RESPONSE PROPERTIES OF NEURONS IN THE INFERIOR COLLICULUS OF THE MONAURALLY DEAFENED FERRET TO ACOUSTIC STIMULATION OF THE INTACT EAR

Response properties of neurons in the central nucleus of the inferior colliculus (ICC) were investigated after unilateral cochlear removal a t various ages during infancy. Nineteen ferrets had the right cochlea surgically ablated, either in adulthood or on postnatal day (P) 5, 25, or 40, 3-18 mo before recording. Adult ablations were made on the sam e day as (''acute,'' n = 3), or 2-3 mo before (''chronic,'' n = 3), re cording. Two ferrets were left binaurally intact. Single-unit (n = 702 ) and multiunit (n = 1,819) recordings were made in the ICC of barbitu rate-anesthetized ferrets ipsilateral (all ages) or contralateral (P5 and acute adult only) to the intact ear. In binaurally intact animals, tonal stimulation of the contralateral ear evoked excitatory activity at the majority (94%) of recording loci, whereas stimulation of the i psilateral ear evoked activity at only 33% of recording loci. In acute ly ablated animals, the majority of contralateral (90%) and ipsilatera l (70%) loci were excited by tonal stimulation of the intact ear. In c hronically ablated animals, 80-90% of loci were excited by ipsilateral stimulation. Single-unit thresholds were generally higher for low-bes t frequency (BF) than for high-BF units, and higher in the ipsilateral than in the contralateral ICC. Analysis of covariance showed highly s ignificant differences between all of the ipsilateral and contralatera l groups, but no effects of age at ablation or survival time following ablation, other than that the group ablated at P25 had higher mean ip silateral thresholds than the groups ablated at P5 or, acutely, in adu lthood. Cochlear ablation at P5, 25, or 40 resulted in a significant i ncrease in dynamic ranges of ipsilateral ICC unit rate-intensity funct ions relative to acutely ablated animals. Dynamic ranges of units in t he contralateral ICC of P5-ablated ferrets were also significantly inc reased compared with those of acutely ablated animals. Cochlear ablati on at P5, 25, or 40 resulted in a significant increase in single-unit spontaneous discharge rates in the ICC ipsilateral but not contralater al (P5 only) to the intact ear. These data show that unilateral cochle ar removal in adult ferrets leads to a rapid and dramatic increase in the proportion of neurons in the ICC ipsilateral to the intact ear tha t is excited by acoustic stimulation of that ear. In addition, the dat a confirm that, in ferrets, cochlear removal in infancy leads to a fur ther increase in responsiveness of individual neurons in the ipsilater al ICC. Finally, the data show that responses in the ICC contralateral to the intact ear are largely but not completely unchanged by unilate ral cochlear removal.