Responses of Cat Auditory Nerve Fibers to Biphasic Electrical Current Pulses

Abstract

Discharge patterns of single auditory nerve fibers were recorded from normal-hearing cats implanted with a 12-band intracochlear electrode array. Stimuli were biphasic current pulses of specifiable width, amplitude, and rate. Acoustic tuning curves were obtained to determine the cochlear positions of the fibers. Response latencies to electrical stimuli formed two groups. Short latency (0.3 to 0.7 ms) responses were attributed to direct activation of spiral ganglion neurons. At high stimulus intensities, these often exhibited abrupt shifts toward even shorter latencies. Long latency (> 1.5 ms) responses were probably caused by electrophonic activation of functional hair cells. Response thresholds to electrical stimuli depended on a fiber's proximity to the stimulating electrodes, and they did not depend on a fiber's acoustic response threshold or spontaneous discharge rate. High intensity (> 1.5 mA) stimuli could excite fibers over a wide range of characteristic frequencies, even for the narrowest (0.45 mm) electrode separations. Response threshold was an exponentially decreasing function of pulse width for widths up to 300 μs/phase. Fiber discharges were highly phase-locked at all suprathreshold intensities, and saturation discharge rates usually equaled stimulus pulse rates for rates up to at least 800 pulses/s. Dynamic ranges were small (1 to 6 dB), increased with pulse rate, and were uncorrelated with electrical response threshold. Within the dynamic range, shapes of poststimulus time and interspike interval histograms resembled those obtained in response to acoustic stimuli. Depolarization block caused fiber activity to cease in 2 to 5 seconds for sustained stimuli presented at high (> 600 pulses/s) pulse rates and intensities.

Cortical Responses to Cochlear Implant Stimulation: Channel Interactions

“…One possible explanation for that observation is that the leading pulse would have had time to trigger activation of voltagesensitive potassium channels and inactivation of voltage-sensitive sodium channels, resulting in a partial refractory state and elevating the threshold for the trailing pulse (Hille 2001). Published time constants for refraction for scala tympani stimulation are around 1.5 ms (Dynes 1986;Javel et al 1987;Parkins 1989;Bruce et al 1999) and 0.7 ms for meatal stimulation (Cartee et al 2000).…”

Changes Across Time in Spike Rate and Spike Amplitude of Auditory Nerve Fibers Stimulated by Electric Pulse Trains

“…Acoustic and electric excitation of auditory nerve fibers (ANFs) produce markedly different response characteristics (Kiang et al 1965;Hartmann et al 1984;van den Honert and Stypulkowski 1984;Javel et al 1987;Javel and Shepherd 2000;Parkins and Colombo 1987;Litvak et al 2001). However, our understanding of how ANFs respond to repetitive electric stimuli is limited.…”

Development of a voltage-dependent current noise algorithm for conductance-based stochastic modelling of auditory nerve fibres

“…One of the important properties of a nerve fibre affecting temporal characteristics such as latency is variance in threshold stimulus intensity (Hales et al 2004;Javel et al 1987;Verveen 1962;Verveen and Derksen 1968). This activation threshold variability has been shown to be primarily caused by the stochastic behaviour of the node of Ranvier's sodium channels (Hales et al 2004) or as noted by Sigworth (1980) and Rubinstein (1995): the macroscopic fluctuation of threshold can sufficiently be accounted for by the microscopic fluctuations of the voltage dependent sodium channels.…”

“…The relationship between activation or discharge probability and stimulus intensity in the stochastic nerve has experimentally been shown to be Gaussian (Javel et al 1987;Shepherd and Javel 1997;Verveen 1962;Verveen and Derksen 1968). Both the threshold (mean, μ) and spread (standard deviation, σ) of the Gaussian discharge probability function (DPF) proved dependent on stimulus duration, but the relative spread ( ) was found to be independent of stimulus duration for pulse durations between 100 μs and 3 ms. RS thus characterises the excitability fluctuation as a measure of the threshold region width or spread (σ) relative to the mean threshold value (μ) (Rubinstein 1995;Verveen 1962).…”