The Journal of Neuroscience, November 1, 1999, 19(21):9674-9686
Coding of Sound Pressure Level in the Barn Owl's Auditory
Nerve
Christine
Köppl1 and
Graeme
Yates2
1 Institut für Zoologie, Technische
Universität München, 85747 Garching, Germany, and
2 The Auditory Laboratory, Department of Physiology,
University of Western Australia, Nedlands 6907, Western Australia
Rate-intensity functions, i.e., the relation between discharge
rate and sound pressure level, were recorded from single auditory nerve
fibers in the barn owl. Differences in sound pressure level between the
owl's two ears are known to be an important cue in sound localization.
One objective was therefore to quantify the discharge rates of auditory
nerve fibers, as a basis for higher-order processing of sound pressure
level. The second aim was to investigate the rate-intensity functions
for cues to the underlying cochlear mechanisms, using a model developed
in mammals.
Rate-intensity functions at the most sensitive frequency mostly showed
a well-defined breakpoint between an initial steep segment and a
progressively flattening segment. This shape has, in mammals, been
convincingly traced to a compressive nonlinearity in the cochlear
mechanics, which in turn is a reflection of the cochlear amplifier
enhancing low-level stimuli. The similarity of the rate-intensity
functions of the barn owl is thus further evidence for a similar
mechanism in birds. An interesting difference from mammalian data was
that this compressive nonlinearity was not shared among fibers of
similar characteristic frequency, suggesting a different mechanism with
a more locally differentiated operation than in mammals.
In all fibers, the steepest change in discharge rate with rising sound
pressure level occurred within 10-20 dB of their respective thresholds. Because the range of neural thresholds at any one characteristic frequency is small in the owl, auditory nerve fibers were collectively most sensitive for changes in sound pressure level
within ~30 dB of the best thresholds. Fibers most sensitive to high
frequencies (>6-7 kHz) showed a smaller increase of rate above
spontaneous discharge rate than did lower-frequency fibers.
Key words:
hearing; cochlea; basilar papilla; interaural intensity
difference; rate-intensity function; bird
Copyright © 1999 Society for Neuroscience 0270-6474/99/19219674-13$05.00/0
