Auditory cortical onset responses revisited: I. First-spike timing.
Peter Heil.
Department of Psychology, Monash University, Clayton, Victoria 3168,
Australia.
APStracts 4:0007N, 1997.
ABSTRACT
Sound onsets are salient and behaviorally relevant, and most auditory neurons
discharge spikes locked to such transients. The acoustic parameters of sound
onsets which shape such onset responses are unknown. This paper analyzes the
timing of spikes of single neurons in the primary auditory cortex of
barbiturate-anesthetized cats to the onsets of tone bursts. By parametric
variation of sound pressure level, rise time, and rise function (linear or
cosine-squared), the time courses of peak pressure, rate of change of peak
pressure, and acceleration of peak pressure during the tones' onsets were
systematically varied. For cosine-squared rise function tones of a given
frequency and laterality any neuron's mean first-spike latency was an
invariant and inverse function of the maximum acceleration of peak pressure
occuring at tone onset. For linear rise function tones, latency was an
invariant and inverse function of the rate of change of peak pressure. Thus,
latency is independent of rise time or sound pressure level per se. Latency -
acceleration functions, obtained with cosine-squared rise function tones under
different stimulus conditions (frequency, laterality) from any given neuron
and across the neuronal pool, were of strikingly similar shape. The same was
true for latency - rate of change of peak pressure functions obtained with
linear rise function tones. Latency - acceleration/rate of change of peak
pressure functions could differ in their extent and in their position within
the coordinate system. The positional differences reflect neuronal differences
in minimum latency Lmin and in a sensitivity S to acceleration and rate of
change of peak pressure, a hitherto unrecognized neuronal property which is
distinctly different from firing threshold. Estimates of Lmin and S, which
were derived by fitting a simple function to the neuronal latency -
acceleration/rate of change of peak pressure functions, were independent of
rise function. On average, Lmin decreased with increasing characteristic
frequency (CF), but varied widely for neurons with the same CF. S varied with
CF in a fashion similar to the cat's audiogram and, for a given neuron, varied
with frequency. Standard deviation of first-spike latency was roughly
proportional to the slope of the functions relating latency to
acceleration/rate of change of peak pressure. Hence, standard deviation
increased exponentially, rather than linearly, with mean latency, and did so
at about twice the rate for linear than for cosine-squared rise function
tones. The proportionality coefficients were quite similar across the neuronal
pool and similar for both rise functions. Minimum standard deviation increased
non-linearly with increasing Lmin. These findings suggest a peripheral origin
of the sensitivity S and a peripheral establishment of latency -
acceleration/rate of change of peak pressure functions. Because of the
striking similarity in the shapes of such functions across the neuronal pool,
sound onsets will produce orderly and predictable spatio-temporal patterns of
first-spike timing, which could be used to instantaneously track rapid
transients and to represent transient features by partly scale-invariant
temporal codes.
Received 5 August 1996; accepted in final form 26 December 1996.
APS Manuscript Number J623-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 21 January 1997