To make a comprehensive percept of the environment, the brain must take into account inputs from all of its sensory systems. Therefore, at some level in the nervous system, inputs from different sensory modalities must converge onto individual neurons. At present, there is essentially a single model of multisensory processing at the neuronal level: cells in the superior colliculus that receive excitatory afferents from different sensory modalities. However, it is unlikely that the architecture of the midbrain or the excitatory-excitatory nature of its convergent sensory inputs represent the full range of multisensory possibilities. Numerous reports indicate that multisensory convergence also occurs within the cerebral cortex. Here, newly identified connections that violate modality-specific borders may provide a different model to examine other possible forms of multisensory interactions. Specifically, projections within the Anterior Ectosylvian Sulcal (AES) cortex from auditory FAES to somatosensory AES (SIV) cross modality boundaries, but the expected bimodal neurons are not evident. This apparent contradiction (and preliminary data) leads to the hypothesis that the FAES-SIV connection represents a form of excitatory-inhibitory multisensory convergence. Furthermore, these intermodal connections may provide a ready-made substrate on which cross-modal plasticity can it an occur as a result of abnormal postnatal sensory experience. These possibilities will be tested using neuroanatomical and pharmacophysiological methods both in normal somatosensory (SIV) cortex and in that which has been reorganized by neonatal deafening. Because current search paradigms used to identify multisensory neurons are biased toward neurons which receive convergent excitatory inputs, the proposed experiments are likely to reveal a vastly underestimated source of multisensory processing. The proposed studies will also elucidate the circuitry not only for multisensory convergence, but for AES/""""""""association"""""""" cortex as well. Ultimately, this excitatory-inhibitory model of multisensory convergence will contribute to a comprehensive conceptual framework in which the mechanisms of perceptual, developmental, and plastic features of a broad range of multisensory cortical processes may be interpreted.
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