Auditory brainstem of the ferret: bilateral cochlear lesions in infancy do not affect the number of neurons projecting from the cochlear nucleus to the inferior colliculus.

Unilateral cochlear removal in infancy produces a number of changes in the structure and physiology of the central auditory system. To test the role that interaural competition plays in this plasticity, ferrets were reared with bilateral cochlear removals from the fourth postnatal week; before the time of the normal onset of auditory function. The totality of the removals was confirmed using brainstem response audiometry and cochlear histology. Following 3-month survival, the animals were injected with wheatgerm horseradish peroxidase in the left inferior colliculus. Retrogradely labeled neurons in the cochlear nucleus on each side were counted. Neither the absolute number nor the bilateral symmetry of the labeled neurons differed significantly from normal adult ferrets. Thus, bilateral cochlear removal does not produce the same change in brainstem connections as unilateral removal or unilateral conductive hearing loss. The results are consistent with the competitive hypothesis.