Auditory cortical onset responses revisited: II. Response strength.
Peter Heil.
Department of Psychology, Monash University, Clayton, Victoria 3168,
Australia.
APStracts 4:0008N, 1997.
ABSTRACT
Most neurons of the auditory pathway discharge spikes locked to the onset of
an acoustic stimulus, but it is largely unknown in which way the acoustic
parameters of sound onsets shape the neuronal responses. This paper analyzes
the number of spikes discharged by single neurons in primary auditory cortex
of barbiturate-anesthetized cats to the onsets of tones of characteristic
frequency. The time course of the peak pressure was altered by parametrically
varying sound pressure level (SPL), rise time, and rise function (linear or
cosine-squared). For both rise functions, rise time had manifold, and in some
cases dramatic, effects on conventional spike count - level functions. In
general, threshold SPL, dynamic range and the lowest SPL at which monotonic
spike count functions saturated increased with prolongation of the rise time.
In neurons with mostly non-monotonic spike count - level functions, "best SPL"
increased and the descending high-SPL arms flattened, so that functions
obtained with long rise times were often monotonic whereas those obtained with
shorter rise times were highly non-monotonic. Consequently, the "tuning" to
SPL was less sharp for longer rise time tones, and spike count versus rise
time functions changed from "short-pass" to "long-pass" with an increase in
SPL. Systematic effects of rise time persisted, when spike counts were plotted
against the rate of change of peak pressure or against the maximum
acceleration of peak pressure. However, when spike counts were plotted as a
function of the instantaneous peak pressure at the time of response
initiation, the functions obtained with different rise times, and even with
different rise functions, were in close register. This suggests that the
stimulus-dependent component of first-spike latency (Heil 1996) can be viewed
as an integration window, during which rate of change of peak pressure is
integrated. The window commences with tone onset and its duration is inversely
related to the maximum acceleration (or, for linear rise functions, the rate
of change) of peak pressure and the neuron's sensitivity. The present findings
seriously question, for onset responses, the usefulness of the spike count -
level function and measures derived from it, such as threshold SPL, dynamic
range, best SPL, or degree of non-monotonicity. They further cast doubt onto
the validity of current concepts of intensity coding at cortical levels, as
most neurons´ onset responses are not indicative of a signal´s steady-state
SPL. However, they suggest a mechanism by which a neuronal population will
sample a given transient in an orderly, sensitivity dependent, temporal
sequence. The sampling rate is automatically adjusted to, and adjusted by, the
rapidity of the signal's change. And the instantaneous properties of the
transient could be represented by the ratios and spatial distribution of
responses across the simultaneously active subpopulation. Such a mechanism
could provide the basis for the demonstrated capability of discrimination of
rapid transients.
Received 5 August 1996; accepted in final form 26 December 1996.
APS Manuscript Number J624-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 21 January 1997