Rapid adaptation depends on the characteristic frequency of auditory nerve fibers

Specificity of the Human Frequency Following Response for Carrier and Modulation Frequency Assessed Using Adaptation

The frequency following response (FFR) is a scalp-recorded measure of phase-locked brainstem activity to stimulus-related periodicities. Three experiments investigated the specificity of the FFR for carrier and modulation frequency using adaptation. FFR waveforms evoked by alternating-polarity stimuli were averaged for each polarity and added, to enhance envelope, or subtracted, to enhance temporal fine structure information. The first experiment investigated peristimulus adaptation of the FFR for pure and complex tones as a function of stimulus frequency and fundamental frequency (F0). It showed more adaptation of the FFR in response to sounds with higher frequencies or F0s than to sounds with lower frequency or F0s. The second experiment investigated tuning to modulation rate in the FFR. The FFR to a complex tone with a modulation rate of 213 Hz was not reduced more by an adaptor that had the same modulation rate than by an adaptor with a different modulation rate (90 or 504 Hz), thus providing no evidence that the FFR originates mainly from neurons that respond selectively to the modulation rate of the stimulus. The third experiment investigated tuning to audio frequency in the FFR using pure tones. An adaptor that had the same frequency as the target (213 or 504 Hz) did not generally reduce the FFR to the target more than an adaptor that differed in frequency (by 1.24 octaves). Thus, there was no evidence that the FFR originated mainly from neurons tuned to the frequency of the target. Instead, the results are consistent with the suggestion that the FFR for low-frequency pure tones at medium to high levels mainly originates from neurons tuned to higher frequencies. Implications for the use and interpretation of the FFR are discussed.

Auditory nerve fibre responses in the ferret

The ferret (Mustela putorius) is a medium-sized, carnivorous mammal with good low-frequency hearing; it is relatively easy to train, and there is therefore a good body of behavioural data detailing its detection thresholds and localization abilities. However, despite extensive studies of the physiology of the central nervous system of the ferret, even extending to the prefrontal cortex, little is known of the functioning of the auditory periphery. Here, we provide an insight into this peripheral function by detailing responses of single auditory nerve fibres. Our expectation was that the ferret auditory nerve responsiveness would be similar that of its near relative, the cat. However, by comparing a range of variables (the frequency tuning, the variation of rate-level functions with spontaneous rate, and the high-frequency cut-off of phase locking) across several species, we show that the auditory nerve (and hence cochlea) in the ferret is more similar to that of the guinea-pig and chinchilla than to that of the cat. Animal models of hearing are often chosen on the basis of the similarity of their audiogram to that of the human, particularly in the low-frequency region. We show here that whereas the ferret hears well at low frequencies, this is likely to occur via fibres with higher characteristic frequencies. These qualitative differences in response characteristics in auditory nerve fibres are important in interpreting data across all of auditory science, as it has been argued recently that tuning in animals is broader than in humans.

Rapid neural adaptation to sound level statistics

Auditory neurons must represent accurately a wide range of sound levels using firing rates that vary over a far narrower range of levels. Recently, we demonstrated that this "dynamic range problem" is lessened by neural adaptation, whereby neurons adjust their input-output functions for sound level according to the prevailing distribution of levels. These adjustments in input-output functions increase the accuracy with which levels around those occurring most commonly are coded by the neural population. Here, we examine how quickly this adaptation occurs. We recorded from single neurons in the auditory midbrain during a stimulus that switched repeatedly between two distributions of sound levels differing in mean level. The high-resolution analysis afforded by this stimulus showed that a prominent component of the adaptation occurs rapidly, with an average time constant across neurons of 160 ms after an increase in mean level, much faster than our previous experiments were able to assess. This time course appears to be independent of both the timescale over which sound levels varied and that over which sound level distributions varied, but is related to neural characteristic frequency. We find that adaptation to an increase in mean level occurs more rapidly than to a decrease. Finally, we observe an additional, slow adaptation in some neurons, which occurs over a timescale of tens of seconds. Our findings provide constraints in the search for mechanisms underlying adaptation to sound level. They also have functional implications for the role of adaptation in the representation of natural sounds.

Influence of cochlear traveling wave and neural adaptation on auditory brainstem responses

The present study investigates the relationship between evoked responses to transient broadband chirps and responses to the same chirps when embedded in longer-duration stimuli. It examines to what extent the responses to the composite stimuli can be explained by a linear superposition of the responses to the single components, as a function of stimulus level. In the first experiment, a single rising chirp was temporally and spectrally embedded in two steady-state tones. In the second experiment, the stimulus consisted of a continuous alternating train of chirps: each rising chirp was followed by the temporally reversed (falling) chirp. In both experiments, the transitions between stimulus components were continuous. For stimulation levels up to approximately 70 dB SPL, the responses to the embedded chirp corresponded to the responses to the single chirp. At high stimulus levels (80-100 dB SPL), disparities occurred between the responses, reflecting a nonlinearity in the processing when neural activity is integrated across frequency. In the third experiment, the effect of within-train rate on wave-V response was investigated. The response to the chirp presented at a within-train rate of 95 Hz exhibited the same amplitude as that to the chirp presented in the traditional single-stimulus paradigm at a rate of 13 Hz. For a corresponding experiment with bandlimited chirps of 4 ms duration, where the within-train rate was 250 Hz, a clear reduction of the response amplitude was observed. This nonlinearity in terms of temporal processing most likely reflects effects of short-term adaptation. Overall, the results of the present study further demonstrate the importance of cochlear processing for the formation of brainstem potentials. The data may provide constraints on future models of peripheral processing in the human auditory system. The findings might also be useful for the development of effective stimulation paradigms in clinical applications.

Spike-frequency adaptation in the inferior colliculus

We investigated spike-frequency adaptation of neurons sensitive to interaural phase disparities (IPDs) in the inferior colliculus (IC) of urethane-anesthetized guinea pigs using a stimulus paradigm designed to exclude the influence of adaptation below the level of binaural integration. The IPD-step stimulus consists of a binaural 3,000-ms tone, in which the first 1,000 ms is held at a neuron's least favorable ("worst") IPD, adapting out monaural components, before being stepped rapidly to a neuron's most favorable ("best") IPD for 300 ms. After some variable interval (1-1,000 ms), IPD is again stepped to the best IPD for 300 ms, before being returned to a neuron's worst IPD for the remainder of the stimulus. Exponential decay functions fitted to the response to best-IPD steps revealed an average adaptation time constant of 52.9 +/- 26.4 ms. Recovery from adaptation to best IPD steps showed an average time constant of 225.5 +/- 210.2 ms. Recovery time constants were not correlated with adaptation time constants. During the recovery period, adaptation to a 2nd best-IPD step followed similar kinetics to adaptation during the 1st best-IPD step. The mean adaptation time constant at stimulus onset (at worst IPD) was 34.8 +/- 19.7 ms, similar to the 38.4 +/- 22.1 ms recorded to contralateral stimulation alone. Individual time constants after stimulus onset were correlated with each other but not with time constants during the best-IPD step. We conclude that such binaurally derived measures of adaptation reflect processes that occur above the level of exclusively monaural pathways, and subsequent to the site of primary binaural interaction.

Dendrotoxin-sensitive K(+) currents contribute to accommodation in murine spiral ganglion neurons

We have previously identified two broad electrophysiological classes of spiral ganglion neuron that differ in their rate of accommodation (Mo & Davis, 1997a). In order to understand the underlying ionic basis of these characteristic firing patterns, we used alpha-dendrotoxin (alpha-DTX) to eliminate the contribution of a class of voltage-gated K(+) channels and assessed its effects on a variety of electrophysiological properties by using the whole-cell configuration of the patch-clamp technique. Exposure to alpha-DTX caused neurons that initially displayed rapid accommodation to fire continuously during 240 ms depolarizing test pulses within a restricted voltage range. We found a non-monotonic relationship between number of action potentials fired and membrane potential in the presence of alpha-DTX that peaked at voltages between -40 to -10 mV and declined at more depolarized and hyperpolarized test potentials. The alpha-DTX-sensitive current had two components that activated in different voltage ranges. Analysis of recordings made from acutely isolated neurons gave estimated half-maximal activation voltages of -63 and 12 mV for the two components. Because alpha-DTX blocks the Kv1.1, Kv1.2 and Kv1.6 subunits, we examined the action of the Kv1.1-selective blocker dendrotoxin K (DTX-K). We found that this antagonist reproduced the effects of alpha-DTX on neuronal firing, and that the DTX-K-sensitive current also had two separate components. These data suggest that the transformation from a rapidly adapting to a slowly adapting firing pattern was mediated by the low voltage-activated component of DTX-sensitive current with a potential contribution from the high voltage-activated component at more depolarized potentials. In addition, the effects of DTX-K indicate that Kv1.1 subunits are important constituents of the underlying voltage-gated potassium channels.

A phenomenological model for the responses of auditory-nerve fibers: I. Nonlinear tuning with compression and suppression

A phenomenological model was developed to describe responses of high-spontaneous-rate auditory-nerve (AN) fibers, including several nonlinear response properties. Level-dependent gain (compression), bandwidth, and phase properties were implemented with a control path that varied the gain and bandwidth of tuning in the signal-path filter. By making the bandwidth of the control path broad with respect to the signal path, the wide frequency range of two-tone suppression was included. By making the control-path filter level dependent and tuned to a frequency slightly higher than the signal-path filter, other properties of two-tone suppression were also included. These properties included the asymmetrical growth of suppression above and below the characteristic frequency and the frequency offset of the suppression tuning curve with respect to the excitatory tuning curve. The implementation of this model represents a relatively simple phenomenological description of a single mechanism that underlies several important nonlinear response properties of AN fibers. The model provides a tool for studying the roles of these nonlinearities in the encoding of simple and complex sounds in the responses of populations of AN fibers.

Temporal aspects of the effects of cooling on responses of single auditory nerve fibers

During an investigation of the effects of cochlear cooling on frequency tuning and input/output relations of single auditory nerve fibers in gerbil (Ohlemiller and Siegel (1994) Hear. Res. 80, 174-190), cooling-related changes in post-stimulus time histogram (PSTH) shape and phase-locking to tonebursts were characterized in a small sample of neurons. Local cochlear cooling by 5-10 degrees C below normal core temperature did not alter overall PSTH shape, although some evidence was found for a reduction in the time constants of rapid and short term rate adaptation. The relative contributions of rapid and short term response components appeared unaltered. Effects of cooling on phase-locking were assessed by calculating the synchronization index for responses to intense ( > 70 dB SPL) tonebursts at 0.5, 1.0, and 2.0 kHz. Synchronization filter functions exhibited modest reductions in both magnitude and the upper frequency limit of phase-locking. The effects of cooling on the temporal character of responses appear distinct from those of a simple reduction in stimulus intensity. Results are interpreted in terms of cooling-related changes in responses of cochlear hair cells and afferent neurons, and suggest that temperature artifacts are unlikely to underlie reported species differences in PSTH shape and phase-locking.

Modeling interaural-delay sensitivity to frequency modulation at high frequencies

Interaural-delay sensitivity to high-frequency (> or = 3 kHz) sinusoidal-frequency-modulated (SFM) tones is examined for rates from 25 to 800 Hz and depths of -12 to 18 dB. Comparison is made to thresholds obtained for sinusoidal-amplitude-modulated (SAM) tones for the same observers and modulation rates. Both SAM and SFM threshold-by-rate functions are U-shaped with optimum sensitivity to SFM tones occurring at higher rates (fm = 200-400 Hz) compared to those for SAM tones (fm = 100-200 Hz). Effects of modulation depth were examined for rates from 50 to 300 Hz. In all cases thresholds improved considerably with increasing modulation depth. It is also shown that a hybrid dichotic signal composed of an SFM tone presented to one ear and an SAM tone to the other, can perceptually fuse and be lateralized, with the contingency that both stimuli have equal modulation rates but not necessarily equal carrier frequencies. Using bandpass noise to restrict off-frequency listening, it was shown that for this stimulus, observers can use information from filters either below or above the carrier frequency. Consistent with FM-to-AM conversion from cochlear bandpass filtering, several important differences between the SAM- and SFM-tone data can be predicted from a nonstationary stochastic model of binaural interaction whose parameters are uniquely determined from the SAM-tone data.

First-spike timing of auditory-nerve fibers and comparison with auditory cortex

First-spike timing of auditory-nerve fibers and comparison with auditory cortex. J. Neurophysiol. 78: 2438-2454, 1997. The timing of the first spike of cat auditory-nerve (AN) fibers in response to onsets of characteristic frequency (CF) tone bursts was studied and compared with that of neurons in primary auditory cortex (AI), reported previously. Tones were shaped with cosine-squared rise functions, and rise time and sound pressure level were parametrically varied. Although measurement of first-spike latency of AN fibers was somewhat compromised by effects of spontaneous activity, latency was an invariant and inverse function of the maximum acceleration of peak pressure (i.e., a feature of the 2nd derivative of the stimulus envelope), as previously found in AI, rather than of tone level or rise time. Latency-acceleration functions of all AN fibers were of very similar shape, similar to that observed in AI. As in AI, latency-acceleration functions of different fibers were displaced along the latency axis, reflecting differences in minimum latency, and along the acceleration axis, reflecting differences in sensitivity to acceleration [neuronal transient sensitivity (S)]. S estimates increased with spontaneous rate (SR), but values of high-SR fibers exceeded those in AI. This suggests that S estimates are biased by SR per se, and that unbiased true S values would be less tightly correlated with response properties covarying with SR, such as firing threshold. S estimates varied with CF in a fashion similar to the cat's audiogram and, for low- and medium-SR fibers, matched those for AI neurons. Minimum latency decreased with increasing SR and CF. As in AI, the standard deviation of first-spike timing (SD) in AN was also an inverse function of maximum acceleration of peak pressure. The characteristics of the increase of SD with latency in a given AN fiber/AI neuron and across AN fibers/AI neurons revealed that the precision of first-spike timing to some stimuli can actually be higher in AI than in AN. The data suggest that the basic characteristics of the latency-acceleration functions of transient onset responses seen in cortex are generated at inner hair cell-AN fiber synapses. Implications for signal processing in the auditory system and for first-spike generation and adaptation in AN are discussed.

Cochlear basal and apical differences reflected in the effects of cooling on responses of single auditory nerve fibers

Responses of single auditory nerve fibers in the Mongolian gerbil were examined before and during rapid, moderate cooling of the cochlea. Reducing cochlear temperature from 35-39 degrees C to 29-32 degrees C led to stable, reversible changes in spontaneous firing rates (SRs), and responses to tonebursts, as characterized by frequency tuning curves and rate-versus-intensity curves. The nature and extent of effects of cooling were strongly linked to characteristic frequency (CF). Rate thresholds at the CF were increased by 0-15 dB for fibers with CFs below 8 kHz, and by 10-30 dB for higher CFs. Although SRs were generally reduced, the percent reduction in SR was striking CF dependent. For fibers with CFs below 4 kHz, the reduction did not exceed 50% of the initial SR. For higher CFs, the reduction was always greater than 50%. The effects of cooling on intensity curve shape differed qualitatively for fibers with CFs below and above 3-4 kHz. The slope of the curve was reduced by an average of 50% for lower CFs, but on average was unchanged for higher CFs. Cooling-related increases in CF threshold probably reflect impairment of active mechanical processes. The CF dependence of these increases suggests either that active mechanical processes are more impaired in the cochlear base relative to the apex, or that they play a more crucial role in determining sensitivity in the base. The CF-dependent changes in SR and in the shape of rate-intensity curves caused by cooling correspond to an enhancement of basal/apical differences seen at normal temperatures. They are best explained by longitudinal gradients in the properties of the inner hair cells and their afferent synapses. Basal and apical differences in the distribution of SRs and in supra-threshold response properties suggest that stimulus coding strategies differ between low and high frequency regions of the cochlea.