The Journal of Neuroscience, September 15, 2001, 21(18):7404-7415
Temporal Integration of Sound Pressure Determines Thresholds of
Auditory-Nerve Fibers
Peter
Heil and
Heinrich
Neubauer
Leibniz Institute for Neurobiology, D-39118 Magdeburg, Germany
Current propositions of the quantity of sound driving the central
auditory system, specifically around threshold, are diverse and at
variance with one another. They include sound pressure, sound power, or
intensity, which are proportional to the square of pressure, and
energy, i.e., the integral of sound power over time. Here we show that
the relevant sound quantity and the nature of the threshold can be
obtained from the timing of the first spike of auditory-nerve (AN)
fibers after the onset of a stimulus. We reason that the first spike is
triggered when the stimulus reaches threshold and occurs with fixed
delay thereafter. By probing cat AN fibers with characteristic
frequency tones of different sound pressure levels and rise times, we
show that the differences in relative timing of the first spike
(including latencies >100 msec of fibers with low spontaneous rates)
can be well accounted for by essentially linear integration of pressure
over time. The inclusion of a constant pressure loss or gain to the
integrator improves the fit of the model and also accounts for most of
the variation of spontaneous rates across fibers. In addition, there are tight correlations among delay, threshold, and spontaneous rate.
First-spike timing cannot be explained by models based on a fixed
pressure threshold, a fixed power or intensity threshold, or an energy
threshold. This suggests that AN fiber thresholds are best measured in
units of pressure by time. Possible mechanisms of pressure integration
by the inner hair cell-AN fiber complex are discussed.
Key words:
hearing; sound; mammal; latency; spike timing; pressure; intensity; energy; modeling; inner hair cell; auditory nerve
Copyright © 2001 Society for Neuroscience 0270-6474/01/21187404-12$05.00/0
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