Effect of Chronic Electrical Stimulation on Cochlear Nucleus Neuron Size in Normal Hearing Kittens

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Abstract
Very young cochlear-implant candidates may have undetected islands of residual hearing. Would the maturation of these functioning auditory neurons be affected by chronic cochlear stimulation? This was tested by examining neuron sizes in the cochlear nuclei of young, normal hearing kittens with and without chronic cochlear stimulation. Six animals received bilateral intra- or extracochlear implants and were electrically stimulated unilaterally for periods of 1,000-1,500 hours. After sacrifice, cross-sectional areas of approximately 11,000 neuron somata in the cochlear nuclei were measured with an image-analysis system. There were statistically significant differences between stimulated and unstimulated nuclei, especially the posteroventral cochlear nucleus (PVCN), in individual cats, but the directions of the differences were inconsistent. Overall, there was no significant effect of electrical stimulation on soma size. These results indicate that chronic electrical stimulation of the auditory nerve has no positive or negative trophic effects on otherwise innervated, maturing cochlear nucleus neurons.