Diversity of characteristic frequency rate-intensity functions in guinea pig auditory nerve fibres

Rate-intensity functions at characteristic frequency (CF) were recorded from single fibres in the auditory nerve of anaesthetised guinea pigs. Within the same animal, CF rate-intensity functions, although probably forming a continuum, could be conveniently divided into three groups; (1) Saturating; reach maximum discharge rate within 30 dB of threshold, (2) Sloping-saturation; initially rapid growth in discharge rate leading to a slower growth in discharge rate but not saturating and (3) Straight; approximately constant increase in firing rate per decibel increase in sound pressure up to the maximum sound pressures used. Thresholds for individual fibres were plotted relative to compound action potential thresholds at the appropriate frequency. Fibres with straight CF rate-intensity functions had the highest thresholds. Fibres of the saturating CF sloping-saturation CF rate-intensity type had thresholds intermediate between saturating and straight. There was a close relationship between the type of CF rate-intensity function exhibited by a fibre and its spontaneous discharge rate. Fibres with saturating CF rate-intensity functions generally had high spontaneous discharge rates (greater than 18/s), whereas those with straight CF rate-intensity functions generally had low spontaneous discharge rates (less than 0.5/s). The majority of fibres with sloping-saturation CF rate-intensity functions had spontaneous rates between 0.5/s and 18/s. There was a negative correlation (r = -0.59) between the logarithm of the spontaneous discharge rate and relative threshold at CF with the lowest spontaneous rate fibres having the highest thresholds and vice-versa. This diversity of CF rate-intensity functions has functional implications for both frequency and intensity coding at high sound pressures in the mammalian auditory system.     

Temporal modulation transfer functions in cochlear implantees using a method that limits overall loudness cues

Temporal modulation transfer functions (TMTFs) were measured for six users of cochlear implants, using different carrier rates and levels. Unlike most previous studies investigating modulation detection, the experimental design limited potential effects of overall loudness cues. Psychometric functions (percent correct discrimination of modulated from unmodulated stimuli versus modulation depth) were obtained. For each modulation depth, each modulated stimulus was loudness balanced to the unmodulated reference stimulus, and level jitter was applied in the discrimination task. The loudness-balance data showed that the modulated stimuli were louder than the unmodulated reference stimuli with the same average current, thus confirming the need to limit loudness cues when measuring modulation detection. TMTFs measured in this way had a low-pass characteristic, with a cut-off frequency (at comfortably loud levels) similar to that for normal-hearing listeners. A reduction in level caused degradation in modulation detection efficiency and a lower-cut-off frequency (i.e. poorer temporal resolution). An increase in carrier rate also led to a degradation in modulation detection efficiency, but only at lower levels or higher modulation frequencies. When detection thresholds were expressed as a proportion of dynamic range, there was no effect of carrier rate for the lowest modulation frequency (50 Hz) at either level.     

Comodulation Masking Release Determined in the Mouse (Mus musculus) using a Flanking-band Paradigm

Comodulation masking release (CMR) has been attributed to auditory processing within one auditory channel (within-channel cues) and/or across several auditory channels (across-channel cues). The present flanking-band (FB) experiment—using a 25-Hz-wide on-frequency noise masker (OFM) centered at the signal frequency of 10 kHz and a single 25-Hz-wide noise FB—was designed to separate the amount of CMR due to within- and across-channel cues and to investigate the role of temporal cues on the size of within-channel CMR. The results demonstrated within-channel CMR in the Naval Medical Research Institute mouse, while no unambiguous evidence could be found for CMR occurring due to across-channel processing (i.e., “true CMR”). The amount of within-channel CMR was dependent on the frequency separation between the FB and the OFM. CMR increased from 4 to 6 dB for a frequency separation of 1 kHz to 18 dB for a frequency separation of 100 Hz. The large increase for a frequency separation of 100 Hz is likely to be due to the exploitation of changes in the temporal pattern of the stimulus upon the addition of the signal. Temporal interaction between both masker bands results in modulations with a large depth at a modulation frequency equal to the beating rate. Adding a signal to the maskers reduces the depth of the modulation. The auditory system of mice might be able to use the change in modulation depth at a beating frequency of 100 Hz as a cue for signal detection, while being unable to detect changes in modulation depth at high modulation frequencies. These results are consistent with other experiments and model predictions for CMR in humans which suggested that the main contribution to the CMR effect stems from processing of within-channel cues.     

Gamma-aminobutyric acidergic and glycinergic inputs shape coding of amplitude modulation in the chinchilla cochlear nucleus.

Amplitude modulation is a prominent acoustic feature of biologically relevant sounds, such as speech and animal vocalizations. Enhanced temporal coding of amplitude modulation signals is found in certain dorsal and posteroventral cochlear nucleus neurons when they are compared to auditory nerve. Although mechanisms underlying this improved temporal selectivity are not known, involvement of inhibition has been suggested. gamma-Aminobutyric acid- and glycine-mediated inhibition have been shown to shape the dorsal cochlear nucleus and posteroventral cochlear nucleus response properties to other acoustic stimuli. In the present study, responses to amplitude modulation tones were obtained from chinchilla dorsal cochlear nucleus and posteroventral cochlear nucleus neurons. The amplitude modulation carrier was set to the neuron's characteristic frequency and the modulating frequency varied from 10 Hz. Rate and temporal modulation transfer functions were compared across neurons. Bandpass temporal modulation transfer functions were observed in 74% of the neurons studied. Most cochlear nucleus neurons (90%) displayed flat or lowpass rate modulation transfer functions to amplitude modulation signals presented at 2540 dB (re: characteristic frequency threshold). The role of inhibition in shaping responses to amplitude modulation stimuli was examined using iontophoretic application of glycine or gamma-aminobutyric acidA receptor agonists and antagonists. Blockade of gamma-aminobutyric acidA or glycine receptors increased stimulus-evoked discharge rates for a majority of neurons tested. Synchronization to the envelope was reduced, particularly at low and middle modulating frequencies, with temporal modulation transfer functions becoming flattened and less bandpass in appearance. Application of glycine, gamma-aminobutyric acid or muscimol increased the modulation gain over the low- and mid-modulation frequencies and reduced the discharge rate across envelope frequencies for most neurons tested. These findings support the hypothesis that glycinergic and gamma-aminobutyric acidergic inputs onto certain dorsal cochlear nucleus and posteroventral cochlear nucleus neurons play a role in shaping responses to amplitude modulation stimuli and may be responsible for the reported preservation of amplitude modulation temporal coding in dorsal cochlear nucleus and posteroventral cochlear nucleus neurons at high stimulus intensities or in background noise.     

Responses of neurons in the inferior colliculus of the rat to AM and FM tones

The responses of units in the inferior colliculus of the urethane-anaesthetized rat were recorded extracellularly. They responded to sinusoidal AM and FM tones with a modulation of their spike discharge usually at the same, or occasionally at twice, the modulation rate of the stimulus. The modulation depth of the response initially increased with the modulation depth of the stimulus, hut usually saturated or decreased at higher stimulus depths. The units showed a bandpass tuning to stimulus modulation rate which was independent of modulation depth and, in all cases, the most effective modulation rate was below 120 Hz. The modulated response to temporally varying stimuli could not be predicted from the pure tone discharge patterns or, in some cases, the unit's mean firing rate to modulated tones; temporally varying stimuli gave temporally varying responses. When compared with the data available from units at other levels in the auditory system, the results indicate a trend in which units at successively higher levels in the pathway respond most effectively to progressively lower rates of modulation.     

等调制深度时间调制转换曲线评估豚鼠下丘和听皮层的时间分辨率

目的探讨以听觉系统对调幅信号响应幅值随调制频率的改变计算等调制深度时间调制转换函数（temporal modulation transfer function，TMTF）来客观评估听觉系统时间分辨率的可行性。方法豚鼠下丘和听皮层分别埋植慢性电极，记录正弦调幅纯音（载频为8kHz，调制深度固定为100％）诱发电位，反应幅值经快速傅立叶变换（fast Fourier transform，F丌）得出相对反应幅值，并以相对反应幅值和调制频率绘制出等调制深度TMTF。记录正弦调幅纯音每个调制频率的调制深度从100％至10％的诱发电位，得出与传统的调制深度阈值TMTF方法相当的等幅值TMTF，与等调制深度TMTF相比较，判断等调制深度TMTF方法的有效性。结果豚鼠下丘和听皮层的等调制深度TMTF与等幅值TMTF都分别表现为带通和低通特性；等调制深度TMTF的截止频率与等幅值TMTF的截止频率差异无统计学意义（P值均〉0．05）。豚鼠听皮层等调制深度TMTF的截止频率与传统的行为学结果基本一致。结论以100％调制深度的正弦调幅纯音诱发反应幅值与调制频率绘制等调制深度TMTF是一种有效的客观评估听觉系统时间分辨率的方法，其中豚鼠听皮层的等调制深度TMTF可用于行为学预测。     

Group delay measurement from spiral ganglion cells in the guinea pig cochlea

Summary Measurements of group delay were made extracellularly from spiral ganglion cells in the basal turn of the guinea pig cochlea, using sinusoidally amplitude modulated tones, with constant modulating frequency and modulation depth at the microphone input. Threshold cochlear tuning was accompanied by a frequency dependent group delay, with relative peak proportional to Q₁₀. For sensitive units with steep intensity functions, the group delay decreased with increasing sound pressure level above threshold, without a significant change of Q₁₀.Supported by a grant from the Australian Research Grants Commission to BMJ and a University of Western Australia Research Studentship to AWGPresented at the 18th Workshop on Inner Ear Biology in Montpellier/La Grande Motte, September 14–16, 1981     

Processing of modulation frequency in the dorsal cochlear nucleus of the guinea pig: Amplitude modulated tones

The modulation frequency (Fm), particularly high Fm (> 200 Hz), in amplitude modulated (AM) tones can elicit the perception of the periodicity pitch (Langner, 1992). In this study, single unit responses to the Fms of the sinusoidal AM tones were investigated at 50 to 90 dB SPL. The recordings were made from the dorsal cochlear nucleus (DCN) of neuroleptic anesthetized guinea pigs with an intact cerebellum. The DCN units show a good capability of phase-locking to Fm at 400–1200 Hz. On-S-type II and Pauser/Buildup (P/B) units have a high modulation gain (7.2–8.3 dB). P/B units can preserve the high modulation gain (5–9 dB) up to 90 dB SPL. The modulation gain exponentially increases with decreasing modulation depth (Dm) and the phase-locking is detectable even at the Dm as low as 2–5%. The ‘central skipping’ of the phase-locking peak has been found at deep Dms in a few cases. The synchronization is independent of the discharge rate and can remain high even when the responses to AM tones are inhibited below the spontaneous activity. Such encoding behaviors over the unit's response area show that the Fm phase-locking is strong near or at its characteristic frequency (CF). The synchronization index (SI) versus carrier frequency (Fc) curve is similar to the inverse shape of tuning curve but more narrowly tuned than the iso-intensity function of pure tones at moderate to high intensity levels. The phase-locking is related to the unit's spontaneous rate (SR). The average modulation gain of the lower SR (≤ 2 spikes/s) units is 5 dB higher than that of the higher SR (> 2 spikes/s) units (8.16 and 2.92 dB, respectively) at 70 dB SPL. These results suggest that AM information is temporally encoded over broad ranges of modulation parameters in the DCN and is conveyed by the Fc channel. Such a timing mechanism can play an important role in processing of complex sounds under normal acoustic conditions.     

Electro-Cochlear Potentials Elicited by Sinusoidally Modulated Signals

Responses of the guinea pig cochlea to amplitude-modulated stimuli were measured with the aid of a gross electrode. The dynamic characteristics of this part of the auditory system was studied by varying several parameters of the applied signal. The signals used as carriers in our experiments were either white noise or pure tones of 1 and 4 kHz. The modulation frequency, dynamic and intensity characteristics were determined by varying the modulating frequency, the modulation depth and the intensity of the applied signal. To get an idea about possible non-linear aspects of the system under investigation, we always computed the Fourier transform of the response data and plotted the amplitude of the various harmonics and the phase of the fundamental separately as functions of the signal parameter in question. The greatest response was always found at a modulation frequency of about 200 Hz, with a relatively gradual rise up to this frequency and a sharper drop above 200 Hz. The phase of the fundamental changes very rapidly at frequencies above 200 Hz. The distortion is mainly second-harmonic and has a maximum about 1 octave lower than the fundamental. The carrier frequency and the intensity of the stimulus were not found to have a great influence on the frequency characteristic. For small modulation depths, the system is nearly linear; at higher intensities and modulation depths saturation occurs, coinciding with a relative increase in the intensity of the second harmonic with respect to the fundamental.     

Steady-state evoked responses to sinusoidally amplitude-modulated sounds recorded in man

Steady-state potentials evoked in response to binaural, sinusoidally amplitude-modulated (AM) pure tones and broadband noise signals were recorded differentially from position F4 and the ipsilateral mastoid on the human scalp. The responses elicited by the AM stimuli were approximately periodic waveforms whose energy was predominantly at the modulation frequency of the stimulus. The magnitude of responses was between 0.1 and 4 microV for modulation frequencies between 2 and 400 Hz imposed on a 1-kHz carrier signal. The magnitude of the responses increased linearly with log modulation depth for low (4 Hz) and high (80 Hz) modulation rates. The response magnitude also increased linearly with the mean intensity of the sound for intensities up to 60 dB above the subject's pure tone threshold; at higher levels the response saturated. The relationship between response magnitude and modulation frequency (the modulation transfer function) was a lowpass function for both pure tone and broadband noise carrier signals. The modulation transfer functions were similar to those obtained from human psychophysical measurements where spectral cues are either unavailable or not used by the subject. The responses also contained a significant component at the second harmonic of the modulation frequency. The magnitude of this component was greatest at modulation rates between 5 and 20 Hz. The responses elicited by ipsilateral and contralateral monaural stimulation were approximately equal in magnitude, and binaural stimulation produced a potential 30% greater than the individual monaural responses. It is suggested that the evoked response represents the entrained neural activity to temporal amplitude fluctuations, and reflects the psychophysically measured performance of the auditory system for the detection and analysis of amplitude modulation.     

GABAergic inputs shape responses to amplitude modulated stimuli in the inferior colliculus

The inferior colliculus (IC) is an important auditory processing center receiving inputs from lower brainstem nuclei, higher auditory and nonauditory structures, and contralateral IC. The IC, along with other auditory structures, is involved in coding information about the envelope of complex signals. Biologically relevant acoustic signals, including animal vocalizations and speech, are spectrally and temporally complex and display amplitude and frequency variations over time. Certain IC neurons respond selectively over a narrow range of modulation frequencies to sinusoidally amplitude modulated (SAM) stimuli. Responses to SAM stimuli can be measured in terms of discharge rate, with rate plotted against the modulation frequency to generate rate modulation transfer functions (rMTF). A role for the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), in shaping selective responses to SAM stimuli has been suggested. The present study examined the role of GABA in shaping responses to SAM stimuli in the IC of anesthetized chinchilla. Responses from 94 IC neurons were obtained before, during and after iontophoretic application of the GABA(A) receptor antagonist bicuculline methiodide. Complete responses to SAM stimuli were obtained from 55 extensively tested neurons, displaying band-pass (38) and low-pass rMTFs (17). For neurons showing band-pass rMTFs, GABA(A) receptor blockade selectively increased discharge rate at low modulation frequencies for 14 units, increased discharge near the best modulation frequency for 12 units. For neurons showing low-pass rMTFs, GABA(A) receptor blockade selectively increased discharge rate at low modulation frequencies for nine units. GABA(A) receptor blockade consistently reduced peak modulation gain, producing low-pass gain functions in a subset of IC neurons. In support of previous findings suggesting that selective temporal responses to SAM stimuli are coded in lower brainstem nuclei, temporal responses to SAM stimuli were relatively unaffected by GABA(A) receptor blockade. These findings support a role for GABA in shaping selective rate responses to SAM stimuli for a subset of chinchilla IC neurons.     

The envelope following response: scalp potentials elicited in the Mongolian gerbil using sinusoidally AM acoustic signals.

Scalp potentials which follow the low frequency envelope of a sinusoidally amplitude modulated stimulus waveform were evoked and recorded in anesthetized gerbils. This envelope following response (EFR) is presumably due to the synchronized discharge of populations of neurons in the auditory pathway. The magnitude of the EFR increased and the latency decreased in a near monotonic fashion with increased stimulus intensity and modulation depth. The modulation rate transfer function (MRTF) was determined for modulation frequencies between 10 and 920 Hz imposed on carrier frequencies ranging from 1 to 7 kHz. The MRTF was low pass in character having a corner frequency of 100-120 Hz. Measurements of the group delay, determined from the phase of the response relative to the stimulus phase, indicate that the response is generated in at least three distinct regions within the auditory pathway.     

Effects of randomizing phase on the discrimination between amplitude-modulated and quasi-frequency-modulated tones

This study investigated the bandwidth of phase sensitivity. Subjects discriminated amplitude-modulated tones (AM), and quasi-frequency-modulated tones (QFM) in a two-interval, forced-choice task. An adaptive threshold procedure was used to estimate the modulation depth needed to discriminate the stimuli as a function of carrier and modulation frequency. Non-monotonicities in threshold-bandwidth functions were often observed at higher modulation frequencies. The results are discussed in terms of two potential cues: (1) waveform envelope, (2) cubic distortion products. In order to degrade the information obtained from auditory distortions, the phase for the carrier frequency was randomly sampled from a uniform distribution, which diminished the non-monotonicities with minimal effect at lower modulation frequencies. Model simulations demonstrated that phase randomization degrades distortion product cues with only a modest effect on temporal cues. Final results show that maximum bandwidths for phase sensitivity (BW(max)) were not proportional to carrier frequencies.     

Coherence of frequency modulation is encoded by cochlear-generated distortion.

Nonlinearities of the peripheral auditory system generate distortion products which present to the central auditory system as apparent acoustic stimuli. The frequency and amplitude of distortion products reflect the frequency, phase and amplitude relationship of the components of a complex stimulus. When the stimulus consists of harmonically-related primaries, the amplitudes of the major distortion products are a function of the relative phase of the presented (primary) tones. We have previously shown (McAnally and Calford, 1990) that the variation of amplitude of the distortion as a function of the relative phase of a pair of harmonically-related primaries is well modelled as a function of the interaction of the multiple modes of distortion which fall at the same frequency (e.g. difference frequency and cubic difference frequency). A possibility raised by this result is that coherence of frequency modulation (that which maintains harmonicity) could be encoded in the amplitude of distortion. This was examined in measurements of both the cochlear microphonic potential (CM) and the responses of auditory nerve fibres in anaesthetized cats. Very small deviations from coherence of frequency modulation produced changes in the amplitude of the CM potential at the frequency of distortion. Also the discharges of auditory nerve fibres tuned to the frequency of distortion were found to be modulated at the same frequency as the amplitude changes observed in the CM. There was no variation in distortion amplitude in the CM and no modulation of auditory nerve discharges when primaries were frequency modulated coherently. It is suggested that amplitude modulation of distortion gives the auditory system its demonstrated sensitivity to minor departures from coherence of frequency-modulated, harmonically-related tones.     

Measurement of first- and second-order modulation detection thresholds in listeners with cochlear hearing loss

First- and second-order modulation detection thresholds were measured in normal hearing and hearing-impaired listeners. 'First-order' modulation detection thresholds correspond to the ability of listeners to detect sinusoidal amplitude modulation (SAM); they are measured as a function of the frequency of that modulation, f(m). 'Second-order' modulation detection thresholds correspond to the ability to detect sinusoidal modulation applied to the modulation depth of a SAM signal; they are measured as a function of the frequency of the modulation applied to the modulation depth (referred to as f(m)'). In this case, the SAM signal acts as a 'carrier' stimulus of frequency f(m) and sinusoidal modulation of the SAM-signal's modulation depth (at rate f(m)') generates two additional components in the modulation spectrum at f(m)-f(m)' and f(m)+f(m)'. In both groups of listeners, first-order modulation detection thresholds were measured for modulation frequencies f(m) ranging between 4 Hz and 32 Hz, and second-order modulation detection thresholds were measured for second-order modulation frequencies f(m)' ranging between 1 Hz and 11 Hz, using a fixed first-order modulation frequency f(m) of 16 Hz. The results showed that, in hearing-impaired listeners: first-order modulation detection thresholds were within the normal range up to f(m) = 16 Hz and poorer than normal at f(m) = 32 Hz; second-order modulation detection thresholds were within the normal range at f(m)' = 3, 5 and 11 Hz, and poorer than normal at f(m)' = 1 Hz and 7 Hz. These results suggest that cochlear damage has little effect on the detection of both sinusoidal and complex temporal envelopes.     

Stimulus properties influencing the responses of inferior colliculus neurons to amplitude-modulated sounds

The temporal pattern of the responses of neurons in the inferior colliculus of the anesthetized rat were studied using continuous tone or noise carrier signals, amplitude modulated by pseudorandom noise. Period histograms of the responses, cross-correlated with the pseudorandom noise, gave an estimate of the unit's impulse responses to modulation. The amplitude-modulation rate transfer function (MTF) was obtained by Fourier transforming the correlograms. At sound levels within approximately 15 dB of the unit threshold, the MTFs were near lowpass functions between 6 and 200 Hz but became more bandpass-like as the intensity was increased. There was a steep decline in the response to modulation at modulation frequencies above 200 Hz for all stimulus intensities. For the bandpass-type MTFs the greatest modulation of the discharge pattern occurred at modulation frequencies between 10 and 200 Hz with a maximum in the distribution of MTF peak values between 100 and 120 Hz. There was no consistent relationship with characteristic frequency of either the position of the MTF peak or the high-frequency cutoff of the MTF. The cross-correlograms obtained at high stimulus intensities (30-60 dB above threshold) often showed a negative peak, representing a decrease in the probability of firing in response to intensity increments in the stimulus, and denoting a nonmonotonic rate-intensity function. The MTFs for units responding to amplitude-modulated broadband noise were often flatter in the low frequency region than those generated with tone carriers at corresponding intensities. For some units addition of a broadband noise background to the modulated tone changed the response characteristic of the MTF from bandpass to lowpass and shifted the MTF peak to a lower modulation frequency. The results demonstrate that although neurons in the inferior colliculus are selectively sensitive to the modulation frequency of dynamic stimuli, the response characteristics are not invariant, but instead are closely dependent on the conditions under which the modulation is presented.     

Latencies of eighth nerve fibre responses with respect to their relative contribution to the compound action potential in the guinea pig

Summary We present a study of the latencies of click-evoked post-stimulus time histograms (PSTHs) in the guinea pig in the context of the auditory nerve fibre's contribution to the compound action potential (CAP) recorded at the round window. The latencies of the dominant PSTH peak were studied as a function of relevant physiological fibre properties, in particular the characteristic frequency (CF) and the spontaneous discharge rate (SR). We found that high-SR fibres have shorter latencies than low-SR fibres. These findings are discussed in the context of correlation between synaptic morphology and SR as described in the literature. The PSTH latency as a function of CF is described separately for low- and high-CF fibres for each of the two SR sub-groups. Finally, we discuss to what extent the various subgroups of fibres contribute to the N₁ peak of the CAP, the most commonly studied component.Presented at the 25th Workshop on Inner Ear Biology in London, England, 4–7 September 1988     

Responses of young and aged rat inferior colliculus neurons to sinusoidally amplitude modulated stimuli

The inferior colliculus (IC) is a processing center for monaural and binaural auditory signals. Many units in the central nucleus of the inferior colliculus (CIC) respond to amplitude and frequency modulated tones, features found in communication signals. The present study examined potential effects of age on responses to sinusoidally amplitude modulated (SAM) tones in CIC and external cortex of the inferior colliculus (ECIC) units in young and aged F344 rats. Extracellular recordings from 154 localized single units of aged (24 month) rats were compared to recordings from 135 IC units from young adult (3 month) animals. SAM tones were presented at 30 dB above threshold. Comparisons were made between CIC and ECIC regarding the percentage of units responding to SAM stimuli, the relationship between SAM responsiveness and temporal response patterns, maximum discharge rates and maximum modulation gains, shapes of rate transfer functions and synchronization modulation transfer functions (MTFs) in response to SAM tones. Sixty percent of units in young and aged rat IC were selectively responsive to SAM stimuli. Eighty-one percent of units classified as onset temporal response patterns were not tonically responsive to SAM stimuli. Median maximum discharge rate in response to SAM tones was 17.6/s in young F344 rats; median maximum modulation gain was 3.85 dB. These measurements did not change significantly with age. Thirty-seven percent of young rat units displayed bandpass MTFs and 53% had lowpass MTFs. There was a significant age-related shift in the distribution of MTF shapes in both the CIC and ECIC. Aged animals showed a lower percentage of bandpass functions and a higher percentage of lowpass functions. Age-related changes observed in SAM coding may reflect an altered balance between excitatory/inhibitory neurotransmitter efficacy in the aged rat IC, and/or possibly a change in the functional dynamic range of IC neurons.     

The Role of Suppression in the Upward Spread of Masking

The upward spread of masking refers to the higher growth rate of masking for maskers lower in frequency than the signal, compared to maskers at the signal frequency (Wegel RL, Lane CE. The auditory masking of one pure tone by another and its possible relation to the dynamics of the inner ear. Physics Rev. 23:266–285, 1924; Egan JP, Hake HW. On the masking pattern of a simple auditory stimulus. J. Acoust. Soc. Am. 22:622–630, 1950; Delgutte B. Physiological mechanisms of psychophysical masking: Observations from auditory-nerve fibres. J. Acoust. Soc. Am. 87:791–809, 1990a, Delgutte B. Two-tone rate suppression in auditory-nerve fibres: Dependence on suppressor frequency and level. Hear Res. 49:225–246, 1990b). The upward spread of simultaneous masking may arise from a combination of excitatory and suppressive effects. In this study, growth of masking functions were obtained for a 4-kHz signal masked by an on-frequency (4 kHz) or off-frequency (2.4 kHz), simultaneous or forward masker, in the presence of a notched noise with a center frequency of 4 kHz presented to restrict off-frequency listening. Compression was estimated from the slopes of the off-frequency growth of masking functions. Suppression was estimated by comparing the off-frequency simultaneous- and forward-masked growth of masking functions. Results showed that, for mid-level signals (35–60 dB SPL), the compression exponent estimated from simultaneous and forward masking averaged 0.31 and 0.26, respectively. The maximum amount of suppression (defined as the decrease in the basilar-membrane response to the signal) was variable, ranging from about 6 to 17 dB across subjects. Despite the substantial reduction in the response to the signal, the results suggest that suppression has a minimal effect on the slope of the masking function at mid levels. Rather, upward spread of masking seems to be mainly determined by the compressive basilar-membrane response to the signal in relation to the linear response to the lower-frequency masker.     

Excitation and suppression of primary auditory fibres in the pigeon

Spike potentials were recorded from single fibres in the auditory nerve of the pigeon. In fibres with recognizable responses to sound, spontaneous activity and properties of responses to tonal stimuli were studied in quiet background conditions. Mean spontaneous rate in the sample of fibres was 35 spikes/s. Tuning of spike response to tones was manifest as a single peak in rate at each sound pressure level (SPL) in the frequency-intensity plane. The majority of fibres showed only excitation of spike rate above spontaneous rate. Post stimulus time histograms (PSTs) in such cases were typical of excitatory responses, previously described in birds and mammals showing pronounced adaptation and post-stimulus suppression of spike rate. In most cases of excitation-only responses, however, slopes of rate functions depended on stimulus frequency. Close to characteristic frequency (CF), slopes tended to decrease with increasing SPL, whereas away from CF, slopes tended to increase with SPL. In a minority of excitation-only responses, slopes of rate functions were parallel. In some fibres, tones adjacent to the response area caused overt suppression of spontaneous firing. For these fibres, the slopes of rate functions were more-strongly frequency-dependent, being negative at low SPL when rate suppression occurred. Suppression of spontaneous activity at low SPL was non-monotonic and quite different from suppression of spike rate at stimulus intensities above rat saturation. In PSTs of suppressed spontaneous activity, rebound occurred at the termination of the tone. The results clarify previous observations of suppression of primary auditory responses in birds. We conclude that responses in the majority of auditory fibres in the pigeon are the product of opposing excitatory and suppressive influences in the cochlea, generated by single tones in quite.