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

Abstract

We undertook a systematic evaluation of spike rates and spike amplitudes of auditory nerve fiber (ANF) responses to trains of electric current pulses. Measures were obtained from acutely deafened cats to examine time-related changes free from the effects of hair-cell and synaptic adaptation. Such data relate to adaptation that likely occurs in ANFs of cochlear-implant users. A major goal was to determine and compare rate adaptation observed at different pulse rates (primarily 250, 1000, and 5000 pulse/s) and describe them using decaying exponential models similar to those used in acoustic studies. Rate-vs.-time functions were best described by two-exponent models and produced time constants similar to (although slightly greater than) the Brapid^and Bshort-term^components described in acoustic studies. There was little dependence of these time constants on onset spike rate, but pulse-rate effects were noted. Spike amplitude changes followed a time course different from that of rate adaptation consistent with a process related to ANF interspike intervals. The fact that two time constants governed rate adaptation in electrically stimulated and deafened fibers suggests that future computational models of adaptation should not only include hair cell and synapse components, but also components determined by fiber membrane characteristics.

Temporal Considerations for Stimulating Spiral Ganglion Neurons with Cochlear Implants

“…The results of similar experiments by Zhang et al (2007) also showed evidence of reduced excitability due to subthreshold responses although it was not explicitly stated by the authors. True spike rate adaptation should only depend on the onset spike rate.…”

“…5D-I of that article). In fact, all three experimental studies (Litvak et al 2001;Heffer 2010;Zhang et al 2007) showed that the normalized spike rate decrement (or spike reduction ratio) increased concomitantly with stimulus frequency. This indicates that accommodation is contributing to the spike rate decrement in parallel to spike rate adaptation.…”

“…More specifically, spike rate adaptation is a neuron's tendency to lower its excitability in response to ongoing action potentials. This is generally observed across timescales on the order of 10 to 100 ms (Zhang et al 2007;Heffer et al 2010;Miller et al 2011) or even minutes (Litvak et al 2003) but typically greater than those for refractoriness, facilitation, and accommodation. For example, one trial of a neuron's response to high-rate stimulation in Figure 2D shows that initially, the neuron fires multiple consecutive action potentials, then later in the pulse train, the occurrence of spikes diminishes.…”

“…Beyond some initial spiking activity, the final spike rate reached 0 spikes/s (see the upper-rightmost panel of Fig. 2 of Zhang et al 2007), which implied an ongoing accommodation to the subthreshold pulse train.…”

“…However, as we have seen in previous sections, the neuron's refractoriness, facilitation, and accommodation can also contribute to the shape of the PSTH, especially at small timescales (or high stimulation rates). Zhang et al (2007) explored effects on the spike rate by applying pulse trains at various stimulation rates and current levels to cat SGNs. Generally, the shape of the PSTHs across all conditions was described by a decay from an initial maximum spike rate towards a stabilized lower spike rate.…”