FIRST-SPIKE TIMING OF AUDITORY-NERVE FIBERS AND COMPARISON WITH AUDITORY-CORTEX

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.