediate communication between primary sensory cortical areas in mice", is available to read in Nature Communications.

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