Rate responses of auditory nerve fibers to tones in noise near masked threshold

Abstract

The rate responses of auditory nerve fibers were measured for best frequency (BF) tone bursts in the presence of continuous background noise. Rate functions for BF tones were constructed over a 32-dB range of levels, centered on the behavioral masked thresholds of cats. The tone level at which noticeable rate changes are evoked by the tones corresponds closely to behavioral masked threshold at all noise levels used (-10- to 30-dB spectrum level). As the noise level increases, the response rate to the background noise approaches saturation, and the incremental rate response to tones decreases. At high noise levels, the rate responses to tones of low and medium spontaneous rate fibers are larger than those of high spontaneous rate fibers. Empirical statistics of auditory nerve fiber spike counts are reported; these differ from those expected of a Poisson process in that the variance is smaller than the mean. A new measure of discharge rate is described that allows rate changes to be expressed in units of a standard deviation. This measure allows tone-evoked responses to be interpreted in terms of their detectability in a signal detection task. Rate responses of low and medium spontaneous rate fibers are more detectable than those of high spontaneous rate fibers, especially at high noise levels. There appears to be sufficient information in the rate response of a small number of auditory nerve fibers to support behaviorally observed levels of detection performance.

Spontaneous Activity of Auditory-Nerve Fibers: Insights into Stochastic Processes at Ribbon Synapses

“…Hence, the refractory period has a random, exponential distribution with a mean duration of (t D ϩ 1/ R ). This refractory function has been implemented in many previous modeling studies (Schroeder and Hall, 1974;Lütkenhöner et al, 1980;Young and Barta, 1986;Li and Young, 1993;Prijs et al, 1993;Schoonhoven et al, 1997;Meddis and O'Mard, 2005;Meddis, 2006), and its shape is in excellent agreement with recent physiological data (Brown, 1994;Miller et al, 2001;Morsnowski et al, 2006); it is also illustrated in Figure 1. In this scenario, the ISIs should be distributed according to a general-gamma distribution (Young and Barta, 1986;Li and Young, 1993;Lehmann, 2002), the CDF of which is given by the following:…”

“…This means that the probability of an event occurring at time t (t Ͼ 0), given that the last event occurred at t ϭ 0, is constant and thus independent of the time elapsed since the last release event. This assumption is in accord with a large number of previous studies of AN fiber activity (Kiang et al, 1965;Molnar and Pfeiffer, 1968;Schroeder and Hall, 1974;Manley and Robertson, 1976;Lütkenhöner et al, 1980;Geisler, 1981Geisler, , 1998Geisler et al, 1985;Javel, 1986;Young and Barta, 1986;Bi, 1989;Carney, 1993;Li and Young, 1993;Miller and Wang, 1993;Schmich and Miller, 1997;Zhang et al, 2001;Krishna, 2002;Kuhlmann et al, 2002) (for review, see Delgutte, 1996;Mountain and Hubbard, 1996). The CDF of the IEIs is then given by the exponential distribution as follows:…”

“…For model Ia, f ISI (t) is given by Equation 4 and ISI by Equation 6, so that the SD of the ISIs takes on the simple form: (Young and Barta, 1986) or, when expressed as a function of the mean (Li and Young, 1993):…”

“…The observed deviations from strictly exponential ISI distributions are therefore commonly attributed to the refractoriness of AN fibers. By the same rationale, the slow recovery (over tens of milliseconds) of the discharge probability with time since the last spike (Gray, 1967;Gaumond et al, 1982Gaumond et al, , 1983Li and Young, 1993;Johnson, 1996) has also been attributed to the refractory properties (Kiang et al, 1965;Gaumond et al, 1983;Young and Barta, 1986;Carney, 1993;Li and Young, 1993;Miller and Wang, 1993;Zhang et al, 2001). However, direct studies of the refractory properties of AN fibers by means of electrical stimulation have shown that refractory periods are very short, Ͻ1 or 2 ms (Brown, 1994;Dynes, 1996;Cartee et al, 2000;Miller et al, 2001;Shepherd et al, 2004;Morsnowski et al, 2006).…”

An Improved Model for the Rate–Level Functions of Auditory-Nerve Fibers

“…The subinterval 1/[K AA ⅐ R max ⅐ (P ϩ P 0 ) ␤ ] decreases as the stimulus amplitude, P, increases, whereas the subinterval 1/R max is constant. Such a sum can be interpreted physiologically (Young and Barta, 1986;Heil et al, 2007). The constant subinterval can be conceived of as a rate-limiting mean dead time.…”