Afferent connections of the primary somatosensory cortex of the mouse for contextual and multisensory processing

Abstract

Sensory information is conveyed from peripheral receptors through specific thalamic relays to primary areas of the cerebral cortex. Information is then routed to specialized areas for the treatment of specific aspects of the sensory signals and to multisensory associative areas. Information processing in primary sensory cortices is influenced by contextual information from top-down projections of multiple cortical motor and associative areas as well as areas of other sensory modalities and higher order thalamic nuclei. The primary sensory cortices are thus located at the interface of the ascending and descending pathways. The theory of predictive coding implies that the primary areas are the site of comparison between the sensory information expected as a function of the context and the sensory information that comes from the environment. To better understand the anatomical basis of this model of sensory systems we have charted the cortical and subcortical afferent inputs in the ipsilateral and contralateral hemispheres of the primary somatosensory cortex of adult C57Bl/6 mice. Iontophoretic injections of the b-fragment of cholera toxin were performed inside the mystacial caudal barrel field, more rostral barrel field and somatosensory cortex outside the barrel field to test the hypothesis that differences exist between these three parts and to compare their projections to the subnetworks built from the Mouse Connectome Project data. The laminar distribution of retrogradely labeled cell bodies was used to classify the projections as feedback, feedforward or lateral. Layer indices range between -1 and 1, indicating feedback and feedforward connections respectively. The primary somatosensory cortex and the barrel field have afferent connections with somatosensory areas, non-somatosensory primary sensory areas, multisensory, motor, associative, and neuromodulatory areas. The caudal part of the barrel field displays different and more abundant cortical and subcortical connections compared to the rest of the primary somatosensory cortex. Layer indices of cortical projections to the primary somatosensory cortex and the barrel field were mainly negative and very similar for ipsilateral and contralateral projections. These data demonstrate that the primary somatosensory cortex receives sensory and non-sensory information from cortical and subcortical sources.