A stochastic model of the electrically stimulated auditory nerve: single-pulse response.

Most models of neural response to electrical stimulation, such as the Hodgkin-Huxley equations, are deterministic, despite significant physiological evidence for the existence of stochastic activity. For instance, the range of discharge probabilities measured in response to single electrical pulses cannot be explained at all by deterministic models. Furthermore, there is growing evidence that the stochastic component of auditory nerve response to electrical stimulation may be fundamental to functionally significant physiological and psychophysical phenomena. In this paper we present a simple and computationally efficient stochastic model of single-fiber response to single biphasic electrical pulses, based on a deterministic threshold model of action potential generation. Comparisons with physiological data from cat auditory nerve fibers are made, and it is shown that the stochastic model predicts discharge probabilities measured in response to single biphasic pulses more accurately than does the equivalent deterministic model. In addition, physiological data show an increase in stochastic activity with increasing pulse width of anodic/cathodic biphasic pulses, a phenomenon not present for monophasic stimuli. These and other data from the auditory nerve are then used to develop a population model of the total auditory nerve, where each fiber is described by the single-fiber model.

A stochastic model of the electrically stimulated auditory nerve: pulse-train response.

The single-pulse model of the companion paper [1] is extended to describe responses to pulse trains by introducing a phenomenological refractory mechanism. Comparisons with physiological data from cat auditory nerve fibers are made for pulse rates between 100 and 800 pulses/s. First, it is shown that both the shape and slope of mean discharge rate curves are better predicted by the stochastic model than by the deterministic model. Second, while interpulse effects such as refractory effects do indeed increase the dynamic range at higher pulse rates, both the physiological data and the model indicate that much of the dynamic range for pulse-train stimuli is due to stochastic activity. Third, it is shown that the stochastic model is able to predict the general magnitude and behavior of variance in discharge rate as a function of pulse rate, while the deterministic model predicts no variance at all.

Phase-locking of auditory-nerve discharges to sinusoidal electric stimulation of the cochlea.

The activity of auditory-nerve fibers was recorded in anesthetized cats in response to sinusoidal electric stimuli applied through a bipolar electrode pair inserted about 5 mm into the cochlea through the round window. The synchronization index was calculated from period histograms for frequencies ranging from 0.2 to over 10 kHz. The stimulus artifact was largely eliminated through the use of differential micropipettes and an adaptive digital filter. Measured synchronization indices were many times larger than the indices that could be attributed to the residual stimulus artifact. Synchronization indices at each stimulus frequency varied considerably from fiber to fiber, even in the same animal. The dependence of synchrony on stimulus frequency was also variable, decreasing monotonically in some fibers and nonmonotonically in others. The average electric synchronization index for all fibers did not fall as steeply with frequency as does the average synchrony for acoustic stimuli. The finding of significant phase locking to electric stimuli well above 1 kHz suggests that the poor frequency discrimination of cochlear-implant recipients for single-channel stimulation above this frequency may be due to the inability of the central processor to make effective use of the available phase-locking information for monaural stimulation.