In just the last decade, studies of multisensory processing have doubled all preceding efforts and a multitude of behavioral and perceptual manifestations have already been documented. It is widely accepted that differences in these final products of neural processing depend on differences in the arrangement and weighting of relevant neural inputs. However, investigations into the neural bases for multisensory processing have been relatively uncommon and, consequently, only one model of multisensory convergence has been offered despite these disparate effects. In this model, converging excitatory afferents from different sensory modalities combine to enhance responses to spatially and temporally coincident multisensory stimuli in a fashion that is clearly linked to detection and orientation behaviors. Yet some multisensory effects do not obey these conventions and it is the central hypothesis of this proposal that there is more than one form of multisensory convergence. Accordingly, during the previous grant period, a 'new' form of multisensory convergence was discovered in somatosensory cortex whereby excitatory afferents from one modality were modulated by inhibitory inputs from another. In fact, a high proportion (approximately 70%) of seemingly 'unimodal' neurons in the fourth somatosensory cortical area (S IV) was suppressed by inputs from the auditory Field of the Anterior Ectosylvian Sulcus (FAES) by GABA-ergic mechanisms. The goal of the present proposal is to expose the fundamental properties of this 'new' form of multisensory convergence by exploring the connectional, functional and developmental mechanisms which underlie it. As more models of convergence become available, understanding the relationship between a particular afferent architecture and the resulting behavioral or perceptual effect will help devise testable hypotheses by which the mechanisms of multisensory processing can be further manipulated and assessed. Ultimately, by understanding the neural bases for multiple forms of cross-modal architecture, we will gain insight into not only non-tactile influences on tactual processing, but also into broad features underlying behavior and perception relevant to both normal and clinical conditions.
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