P. Heil et Drf. Irvine, FIRST-SPIKE TIMING OF AUDITORY-NERVE FIBERS AND COMPARISON WITH AUDITORY-CORTEX, Journal of neurophysiology, 78(5), 1997, pp. 2438-2454
The timing of the first spike of cat auditory-nerve (AN) fibers in res
ponse to onsets of characteristic frequency (CF) tone bursts was studi
ed and compared with that of neurons in primary auditory cortex (AI),
reported previously. Tones were shaped with cosine-squared rise functi
ons, and rise time and sound pressure level were parametrically varied
. Although measurement of first-spike latency of AN fibers was somewha
t compromised by effects of spontaneous activity, latency was an invar
iant and inverse function of the maximum acceleration of peak pressure
(i.e., a feature of the 2nd derivative of the stimulus envelope), as
previously found in AI, rather than of tone level or rise time. Latenc
y-acceleration functions of all AN fibers were of very similar shape,
similar to that observed in AI. As in AI, latency-acceleration functio
ns of different fibers were displaced along the latency axis, reflecti
ng differences in minimum latency, and along the acceleration axis, re
flecting differences in sensitivity to acceleration [neuronal transien
t 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 w
ould be less tightly correlated with response properties covarying wit
h SR, such as firing threshold. S estimates varied with CF in a fashio
n similar to the cat's audiogram and, for low-and medium-SR fibers, ma
tched those for Al 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 pr
essure. The characteristics of the increase of SD with latency in a gi
ven AN fiber/AI neuron and across AN fibers/AI neurons revealed that t
he precision of first-spike timing to some stimuli can actually be hig
her in AI than in AN. The data suggest that the basic characteristics
of the latency-acceleration functions of transient onset responses see
n in cortex are generated at inner hair cell-AN fiber synapses. Implic
ations for signal processing in the auditory system and for first-spik
e generation and adaptation in AN are discussed.