Corticocortical Connectivity Subserving Different Forms of Multisensory Convergence

Abstract

Historically, anatomical investigations of multisensory convergence largely confirmed that inputs to multisensory structures arose from areas representing different sensory modalities, and the functional result of such convergence was apparent in the form of bimodal (or trimodal) neurons. Recently, a few reports have observed projections from representations of one modality into that of another, but the traditional forms of bimodal (or trimodal) multisensory neurons were not apparent. These crossmodal projections were generally sparse in density and preferentially terminated in the supragranular cortical layers. Subsequent recordings in regions receiving such projections have revealed a ‘new’ form of multisensory neuron: the subthreshold multisensory neuron. These neurons were activated by a single modality, but showed subtle yet significant modulation of that response when a stimulus from a second modality was present. Furthermore, these subthreshold neurons represent a transitional form along a continuum from overtly bimodal multisensory neurons, at one end, and unisensory neurons at the other. Ultimately, by distributing different proportions of these multisensory neuron types across different neural areas, it is possible that a single multisensory stimulus can elicit different levels of multisensory processing/integration from different regions in the brain.