ADAPTIVE PLASTICITY OF THE AUDITORY SPACE MAP IN THE OPTIC TECTUM OF ADULT AND BABY BARN OWLS IN RESPONSE TO EXTERNAL EAR MODIFICATION

1. This study demonstrates the influence of experience on the establis hment and maintenance of the auditory map of space in the optic tectum of the barn owl. Auditory experience was altered either by preventing the structures of the external ears (the facial ruff and preaural fla ps) from appearing in baby barn owls (baby ruff-cut owls) or by removi ng these structures in adults (adult ruff-cut owls). These structures shape the binaural cues used for localizing sounds in both the horizon tal and vertical dimensions. 2. The acoustic effects of removing the e xternal ear structures were measured using probe tube microphones plac ed in the ear canals. In both baby and adult ruff-cut owls, the spatia l pattern of binaural localization cues was dramatically different fro m normal: interaural level difference (ILD) changed with azimuth inste ad of with elevation, the rate of change of ILD across space was decre ased relative to normal, and the rate of change of interaural time dif ference (ITD) across frontal space was increased relative to normal. 3 . The neurophysiological representations of ITD and ILD in the optic t ectum were measured before and greater than or equal to 3 mo after ruf f removal in adults and beginning at 4.5 months of age in baby ruffcut owls. Multiunit tuning to ITD and to ILD was measured using dichotic stimulation in ketamine-anesthetized owls. The tectal maps of ITD and ILD were reconstructed using visual receptive held location as a marke r for recording site location in the optic tectum. 4. Adjustment of th e tectal map of ITD to the altered spatial pattern of acoustic ITD was essentially complete in adults as well as in baby ruff-cut owls. This adjustment changed the magnification of ITD across the tectum, with r esultant changes in ITD tuning at individual tectal sites of up to sim ilar to 25 mu s (similar to 5% of the physiological range) relative to normal values. 5. Adaptation of the tectal ILD map to the ruff-cut sp atial pattern of acoustic ILD was substantial but clearly incomplete i n both adult and baby ruff-cut owls. Although changes of up to similar to 15 dB (similar to 47% of the physiological range) relative to norm al tuning were observed at certain rectal sites, the topography of the ILD map was always intermediate between normal and that predicted by the ruff-cut spatial pattern of acoustic ILD. 6. In the adult ruff-cut owls, the persisting influence of normal ILD topography on the map co uld be explained by the earlier experience of these animals with the n ormal acoustic pattern. In the baby ruff-cut owls, which never experie nced the normal acoustic ILD pattern, this influence must result from an innate predisposition for normal topography, genetically encoded si nce the recent evolution of the barn owl's asymmetric ears. 7. The pla sticity observed in the adult animals demonstrates that the auditory s ystem retains the capacity to make major modifications in its represen tation of sound-localization cues throughout life.