A site of auditory experience-dependent plasticity in the neural representation of auditory space in the barn owl's inferior colliculus

The barn owl's optic tectum contains a map of auditory space that is based, in part, on a map of interaural time difference (ITD). Previous studies ha ve shown that this ITD map is shaped by auditory experience. In this study, we investigated whether the plasticity responsible for experience-induced changes in ITD tuning in the tectum occurs within the tectum itself or at a n earlier stage in the auditory pathway. We altered auditory experience in young owls by implanting an acoustic filt ering device in one ear that caused frequency-dependent changes in sound ti ming and level. We analyzed the representation of ITD in normal and device- reared owls in two nuclei in the ascending pathway: the external nucleus of the inferior colliculus (ICX), the primary source of ascending auditory in put to the tectum, and the lateral shell of the central nucleus of the infe rior colliculus (ICCls), the primary source of input to the ICX. In the ICX , device rearing caused adaptive, frequency-dependent changes in ITD tuning , as well as changes in frequency tuning. These changes in tuning were simi lar to changes that occurred in the optic tectum in the same owls. In contr ast, in the ICCls, tuning for ITD and frequency was unaffected by device re aring. The data indicate that plasticity at the level of the ICX is largely responsible for the adaptive adjustments in ITD tuning and frequency tunin g that are observed in the optic tecta of owls raised with abnormal auditor y experience.