Frequency tuning and spontaneous activity in the auditory nerve and cochlear nucleus magnocellularis of the barn owl Tyto alba. Christine Kbyppl. Institut fr Zoologie der Technischen Universitt MnchenLichtenbergstr. 4, 85747 Garching, Germany.
APStracts 3:0200N, 1996.
ABSTRACT
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