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Abstract

The sizes of large spherical and fusiform cells of the anteroventral cochlear nucleus were studied in rats reared after unilateral middle ear ossicle removal on postnatal days 10 or 16, or after bilateral removal on postnatal day 16. In animals deprived monaurally from 10 days after birth, large spherical cell areas of the deprived side were reduced by 21% compared to the nondeprived side; the deprived cells were 17% smaller than cells of control animals. Large spherical cells contralateral to the deprived ear were slightly larger than those of controls. Large spherical cells monaurally deprived from the 16th day showed effects similar to those deprived from the onset of hearing, although cell reduction on the deprived side was only 10% relative to controls. In contrast to the consequences of monaural sound deprivation, binaural deprivation from the 16th day resulted in no reduction in size of large spherical cells. Fusiform cells of animals in monaurally and binaurally deprived groups were not significantly smaller than the fusiform cells of controls. Ear histologies confirmed that each cochlea and spiral ganglion was normal in experimental and control animals. Because large spherical cells receive unilateral input directly from the acoustic nerve and are known to project bilaterally to the medial superior olive, the bilateral differences observed in large spherical cell area following rearing under monaural deprivation conditions may reflect a form of imbalanced binaural interactions between axon terminals of spherical cells.