REPRESENTATION OF FREQUENCY IN THE PRIMARY AUDITORY FIELD OF THE BARN OWL FOREBRAIN. Cohen, Yale E. and Eric I. Knudsen. Department of Neurobiology, Stanford University, Stanford, CA 94305- 5401.
APStracts 3:0168N, 1996.
SUMMARY AND CONCLUSIONS
1. The primary auditory field (PAF) constitutes the first telencephalic stage of auditory information processing in the classical auditory pathway. In this study, we investigated the frequency representation in the PAF of the barn owl, a species with a broad frequency range of hearing and a highly advanced auditory system. 2. Single- and multi-unit sites were recorded extracellularly in ketamine-anesthetized owls. The frequency response properties of PAF sites were assessed using digitally synthesized dichotic stimuli. PAF sites (n = 442) were either unresponsive to tonal stimulation (but responsive to noise stimuli), tuned for frequency, or had multi-peaked frequency response profiles. Tuned sites responded best to frequencies between 0.2 - 8.8 kHz, a range that encompasses nearly the entire hearing range of the barn owl. Most sites responding best to frequencies < 4 kHz had relatively broad frequency tuning whereas sites responding best to higher frequencies had either broad or narrow frequency tuning. Sites with multi-peaked frequency response profiles typically had two response peaks: The first peak was usually between 1 - 3 kHz and the second was usually between 5 - 8 kHz; there was no systematic relationship between the two peak frequencies. 3. In dorsoventral electrode penetrations that contained sites with tuned and/or multi-peaked response profiles, a "common frequency" was identified that elicited a maximal response from all of the sites in the penetration. 4. The PAF contains a single tonotopic field. Units tuned to low frequencies are located caudomedially while units tuned to high frequencies are located rostrolaterally. Compared with the frequency representation along the basilar papilla and in other auditory structures, the PAF over-represents low frequencies (<4 kHz) that are important for barn owl vocalizations. Conversely, high frequencies ( 4 kHz), which are necessary for precise sound localization, are under-represented, relative to these more peripheral auditory structures. 5. There was considerable inter-individual variability both in the relative magnification of different frequency ranges and in the orientation of the tonotopic map in the brain. 6. These results suggest that the barn owl PAF, like the mammalian primary auditory cortex, is a general processor of auditory information that is involved in the analysis of both the meaning (such as species-specific vocalizations) and the location of auditory stimuli. In addition, the high- degree of inter-individual variability in the representation of frequency information suggests that the barn owl PAF, like the mammalian auditory cortex, is subject to modification by sensory experience.

Received 22 April 1996; accepted in final form 5 August 1996.
APS Manuscript Number J335-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 29 August 1996