ALTERED SPECTRAL LOCALIZATION CUES DISRUPT THE DEVELOPMENT OF THE AUDITORY SPACE MAP IN THE SUPERIOR COLLICULUS OF THE FERRET

Spectral localization cues provided by the outer ear are utilized in t he construction of the auditory space map in the superior colliculus ( SC). The role of the outer ear in the development of this map was exam ined by recording from the SC of anesthetized, adult ferrets in which the pinna and concha had been removed in infancy. The acoustical conse quences of this procedure were assessed by recording outer ear impulse responses via a probe-tube microphone implanted in the wall of the ea r canal. Both monaural and binaural spectral cues normally show a numb er of asymmetric features within the horizontal plane, which allow azi muthal locations on either side of the interaural axis to be discrimin ated. These features were eliminated or altered by chronic pinnectomy. The responses of auditory units in the SC to noise bursts presented i n the free field were examined at sound levels of similar to 10 and 25 dB above unit threshold. After bilateral pinnectomy, the representati on of auditory space was severely degraded at both sound levels. In co ntrast to normal ferrets, many units had bilobed azimuthal response pr ofiles, indicating that they were unable to resolve sound locations on either side of the interaural axis. There was also much less order in the distribution of best azimuths or elevations of those units that w ere tuned to a single direction. Some units were tuned to locations ha t extended much further into the hemifield ipsilateral to the recordin g side than the normal range of best azimuths. Unilateral removal of t he outer ear, which disrupts the monaural spectral cues for one side o nly, had a much smaller effect on the development of the auditory repr esentation. At supra-and near-threshold sound levels, the: representat ion of sound azimuth in the SC on both sides of the brain was less sca ttered than that found after bilateral pinna removal. Nevertheless, un its with bilobed responses, broader tuning: and inappropriate best azi muths were observed in both the left and right SC of ferrets in which the left pinna and concha had been removed in infancy. These data illu strate that the localization cues provided by the outer ear play a cri tical role in the development of the auditory space map in the SC. In contrast to other manipulations of either auditory or visual inputs, t he map does not appear to adapt to the changes in spectral cues brough t about by pinna removal, suggesting that residual binaural cues are, by themselves, insufficient for its normal maturation.