Frequency tuning and spontaneous activity in the auditory nerve and
cochlear nucleus magnocellularis of the barn owl Tyto alba.
Institut fr Zoologie der Technischen Universitt MnchenLichtenbergstr. 4,
85747 Garching, Germany.
APStracts 3:0200N, 1996.
Single-unit recordings were obtained from the brainstem of the barn owl at the
level of entrance of the auditory nerve. Auditory-nerve and nucleus-
magnocellularis units were distinguished by physiological criteria, using the
response latency to clicks, the spontaneous discharge rate and the pattern of
characteristic frequencies encountered along an electrode track. The response
latency to click stimulation decreased in a logarithmic fashion with
increasing characteristic frequency for both auditory-nerve and nucleus
magnocellularis units. The average difference between these populations was
0.4 to 0.55 ms. The most sensitive thresholds were around 0 dB SPL and varied
little between 0.5 and 9 kHz. Frequency threshold curves showed the simple V-
shape that is typical for birds, with no indication of a low-frequency tail.
Frequency selectivity increased in a gradual, power-law fashion with
increasing characteristic frequency. There was no reflection of the unusual
and greatly-expanded mapping of higher frequencies on the basilar papilla of
the owl. This observation is contrary to the equal-distance hypothesis that
relates frequency selectivity to the spatial respresentation in the cochlea
(Greenwood 1991). On the basis of spontaneous rates and / or sensitivity there
was no evidence for distinct subpopulations of auditory-nerve fibers, such as
the well-known type I afferent response classes in mammals. On the whole, barn
owl auditory-nerve physiology conformed entirely to the typical patterns seen
in other bird species. The only exception was a remarkably small spread of
thresholds at any one frequency, this being only 10-15 dB in individual owls.
Average spontaneous rate was 72.2 spikes/s in the auditory nerve and 219.4
spikes/s for nucleus magnocellularis. This large difference, together with the
known properties of endbuld-of-Held synapses, suggests a convergence of about
2-4 auditory-nerve fibers onto one nucleus magnocellularis neuron. Some
auditory-nerve fibers as well as nucleus magnocellularis units showed a quasi-
periodic spontaneous discharge with preferred intervals in the time-interval
histogram. This phenomenon was observed at frequencies as high as 4.7 kHz.
Received 15 July 1996; accepted in final form 12 September 1996.
APS Manuscript Number J552-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996