Changes Across Time in the Temporal Responses of Auditory Nerve Fibers Stimulated by Electric Pulse Trains

Abstract

Most auditory prostheses use modulated electric pulse trains to excite the auditory nerve. There are, however, scant data regarding the effects of pulse trains on auditory nerve fiber (ANF) responses across the duration of such stimuli. We examined how temporal ANF properties changed with level and pulse rate across 300-ms pulse trains. Four measures were examined: (1) first-spike latency, (2) interspike interval (ISI), (3) vector strength (VS), and (4) Fano factor (FF, an index of the temporal variability of responsiveness). Data were obtained using 250-, 1,000-, and 5,000-pulse/s stimuli. First-spike latency decreased with increasing spike rate, with relatively small decrements observed for 5,000-pulse/s trains, presumably reflecting integration. ISIs to low-rate (250 pulse/s) trains were strongly locked to the stimuli, whereas ISIs evoked with 5,000-pulse/s trains were dominated by refractory and adaptation effects. Across time, VS decreased for low-rate trains but not for 5,000-pulse/s stimuli. At relatively high spike rates (9200 spike/s), VS values for 5,000-pulse/s trains were lower than those obtained with 250-pulse/s stimuli (even after accounting for the smaller periods of the 5,000-pulse/s stimuli), indicating a desynchronizing effect of high-rate stimuli. FF measures also indicated a desynchronizing effect of high-rate trains. Across a wide range of response rates, FF underwent relatively fast increases (i.e., within 100 ms) for 5,000-pulse/s stimuli. With a few exceptions, ISI, VS, and FF measures approached asymptotic values within the 300-ms duration of the low-and high-rate trains. These findings may have implications for designs of cochlear implant stimulus protocols, understanding electrically evoked compound action potentials, and interpretation of neural measures obtained at central nuclei, which depend on understanding the output of the auditory nerve.

Neural Masking by Sub-threshold Electric Stimuli: Animal and Computer Model Results

“…This report builds upon the background and studies of ANF responses to electric pulse trains (Litvak et al 2003;Zhang et al 2007;Miller et al 2008) by adding a preceding masker train and determining its effect on responses to a subsequent probe train. Masker train durations were typically 300 ms, comparable to speech segment durations.…”

“…The filtered potentials were digitally sampled at 100,000 samples/s using an Instrutech ITC-18 data acquisition board (HEKA Instruments, Bellmore, NY, USA) and stored for detailed off-line analysis using Matlab (The MathWorks, Natick, MA, USA) routines written in house. Spikes were extracted from stimulus artifacts using either "template subtraction" or boxcar filtering techniques, as described in Miller et al (2008).…”

Changes in Auditory Nerve Responses Across the Duration of Sinusoidally Amplitude-Modulated Electric Pulse-Train Stimuli

“…Animal anesthesia and maintenance, acute deafening, and surgical preparations were conducted as described in Zhang et al (2007) and Miller et al (2008a). Briefly, cats were sedated with intramuscular ketamine (22 mg/kg) and xylazine (1.1 mg/kg) and kept at surgical anesthesia level with Nembutal (8-13 mg/kg, i.v.).…”

“…Responses within the first 50-ms epoch are strongly shaped by refractory effects, resulting in spike timing unrelated to the modulator and, hence, relatively poor synchronization. This influence of refractoriness on stimulus coding is likely influenced by stimulus level (Miller et al 2008a). PHs relative to the modulator period are shown in Figure 1D for five selected 50-ms epochs.…”

“…However, those studies focused on "steady-state" responses; ANF responses to electric pulse trains are not constant over time, nor are speech stimuli. Zhang et al (2007) and Miller et al (2008a) showed that spike rate and temporal response properties (vector strength, interval statistics) changed over the duration of 300 ms constant-amplitude pulse trains. Rate adaptation was clearly evident over the first 100 ms relative to train onset and greater with high-rate trains than with low-rate trains.…”