Spatial response profiles of posteroventral cochlear nucleus neurons and auditory-nerve fibers in unanesthetized decerebrate cats: response to pure tones

Encoding of 1- and 5-kHz pure tones by auditory-nerve (AN) fibers and choppers of the posteroventral cochlear nucleus (PVCN) was investigated. Neuronal responses were analyzed as the discharge rate, rate change, and the mean and standard deviation (or sigma) of spike counts. The major findings are: (1) Sideband inhibitory areas were observed in spatial profiles of rate changes of PVCN choppers whereas they were absent in those of AN fibers; (2) spatial profiles of rate changes and mean discharge rates of PVCN choppers were sharper than those of high spontaneous-rate (HSR) AN fibers and were comparable to those of low and medium SR (LMSR) AN fibers for 1 kHz at 50 and 70 dB SPL re: 20 microPa; (3) to 5 kHz, 30 dB SPL, PVCN choppers were strongly driven comparable to HSR AN fibers whereas LMSR AN fibers were weakly driven (implying higher thresholds); (4) PVCN choppers exhibited higher maximum discharge rates (300-600 spikes/s) than either LMSR AN fibers (200-250 spikes/s) or HSR AN fibers (150-250 spikes/s); (5) mean-to-sigma ratios of PVCN choppers, particularly at 70 dB SPL, were much higher than those of LMSR or HSR AN fibers; (6) rate-change profiles of LMSR AN fibers were distinct from those of HSR AN fibers, more conspicuously for 1 kHz than for 5 kHz; (7) the neural response profiles to 5 kHz were sharper than those to 1 kHz; and (8) 45% of PVCN choppers in the present study exhibited SR greater than 20 spikes/s whereas only 11%-12% of AVCN choppers in previous studies of anesthetized cats exhibited the same SR, which may represent an effect of anesthesia. The observations support a hypothesis that the transformation of the discharge-rate signal from AN fibers to PVCN choppers leads to an amplification of the mean discharge-rate signal with an increase in the signal-to-noise ratio. The observations suggest that PVCN choppers can encode pure-tone frequency in a spatial profile more accurately than HSR or LMSR AN fibers. The present data on AN and PVCN spatial profiles should be valuable to CN modeling studies by providing the input to the CN and the output of a class of physiologically characterized CN neurons for an identical set of stimuli.