Functional consequences of neonatal unilateral cochlear removal

The physiological consequences of unilateral cochlear removal in infancy were assessed by recording the responses of neurones in the ferret inferior colliculus and superior colliculus to acoustic stimulation of the intact ear. Animals were lesioned between postnatal days P5 and P40 and survived for at least a year prior to recording. In the inferior colliculus ipsilateral to the intact ear, neurones had lower thresholds and wider dynamic ranges following earlier (P5) than following later (P40, adult) cochlear removal. In the superior colliculus contralateral to the intact ear, neurones had broader spatial tuning in response to high-level, free-field stimulation following cochlear removal at P25, than had neurones in normal, unlesioned adults. The neural map of auditory space was also disrupted in the lesioned animals. However, at low stimulus levels the auditory space map was unaffected by the cochlear removal. These results show a developmental sensitive period for the effects of unilateral cochlear removal on the responses of ferret inferior colliculus neurones, and a level-dependent effect of cochlear removal on the responses of superior colliculus neurones.