REPRESENTATION OF SOUND LOCALIZATION CUES IN THE AUDITORY THALAMUS OFTHE BARN OWL

Barn owls can localize a sound source using either the map of auditory space contained in the optic tectum or the auditory forebrain. The au ditory thalamus, nucleus ovoidalis (N.Ov), is situated between these t wo auditory areas, and its inactivation precludes the use of the audit ory forebrain for sound localization, We examined the sources of input s to the N.Ov as well as their patterns of termination within the nucl eus, We also examined the response of single neurons within the N.Ov t o tonal stimuli and sound localization cues. Afferents to the N.Ov ori ginated with a diffuse population of neurons located bilaterally withi n the lateral shell, core, and medial shell subdivisions of the centra l nucleus of the inferior colliculus, Additional afferent input origin ated from the ipsilateral ventral nucleus of the lateral lemniscus. No afferent input was provided to the N.Ov from the external nucleus of the inferior colliculus or the optic tectum, The N.Ov was tonotopicall y organized with high frequencies represented dorsally and low frequen cies ventrally, Although neurons in the N.Ov responded to localization cues, there was no apparent topographic mapping of these cues within the nucleus, in contrast to the tectal pathway. However, nearly all po ssible types of binaural response to sound localization cues were repr esented, These findings suggest that in the thalamotelencephalic audit ory pathway, sound localization is subserved by a nontopographic repre sentation of auditory space.