First-spike timing of auditory-nerve fibers and comparison with auditory
cortex.
Peter Heil and Dexter R.F. Irvine.
Department of Psychology, Monash University, Clayton, Victoria 3168,
Australia.
APStracts 4:128N, 1997.
ABSTRACT
The timing of the first spike of cat auditory-nerve (AN) fibers in response to
onsets of characteristic frequency (CF) tone bursts was studied and compared
with that of neurons in primary auditory cortex (AI), reported previously.
Tones were shaped with cosine-squared rise functions, and rise time and sound
pressure level were parametrically varied. Although measurement of first-spike
latency of AN fibers was somewhat compromised by effects of spontaneous
activity, latency was an invariant and inverse function of the maximum
acceleration of peak pressure (i.e. a feature of the second derivative of the
stimulus envelope), as previously found in AI, rather than of tone level or
rise time. Latency - acceleration functions of all AN fibers were of very
similar shape, similar to that observed in AI. As in AI, latency -
acceleration functions of different fibers were displaced along the latency
axis, reflecting differences in minimum latency, and along the acceleration
axis, reflecting differences in sensitivity to acceleration (transient
sensitivity S). S estimates increased with spontaneous rate (SR), but values
of high-SR fibers exceeded those in AI. This suggests that S estimates are
biased by SR per se, and that unbiased true S values would be less tightly
correlated with response properties covarying with SR, such as firing
threshold. S estimates varied with CF in a fashion similar to the cat's
audiogram and, for low- and medium-SR fibers, matched those for AI neurons.
Minimum latency decreased with increasing SR and CF. As in AI, the standard
deviation of first-spike timing (SD) in AN was also an inverse function of
maximum acceleration of peak pressure. The characteristics of the increase of
SD with latency in a given AN fiber/AI neuron and across AN fibers/AI neurons
revealed that the precision of first-spike timing to some stimuli can actually
be higher in AI than in AN. The data suggest that the basic characteristics of
the latency - acceleration functions of transient onset responses seen in
cortex are generated at inner hair cell - AN fiber synapses. Implications for
signal processing in the auditory system and for first-spike generation and
adaptation in AN are discussed.
Received 24 December 1996; accepted in final form 1 July 1997.
APS Manuscript Number J1002-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 24 July 1997