REPRESENTATION OF FREQUENCY IN THE PRIMARY AUDITORY FIELD OF THE BARN OWL
Cohen, Yale E. and Eric I. Knudsen.
Department of Neurobiology, Stanford University, Stanford, CA 94305-
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 (³ 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