FOREBRAIN PATHWAY FOR AUDITORY SPACE PROCESSING IN THE BARN OWL

The forebrain plays an important role in many aspects of sound localiz ation behavior. Yet, the forebrain pathway that processes auditory spa tial information is not known for any species. Using standard anatomic labeling techniques, we used a ''top-down'' approach to trace the flo w of auditory spatial information from an output area of the forebrain sound localization pathway (the auditory archistriatum, AAr), back th rough the fore brain, and into the auditory midbrain. Previous work ha s demonstrated that AAr units are specialized for auditory space proce ssing. The results presented here show that the AAr receives afferent input from Field L both directly and indirectly via the caudolateral n eostriatum. Afferent input to Field L originates mainly in the auditor y thalamus, nucleus ovoidalis, which, in turn, receives input from the central nucleus of the inferior colliculus. In addition, we confirmed previously reported projections of the AAr to the basal ganglia, the external nucleus of the inferior colliculus (ICX), the deep layers of the optic tectum, and various brain stem nuclei. A series of inactivat ion experiments demonstrated that the sharp tuning of AAr sites for bi naural spatial cues depends on field L input but not on input from the auditory space map in the midbrain ICX: pharmacological inactivation of Field L eliminated completely auditory responses in the AAr, wherea s bilateral ablation of the midbrain ICX had no appreciable effect on AAr responses. We conclude, therefore, that the forebrain sound locali zation pathway can process auditory spatial information independently of the midbrain localization pathway.