This paper investigates the role of the central nucleus of the barn owl's inferior colliculus in determination of the sound-source azimuth. The central nucleus contains many neurons that are sensitive to interaural time difference (ITD), the cue for azimuth in the barn owl. The response of these neurons varies in a cyclic manner with the ITD of a tone or noise burst. Response maxima recur at integer multiples of the period of the stimulating tone, or, if the stimulus is noise, at integer multiples of the period corresponding to the neuron's best frequency. Such neurons can signal, by means of their relative spike rate, the phase difference between the sounds reaching the left and right ears. Since an interaural phase difference corresponds to more than one ITD, these neurons represent ITD ambiguously. We call this phenomenon phase ambiguity. The central nucleus is tonotopically organized and its neurons are narrowly tuned to frequency. Neurons in an array perpendicular to isofrequency laminae form a physiological and anatomical unit; only one ITD, the array-specific ITD, activates all neurons in an array at the same relative level. We, therefore, may say that, in the central nucleus, an ITD is conserved in an array of neurons. Array-specific ITDs are mapped and encompass the entire auditory space of the barn owl. Individual space-specific neurons of the external nucleus, which receive inputs from a wide range of frequency channels (Knudsen and Konishi, 1978), are selective for a unique ITD. Space-specific neurons do not show phase ambiguity when stimulated with noise (Takahashi and Konishi, 1986). Space-specific neurons receive inputs from arrays that are selective for the same ITD. The collective response of the neurons in an array may be the basis for the absence of phase ambiguity in space-specific neurons.

Space-specific neurons, found in the barn owl's inferior colliculus, respond selectively to a narrow range of interaural time and intensity differences. We show that injecting a local anesthetic into one cochlear nucleus, nucleus magnocellularis, alters the space-specific cell's selectivity for interaural time difference, leaving its selectivity for interaural intensity difference intact. Anesthetizing the other cochlear nucleus, nucleus angularis, has the converse effects. We show also that the spacespecific neuron's selectivity for one interaural cue is the same for all effective values of the other cue. We conclude that time and intensity cues are processed in separate neural channels of the barn owl's auditory system and that the two cues operate independently.The barn owl uses differences in the timing and intensity of a sound at its ears to determine, respectively, the azimuth and elevation of the source (Knudsen and Konishi, 1979). Space-specific neurons (formerly termed "limited-field" or "space-mapped" cells) found in its inferior colliculus are highly selective for these sound localization cues. They are excited only by sounds emanating from a circumscribed region of space or by dichotic stimuli having a particular combination of interaural intensity difference (IID) and interaural time difference (ITD) (Moiseff and Konishi, 1981).Recent evidence suggests that the auditory pathways leading to the barn owl's inferior colliculus process time and intensity cues independently. The cochlear nuclei, nucleus angularis and nucleus magnocellularis, are specialized to encode either the intensity or the phase of a sound (Sullivan and Konishi, 1984), and the binaural nuclei innervated by them are sensitive to either IID or ITD (Moiseff and Konishi, 1983). We proved that time and intensity are independently processed by monitoring the activity of single space-specific neurons while reducing neural activity in a cochlear nucleus with a local anesthetic. We also demonstrated that under normal, 1 We thank Drs.

The barn owl uses the interaural difference in the timing of sounds to determine the azimuth of the source. When the sound has a wide frequency band, localization is precise. When localizing tones, however, the barn owl errs in a manner that suggests that it is confused by phantom targets. We report a neural equivalent of these phenomena as they are observed in the space-specific neuron of the owl's inferior colliculus. When stimulated with a tone, the space-specific neuron discharges maximally at interaural time differences (ITDs) that differ by one period of the stimulus tone. Changing the stimulus frequency changes the period of the ITD-response functions, but 1 ITD evokes, in most neurons, a maximal response, regardless of frequency. This ITD is the characteristic delay (CD). When stimulated with noise, there is a maximal response only to ITDs at or near the CD. Thus, the space-specific neuron can unambiguously signal the CD, provided that the signal contains more than 1 frequency. The preferential response to a single ITD, which is observed with noise stimuli, was also observed when the summed waveform of the best frequency and another tone, F2, was presented. The response of the space-specific neuron to these 2-tone stimuli could not be accounted for by the summing or averaging of the ITD-response functions obtained with the best frequency or F2 alone, suggesting that nonlinear neural processes are involved.

The barn owl determines the directions from which sounds emanate by computing the interaural differences in the timing and intensity of sounds. These cues for sound localization are processed in independent channels originating at nucleus magnocellularis (NM) and nucleus angularis (NA), the cochlear nuclei. The cells of NM are specialized for encoding the phase of sounds in the ipsilateral ear. The cells of NA are specialized for encoding the intensity of sounds in the ipsilateral ear. NM projects solely, bilaterally, and tonotopically to nucleus laminaris (NL). NL and NA project to largely nonoverlapping zones in the central nucleus of the inferior colliculus (ICc), thus forming hodological subdivisions in which time and intensity information may be processed. The terminal field of NL occupies a discrete zone in the rostromedial portion of the contralateral ICc, which we have termed the "core" of ICc. The terminal field of NA surrounds the core of ICc and thus forms a "shell" around it. The projection from NL to the core conserves tonotopy. Low-frequency regions of NL project to the dorsal portions of the core whereas higher-frequency regions project to more ventral portions. This innervation pattern is consistent with earlier physiological studies of tonotopy. Physiological studies have also suggested that NL and the core of ICs contain a representation of the location of a sound source along the horizontal axis. Our data suggest that the projection from NL to the core preserves spatiotopy. Thus, the dorsal portion of NL on the left, which contains a representation of eccentric loci in the right hemifield, innervates the area of the right ICc core that represents eccentric right loci. The more ventral portion of the left NL, which represents loci close to the vertical meridian, innervates the more rostral portions of the right core, which also represents loci near the vertical meridian.