Relationship between the dynamic range of cochlear nerve fibres and their spontaneous activity

The dynamic ranges of cochlear nerve fibres in cats were determined automatically and were related to the fibres' rates of spontaneous activity, in both pooled data and data from individual cochlear nerves. The dynamic range represents the range of levels of a tone at the characteristic frequency of the fibre evoking mean discharge rates between spontaneous and saturated activity. In common with the findings of other investigators, the distribution of spontaneous discharge rates was bimodal. The total population could be divided into two sub-populations with spontaneous discharge rates above and below 15 spikes/s, respectively. The mean dynamic range of fibres having spontaneous discharge rates in excess of 15 spikes/s, was 41 dB (+/- 0.65 S.E.); that for fibres with rates below 15 spikes/s was 50 dB (+/- 1.2 S.E.). While the distributions of dynamic ranges of the two populations overlapped, they were significantly different, and dynamic ranges in excess of 60 dB were only found in substantial numbers (23%) in the population having low spontaneous discharge rates. Some of these were not saturated at the highest stimulus levels used.