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

Abstract

Response rates of auditory nerve fibers (ANFs) to electric pulse trains change over time, reflecting substantial spike-rate adaptation that depends on stimulus parameters. We hypothesize that adaptation affects the representation of amplitude-modulated pulse trains used by cochlear prostheses to transmit speech information to the auditory system. We recorded cat ANF responses to sinusoidally amplitude-modulated (SAM) trains with 5,000 pulse/s carriers. Stimuli delivered by a monopolar intracochlear electrode had fixed modulation frequency (100 Hz) and depth (10%). ANF responses were assessed by spike-rate measures, while representation of modulation was evaluated by vector strength (VS) and the fundamental component of the fast Fourier transform (F 0 amplitude). These measures were assessed across the 400 ms duration of pulse-train stimuli, a duration relevant to speech stimuli. Different stimulus levels were explored and responses were categorized into four spike-rate groups to assess level effects across ANFs. The temporal pattern of rate adaptation to modulated trains was similar to that of unmodulated trains, but with less rate adaptation. VS to the modulator increased over time and tended to saturate at lower spike rates, while F 0 amplitude typically decreased over time for low driven rates and increased for higher driven rates. VS at moderate and high spike rates and degree of F 0 amplitude temporal changes at low and moderate spike rates were positively correlated with the degree of rate adaptation. Thus, high-rate carriers will modify the ANF representation of the modulator over time. As the VS and F 0 measures were sensitive to adaptation-related changes over different spike-rate ranges, there is value in assessing both measures.

Relationship Between Peripheral and Psychophysical Measures of Amplitude Modulation Detection in Cochlear Implant Users

“…A future study could compromise between experimental time and the need for similar stimuli for psychophysical and electrophysiologic measures – such as using a 100 ms stimulus for both measures and alternative means to optimize the experimental time need to collect both measures. In addition, as we previously mentioned, adaptation to AM stimuli can still occur beyond 50 ms of stimulation as shown by animal data (Hu et al, 2008, 2010). Although adaptation to longer duration pulse train stimuli have been studied in human CI users (Hay-McCutcheon et al, 2005; Hughes et al, 2012, 2014), the time course of adaptation was not a focus of these studies.…”

“…All of these factors may affect the degree of adaptation and consequently may affect the modulated responses. In prior animal studies (Hu et al, 2008, 2010), ANF responses to SAM pulse trains indicate large spike rate adaptation as well as low vector strength within the first 50 ms. After this initial 50 ms, adaptation still occurs, but the rate of adaptation is reduced. In addition, vector strength of ANF responses improves over time.…”

“…In single auditory nerve fiber recordings, the effect of stimulus modulation on synchrony of neural spiking has been assessed by measuring vector strength (Goldberg & Brown, 1969; Hu et al, 2008, 2010). Vector strength is a measure of the degree to which nerve fibers phase lock to the stimulus (or to the periodic modulation of the stimulus amplitude), and thus is a method of quantifying temporal distribution of neural responses relative to the modulation frequency.…”

“…Generally, vector strength is weaker at the onset of the modulated pulse train, attributed to refractory effects. Vector strength improves over the time course of the AM pulse train (Hu et al, 2008, 2010). Similar to the ECAP MRA data, vector strength increases with modulation frequency up to a cutoff; beyond that, vector strength decreases at higher modulation frequencies (Hu et al, 2008).…”

Effect of stimulus level on the temporal response properties of the auditory nerve in cochlear implants

“…Several studies have evaluated ECAP (Tejani et al, 2017; Wilson et al, 1997) or single-fiber (Hu et al, 2010) responses to amplitude-modulated pulse trains, or modeled spike activity for amplitude-modulated stimuli (Goldwyn et al, 2010). As with the unmodulated pulse-train data, results show that neural activity changes over the course of the modulated pulse train.…”