Discharge rate of the auditory nerve during noise revealed by electrocochlear stimulation

The purpose of this study was to evaluate the average discharge rate of all fibres in the whole auditory nerve (Ii,) when a broadband noise with stead y-state effects is applied to the ear. We assessed the R-wn parameter by de tecting the state of refractoriness of the nerve during noise stimulation u sing an electric stimulus (ES) as a probe. The technique, applied in awake pre-implanted guinea pigs (Charlet de Sauvage et al., 1994), made it possib le to obtain electro-acoustic responses (EARs), from which an estimate of t he R-wn parameter could be deduced. Negative current pulses of 100 mu s dur ation, each followed by an identical pulse of positive polarity for charge balance, were applied between round window and indifferent vertex electrode s at intervals of 160 ms. The 120 ms wide-band noise masker started 92 ms b efore every other negative ES. The signal on the stimulating electrodes was averaged over a 5.12 ms window in synchrony with the negative pulse. EARs were obtained by alternately subtracting recordings during noise from those during silence. The R-wn parameter was determined by comparing experimenta l and computed EAR patterns. For this purpose, a model of unit response inc orporating changes in amplitude and conduction velocity during the relative refractory period was designed. The recovery function of the firing probab ility in response to ES was evaluated. Fibres were classified in different categories according to their background discharge rates. The probability o f response of single fibres to ES in each category was calculated on the ba sis of their interval histograms during silence and noise. Individual spike s were combined accordingly to obtain the computed EAR waveform. R-wn was d etermined by adjusting the EAR amplitude of the model in relation to that o f the experimental EAR. R-wn generally increases in a linear fashion with r espect to noise intensity expressed in dB, thus following the increase in l oudness perception estimated by Weber's law. At the highest noise levels, R -wn tends to saturate. The estimated saturation rate was found to be about 380 spikes/s. (C) 2000 Elsevier Science B.V. All rights reserved.