Recent evidence shows that there is a reciprocal projection between the cochlear nuclei (CN) and the dorsal column nuclei in cat and rat (DCoN) (Itoh et al., 1987, Weinberg and Rustioni, 1987). The targets of somatosensory (DCoN) axons in the CN are in granule cell areas and in the fusiform cell layer of the dorsal division of the CN (DCN). This project is an investigation of somatosensory-auditory interactions in the CN. The principal hypothesis to be investigated is that the somatosensory input to CN is associated with the pinnae and provides information about pinna position to DCN neurons. This information could be useful in correcting for pinna orientation during sound localization. The project has three broad aims. First, the targets of somatosensory axons in the CN will be identified in a survey using single unit recording. Electrical stimulation of the dorsal columns will be used to activate the DCON projection fibers. The goal is to identify which unit types in CN, defined in terms of their auditory response properties, receive somatosensory inputs and what the nature of the effects are (excitation, inhibition, facilitation). We will investigate the possibility that there are cells in superficial DCN which receive somatosensory inputs through the granule cell axons which run there, but only weak auditory input. If such cells are found, we will test, using cross-correlation analysis, whether they are inhibitory inteneurons, as is expected from morphological descriptions of the superficial layers of the DCN (Mugnaini, 1985; Wouterlood and Mugnaini, 1984). Second, the somatosensory receptive fields from which CN units are affected will be determined. Somatosensory fields will be classified in terms of their receptive fields (areas of activation on the skin) and in terms of somatosensory submodalities (hair, cutaneous touch, deep, or joint receptors). Third, the properties of the somatosensory input to DCN principal cells will be compared to known properties of granule cell activation of the same cells (Manis, 1989). One goal is to test whether the granule cell system is the path by which somatosensory inputs reach DCN cells.
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