Sinusoidally Amplitude-modulated Tones Research Articles

Rate dependent neural responses of interaural-time-difference cues in fine-structure and envelope.

Advancements in cochlear implants (CIs) have led to a significant increase in bilateral CI users, especially among children. Yet, most bilateral CI users do not fully achieve the intended binaural benefit due to potential limitations in signal processing and/or surgical implant positioning. One crucial auditory cue that normal hearing (NH) listeners can benefit from is the interaural time difference (ITD), i.e., the time difference between the arrival of a sound at two ears. The ITD sensitivity is thought to be heavily relying on the effective utilization of temporal fine structure (very rapid oscillations in sound). Unfortunately, most current CIs do not transmit such true fine structure. Nevertheless, bilateral CI users have demonstrated sensitivity to ITD cues delivered through envelope or interaural pulse time differences, i.e., the time gap between the pulses delivered to the two implants. However, their ITD sensitivity is significantly poorer compared to NH individuals, and it further degrades at higher CI stimulation rates, especially when the rate exceeds 300 pulse per second. The overall purpose of this research thread is to improve spatial hearing abilities in bilateral CI users. This study aims to develop electroencephalography (EEG) paradigms that can be used with clinical settings to assess and optimize the delivery of ITD cues, which are crucial for spatial hearing in everyday life. The research objective of this article was to determine the effect of CI stimulation pulse rate on the ITD sensitivity, and to characterize the rate-dependent degradation in ITD perception using EEG measures. To develop protocols for bilateral CI studies, EEG responses were obtained from NH listeners using sinusoidal-amplitude-modulated (SAM) tones and filtered clicks with changes in either fine structure ITD (ITDFS) or envelope ITD (ITDENV). Multiple EEG responses were analyzed, which included the subcortical auditory steady-state responses (ASSRs) and cortical auditory evoked potentials (CAEPs) elicited by stimuli onset, offset, and changes. Results indicated that acoustic change complex (ACC) responses elicited by ITDENV changes were significantly smaller or absent compared to those elicited by ITDFS changes. The ACC morphologies evoked by ITDFS changes were similar to onset and offset CAEPs, although the peak latencies were longest for ACC responses and shortest for offset CAEPs. The high-frequency stimuli clearly elicited subcortical ASSRs, but smaller than those evoked by lower carrier frequency SAM tones. The 40-Hz ASSRs decreased with increasing carrier frequencies. Filtered clicks elicited larger ASSRs compared to high-frequency SAM tones, with the order being 40 > 160 > 80> 320 Hz ASSR for both stimulus types. Wavelet analysis revealed a clear interaction between detectable transient CAEPs and 40-Hz ASSRs in the time-frequency domain for SAM tones with a low carrier frequency.

The Lower Limit of Pitch Perception for Pure Tones and Low-Frequency Complex Sounds

The lower limit of pitch (LLP) perception was explored for pure tones, sinusoidally amplitude-modulated (SAM) tones with a carrier frequency of 125 Hz, and trains of 125-Hz tone pips, using an adaptive procedure to estimate the lowest repetition rate for which a tonal/humming quality was heard. The LLP was similar for the three stimulus types, averaging 19 Hz. There were marked individual differences, which were correlated to some extent across stimulus types. The pure-tone stimuli contained a single resolved harmonic, whereas the SAM tones and tone-pip trains contained only unresolved components, whose frequencies did not necessarily form a harmonic series. The similarity of the LLP across stimulus types suggests that the LLP is determined by the repetition period of the sound for pure tones, and the envelope repetition period for complex stimuli. The results are consistent with the idea that the LLP is determined by a periodicity analysis in the auditory system, and that the longest time interval between waveform or envelope peaks for which this analysis can be performed is approximately 53 ms.

Asymmetric temporal envelope encoding: Implications for within- and across-ear envelope comparison.

Separating sound sources in acoustic environments relies on making ongoing, highly accurate spectro-temporal comparisons. However, listeners with hearing impairment may have varying quality of temporal encoding within or across ears, which may limit the listeners' ability to make spectro-temporal comparisons between places-of-stimulation. In this study in normal hearing listeners, depth of amplitude modulation (AM) for sinusoidally amplitude modulated (SAM) tones was manipulated in an effort to reduce the coding of periodicity in the auditory nerve. The ability to judge differences in AM rates was studied for stimuli presented to different cochlear places-of-stimulation, within- or across-ears. It was hypothesized that if temporal encoding was poorer for one tone in a pair, then sensitivity to differences in AM rate of the pair would decrease. Results indicated that when the depth of AM was reduced from 50% to 20% for one SAM tone in a pair, sensitivity to differences in AM rate decreased. Sensitivity was greatest for AM rates near 90 Hz and depended upon the places-of-stimulation being compared. These results suggest that degraded temporal representations in the auditory nerve for one place-of-stimulation could lead to deficits comparing that temporal information with other places-of-stimulation.

Investigating the Effect of Cochlear Synaptopathy on Envelope Following Responses Using a Model of the Auditory Nerve

The healthy auditory system enables communication in challenging situations with high levels of background noise. Yet, despite normal sensitivity to pure tones, many listeners complain about having difficulties in such situations. Recent animal studies demonstrated that noise overexposure that produces temporary threshold shifts can cause the loss of auditory nerve (AN) fiber synapses (i.e., cochlear synaptopathy, CS), which appears to predominantly affect medium- and low-spontaneous rate (SR) fibers. In the present study, envelope following response (EFR) magnitude-level functions were recorded in normal hearing (NH) threshold and mildly hearing-impaired (HI) listeners with thresholds elevated above 2 kHz. EFRs were elicited by sinusoidally amplitude modulated (SAM) tones presented in quiet with a carrier frequency of 2 kHz, modulated at 93 Hz, and modulation depths of 0.85 (deep) and 0.25 (shallow). While EFR magnitude-level functions for deeply modulated tones were similar for all listeners, EFR magnitudes for shallowly modulated tones were reduced at medium stimulation levels in some NH threshold listeners and saturated in all HI listeners for the whole level range. A phenomenological model of the AN was used to investigate the extent to which hair-cell dysfunction and/or CS could explain the trends observed in the EFR data. Hair-cell dysfunction alone, including postulated elevated hearing thresholds at extended high frequencies (EHF) beyond 8 kHz, could not account for the recorded EFR data. Postulated CS led to simulations generally consistent with the recorded data, but a loss of all types of AN fibers was required within the model framework. The effects of off-frequency contributions (i.e., away from the characteristic place of the stimulus) and the differential loss of different AN fiber types on EFR magnitude-level functions were analyzed. When using SAM tones in quiet as the stimulus, model simulations suggested that (1) EFRs are dominated by the activity of high-SR fibers at all stimulus intensities, and (2) EFRs at medium-to-high stimulus levels are dominated by off-frequency contributions.

Stimulus bandwidth impact on auditory evoked potential thresholds and estimated upper-frequency limits of hearing in dolphins.

The frequency range of hearing is important for assessing the potential impact of anthropogenic noise on marine mammals. Auditory evoked potentials (AEPs) are commonly used to assess toothed whale hearing, but measurement methods vary across researchers and laboratories. In particular, estimates of the upper-frequency limit of hearing (UFL) can vary due to interactions between the unintended spread of spectral energy to frequencies below the desired test frequency and a sharp decline in hearing sensitivity at frequencies near the UFL. To assess the impact of stimulus bandwidth on UFL measurement, AEP hearing tests were conducted in four bottlenose dolphins (Tursiops truncatus) with normal and impaired hearing ranges. Dolphins were tested at frequencies near the UFL and at a frequency 1/2-octave below the UFL, where hearing sensitivity was better (i.e., threshold was lower). Thresholds were measured using sinusoidal amplitude modulated (SAM) tones and tone-bursts of varying bandwidth. Measured thresholds varied inversely as a function of stimulus bandwidth near the UFL with narrow-band tone-bursts approximating thresholds measured using SAM tones. Bandwidth did not impact measured thresholds where hearing was more sensitive, highlighting how stimulus bandwidth and the rate of decline of hearing sensitivity interact to affect measured threshold near the UFL.

Masking Differentially Affects Envelope-following Responses in Young and Aged Animals

Age-related hearing decline typically includes threshold shifts as well as reduced wave I auditory brainstem response (ABR) amplitudes due to cochlear synaptopathy/neuropathy, which may compromise precise coding of suprathreshold speech envelopes. This is supported by findings with older listeners, who have difficulties in envelope and speech processing, especially in noise. However, separating the effects of threshold elevation, synaptopathy, and degradation by noise on physiological representations may be difficult. In the present study, the effects of notched, low- and high-pass noise on envelope-following responses (EFRs) in aging were compared when sound levels (aged: 85-dB SPL; young: 60- to 80-dB SPL) were matched between groups peripherally, by matching wave I ABR amplitudes, or centrally by matching EFR amplitudes. Low-level notched noise reduced EFRs to sinusoidally amplitude-modulated (SAM) tones in young animals for notch widths up to 2 octaves. High-pass noise above the carrier frequency reduced EFRs. Young animals showed EFR reductions at lower noise levels. Low-pass noise did not reduce EFRs in either young or aged animals. High-pass noise may affect EFR amplitudes in young animals more than aged by reducing the contributions of high-frequency-sensitive inputs. EFRs to SAM tones in modulated noise (NAM) suggest that neurons of young animals can synchronize to NAM at lower sound levels and maintain dual AM representations better than older animals. The overall results show that EFR amplitudes are strongly influenced by aging and the presence of a competing sound that likely reduces or shifts the pool of responsive neurons.

Erratum to: A Modeling Study of the Responses of the Lateral Superior Olive to Ipsilateral Sinusoidally Amplitude-Modulated Tones.

The lateral superior olive (LSO) is a brainstem nucleus that is classically understood to encode binaural information in high-frequency sounds. Previous studies have shown that LSO cells are sensitive to envelope interaural time difference in sinusoidally amplitude-modulated (SAM) tones (Joris and Yin, J Neurophysiol 73:1043–1062, 1995; Joris, J Neurophysiol 76:2137–2156, 1996) and that a subpopulation of LSO neurons exhibit low-threshold potassium currents mediated by Kv1 channels (Barnes-Davies et al., Eur J Neurosci 19:325–333, 2004). It has also been shown that in many LSO cells the average response rate to ipsilateral SAM tones decreases with modulation frequency above a few hundred Hertz (Joris and Yin, J Neurophysiol 79:253–269, 1998). This low-pass feature is not directly inherited from the inputs to the LSO since the response rate of these input neurons changes little with increasing modulation frequency. In the current study, an LSO cell model is developed to investigate mechanisms consistent with the responses described above, notably the emergent rate decrease with increasing frequency. The mechanisms explored included the effects of after-hyperpolarization (AHP) channels, the dynamics of low-threshold potassium channels (KLT), and the effects of background inhibition. In the model, AHP channels alone were not sufficient to induce the observed rate decrease at high modulation frequencies. The model also suggests that the background inhibition alone, possibly from the medial nucleus of the trapezoid body, can account for the small rate decrease seen in some LSO neurons, but could not explain the large rate decrease seen in other LSO neurons at high modulation frequencies. In contrast, both the small and large rate decreases were replicated when KLT channels were included in the LSO neuron model. These results support the conclusion that KLT channels may play a major role in the large rate decreases seen in some units and that background inhibition may be a contributing factor, a factor that could be adequate for small decreases.

Investigating brain stimulation during amplitude modulation detection: A pilot study

Positive effects of transcranial brain stimulation have been observed across a number of cognitive and sensory domains. This non-invasive external electric stimulation technique has been shown to induce changes in patterns of brain activity, and these changes have been suggested to be responsible for affecting sensory perception. Recent findings have suggested transcranial alternating current stimulation (tACS) can entrain neural oscillations, meaning the externally applied alternating current can affect firing rates by inducing oscillatory coherence. Thus, tACS introduces a methodology to causally investigate the effect of neural oscillations on auditory perception. The purpose of this study was to determine if tACS improves amplitude modulation detection for individuals with normal hearing. In a crossover design, young healthy adults with normal hearing were presented with sinusoidally amplitude-modulated (SAM) tones while undergoing tACS. SAM tones were presented with a modulation rate of 4 Hz or 8 Hz; the modulation depth was set to each participant’s modulation detection threshold. The frequency of tACS was varied across two sessions to match the modulation rate of one of the SAM tones. Preliminary results suggest variable responses to tACS across the participants tested. Considerations for auditory tACS research will be discussed.

Relating Click-Evoked Auditory Brainstem Response Waveforms to Hearing Loss in the Bottlenose Dolphin (Tursiops truncatus)

Hearing sensitivity in captive Atlantic bottlenose dolphins was assessed using a portable electrophysiologic data collection system, a transducer attached to the pan region of the mandible, and non-invasive recording electrodes. The auditory steady-state response (ASSR) was evoked using sinusoidal amplitude-modulated tones at half octave steps from 20–160 kHz and utilized to determine the upper frequency limit of hearing (i.e., the frequency at which threshold was ≤ 120 dB re 1 μPa). An auditory brainstem response (ABR) was then recorded to a moderate-amplitude click (peak-equivalent sound pressure level of 122 dB re 1 μPa) and examined to determine if relationships existed between the upper frequency limit of hearing and the waveform characteristics of the click-evoked ABR. The ASSR and click-evoked ABR were measured in 6 bottlenose dolphins with varying hearing sensitivity and frequency range of hearing. A significant relationship existed between click-evoked ABR wave amplitudes and the upper frequency limit of hearing. Test times for assessment using frequency-specific ASSR and click-evoked ABR were ∼45 minutes and 1 minute, respectively. With further definition of normative data, measurement of click-evoked ABRs could form the basis of an expedited electrophysiologic method for hearing screening in marine mammals.

Relating click-evoked auditory brainstem response waveforms to hearing loss in the bottlenose dolphin (Tursiops truncatus)

Hearing sensitivity in captive Atlantic bottlenose dolphins was assessed using a portable electrophysiologic data collection system, a transducer attached to the pan region of the mandible, and non-invasive recording electrodes. The auditory steady-state response (ASSR) was evoked using sinusoidal amplitude-modulated tones at half octave steps from 20–160 kHz and utilized to determine the upper frequency limit of hearing (i.e., the frequency at which threshold was ≤ 120 dB re 1 μPa). An auditory brainstem response (ABR) was then recorded to a moderate-amplitude click (peak-equivalent sound pressure level of 122 dB re 1 μPa) and examined to determine if relationships existed between the upper frequency limit of hearing and the waveform characteristics of the click-evoked ABR. The ASSR and click-evoked ABR were measured in 6 bottlenose dolphins with varying hearing sensitivity and frequency range of hearing. A significant relationship existed between click-evoked ABR wave amplitudes and the upper frequency limit of hearing. Test times for assessment using frequency-specific ASSR and click-evoked ABR were ∼45 minutes and 1 minute, respectively. With further definition of normative data, measurement of click-evoked ABRs could form the basis of an expedited electrophysiologic method for hearing screening in marine mammals.

Envelope enhancement increases cortical sensitivity to interaural envelope delays with acoustic and electric hearing.

Evidence from human psychophysical and animal electrophysiological studies suggests that sensitivity to interaural time delay (ITD) in the modulating envelope of a high-frequency carrier can be enhanced using half-wave rectified stimuli. Recent evidence has shown potential benefits of equivalent electrical stimuli to deaf individuals with bilateral cochlear implants (CIs). In the current study we assessed the effects of envelope shape on ITD sensitivity in the primary auditory cortex of normal-hearing ferrets, and profoundly-deaf animals with bilateral CIs. In normal-hearing animals, cortical sensitivity to ITDs (±1 ms in 0.1-ms steps) was assessed in response to dichotically-presented i) sinusoidal amplitude-modulated (SAM) and ii) half-wave rectified (HWR) tones (100-ms duration; 70 dB SPL) presented at the best-frequency of the unit over a range of modulation frequencies. In separate experiments, adult ferrets were deafened with neomycin administration and bilaterally-implanted with intra-cochlear electrode arrays. Electrically-evoked auditory brainstem responses (EABRs) were recorded in response to bipolar electrical stimulation of the apical pair of electrodes with singe biphasic current pulses (40 µs per phase) over a range of current levels to measure hearing thresholds. Subsequently, we recorded cortical sensitivity to ITDs (±800 µs in 80-µs steps) within the envelope of SAM and HWR biphasic-pulse trains (40 µs per phase; 6000 pulses per second, 100-ms duration) over a range of modulation frequencies. In normal-hearing animals, nearly a third of cortical neurons were sensitive to envelope-ITDs in response to SAM tones. In deaf animals with bilateral CI, the proportion of ITD-sensitive cortical neurons was approximately a fifth in response to SAM pulse trains. In normal-hearing and deaf animals with bilateral CI the proportion of ITD sensitive units and neural sensitivity to ITDs increased in response to HWR, compared with SAM stimuli. Consequently, novel stimulation strategies based on envelope enhancement may prove beneficial to individuals with bilateral cochlear implants.

Evaluating auditory stream segregation of SAM tone sequences by subjective and objective psychoacoustical tasks, and brain activity

Auditory stream segregation refers to a segregated percept of signal streams with different acoustic features. Different approaches have been pursued in studies of stream segregation. In psychoacoustics, stream segregation has mostly been investigated with a subjective task asking the subjects to report their percept. Few studies have applied an objective task in which stream segregation is evaluated indirectly by determining thresholds for a percept that depends on whether auditory streams are segregated or not. Furthermore, both perceptual measures and physiological measures of brain activity have been employed but only little is known about their relation. How the results from different tasks and measures are related is evaluated in the present study using examples relying on the ABA- stimulation paradigm that apply the same stimuli. We presented A and B signals that were sinusoidally amplitude modulated (SAM) tones providing purely temporal, spectral or both types of cues to evaluate perceptual stream segregation and its physiological correlate. Which types of cues are most prominent was determined by the choice of carrier and modulation frequencies (fmod) of the signals. In the subjective task subjects reported their percept and in the objective task we measured their sensitivity for detecting time-shifts of B signals in an ABA- sequence. As a further measure of processes underlying stream segregation we employed functional magnetic resonance imaging (fMRI). SAM tone parameters were chosen to evoke an integrated (1-stream), a segregated (2-stream), or an ambiguous percept by adjusting the fmod difference between A and B tones (Δfmod). The results of both psychoacoustical tasks are significantly correlated. BOLD responses in fMRI depend on Δfmod between A and B SAM tones. The effect of Δfmod, however, differs between auditory cortex and frontal regions suggesting differences in representation related to the degree of perceptual ambiguity of the sequences.

Neurometric amplitude-modulation detection threshold measured in the chinchilla ventral cochlear nucleus following sensorineural hearing loss

Amplitude modulation is a common feature of natural sounds and an important cue in audition. Modulation supports perceptual segregation of “objects” in complex acoustic scenes, and provides information for speech understanding and pitch perception. Previous work in our laboratory showed increased modulation gain without change in temporal modulation transfer function (tMTF) bandwidth in auditory-nerve fiber responses to sinusoidal amplitude-modulated (SAM) tones measured in chinchillas with noise-induced hearing loss (HL), compared to normal-hearing (NH) controls. The ventral cochlear nucleus (VCN) provides significant input-output transformations with respect to amplitude-modulation representation, with enhanced spike synchrony to the amplitude envelope in several distinct cell types. We recorded spike times in response to SAM tones with modulation depths between 3% and 100% from all major VCN unit types in anesthetized NH and HL chinchillas. HL animals were previously exposed to 116 dBSPL 500 Hz-centered octave-band Gaussian noise. Spike times were analyzed in terms of synchrony to the amplitude envelope, tMTFs were calculated, and a signal-detection theoretic analysis was used to compute modulation-detection and discrimination thresholds. Results will be related to human perceptual studies, which have shown better modulation-detection thresholds in HL. [Work supported by an Action on Hearing Loss Fulbright Commission scholarship (M.S.), and NIH grant R01-DC009838.]

Dual sensitivity of inferior colliculus neurons to ITD in the envelopes of high-frequency sounds: experimental and modeling study

Human listeners are sensitive to interaural time differences (ITDs) in the envelopes of sounds, which can serve as a cue for sound localization. Many high-frequency neurons in the mammalian inferior colliculus (IC) are sensitive to envelope-ITDs of sinusoidally amplitude-modulated (SAM) sounds. Typically, envelope-ITD-sensitive IC neurons exhibit either peak-type sensitivity, discharging maximally at the same delay across frequencies, or trough-type sensitivity, discharging minimally at the same delay across frequencies, consistent with responses observed at the primary site of binaural interaction in the medial and lateral superior olives (MSO and LSO), respectively. However, some high-frequency IC neurons exhibit dual types of envelope-ITD sensitivity in their responses to SAM tones, that is, they exhibit peak-type sensitivity at some modulation frequencies and trough-type sensitivity at other frequencies. Here we show that high-frequency IC neurons in the unanesthetized rabbit can also exhibit dual types of envelope-ITD sensitivity in their responses to SAM noise. Such complex responses to SAM stimuli could be achieved by convergent inputs from MSO and LSO onto single IC neurons. We test this hypothesis by implementing a physiologically explicit, computational model of the binaural pathway. Specifically, we examined envelope-ITD sensitivity of a simple model IC neuron that receives convergent inputs from MSO and LSO model neurons. We show that dual envelope-ITD sensitivity emerges in the IC when convergent MSO and LSO inputs are differentially tuned for modulation frequency.

Neurometric amplitude‐modulation detection threshold in the guinea‐pig ventral cochlear nucleus

Amplitude modulation (AM) is a pervasive feature of natural sounds. Neural detection and processing of modulation cues is behaviourally important across species. Although most ecologically relevant sounds are not fully modulated, physiological studies have usually concentrated on fully modulated (100% modulation depth) signals. Psychoacoustic experiments mainly operate at low modulation depths, around detection threshold (∼5% AM). We presented sinusoidal amplitude-modulated tones, systematically varying modulation depth between zero and 100%, at a range of modulation frequencies, to anaesthetised guinea-pigs while recording spikes from neurons in the ventral cochlear nucleus (VCN). The cochlear nucleus is the site of the first synapse in the central auditory system. At this locus significant signal processing occurs with respect to representation of AM signals. Spike trains were analysed in terms of the vector strength of spike synchrony to the amplitude envelope. Neurons showed either low-pass or band-pass temporal modulation transfer functions, with the proportion of band-pass responses increasing with increasing sound level. The proportion of units showing a band-pass response varies with unit type: sustained chopper (CS) > transient chopper (CT) > primary-like (PL). Spike synchrony increased with increasing modulation depth. At the lowest modulation depth (6%), significant spike synchrony was only observed near to the unit's best modulation frequency for all unit types tested. Modulation tuning therefore became sharper with decreasing modulation depth. AM detection threshold was calculated for each individual unit as a function of modulation frequency. Chopper units have significantly better AM detection thresholds than do primary-like units. AM detection threshold is significantly worse at 40 dB vs. 10 dB above pure-tone spike rate threshold. Mean modulation detection thresholds for sounds 10 dB above pure-tone spike rate threshold at best modulation frequency are (95% CI) 11.6% (10.0-13.1) for PL units, 9.8% (8.2-11.5) for CT units, and 10.8% (8.4-13.2) for CS units. The most sensitive guinea-pig VCN single unit AM detection thresholds are similar to human psychophysical performance (∼3% AM), while the mean neurometric thresholds approach whole animal behavioural performance (∼10% AM).

Steady-state MEG responses elicited by a sequence of amplitude-modulated short tones of different carrier frequencies

The auditory steady-state response (ASSR) is a weak potential or magnetic response elicited by periodic acoustic stimuli with a maximum response at about a 40-Hz periodicity. In most previous studies using amplitude-modulated (AM) tones of stimulus sound, long lasting tones of more than 10 s in length were used. However, characteristics of the ASSR elicited by short AM tones have remained unclear. In this study, we examined magnetoencephalographic (MEG) ASSR using a sequence of sinusoidal AM tones of 0.78 s in length with various tone frequencies of 440–990 Hz in about one octave variation. It was found that the amplitude of the ASSR was invariant with tone frequencies when the level of sound pressure was adjusted along an equal-loudness curve. The amplitude also did not depend on the existence of preceding tone or difference in frequency of the preceding tone. When the sound level of AM tones was changed with tone frequencies in the same range of 440–990 Hz, the amplitude of ASSR varied in a proportional manner to the sound level. These characteristics are favorable for the use of ASSR in studying temporal processing of auditory information in the auditory cortex. The lack of adaptation in the ASSR elicited by a sequence of short tones may be ascribed to the neural activity of widely accepted generator of magnetic ASSR in the primary auditory cortex.

Stream Segregation in the Perception of Sinusoidally Amplitude-Modulated Tones

Amplitude modulation can serve as a cue for segregating streams of sounds from different sources. Here we evaluate stream segregation in humans using ABA- sequences of sinusoidally amplitude modulated (SAM) tones. A and B represent SAM tones with the same carrier frequency (1000, 4000 Hz) and modulation depth (30, 100%). The modulation frequency of the A signals (fmodA) was 30, 100 or 300 Hz, respectively. The modulation frequency of the B signals was up to four octaves higher (Δfmod). Three different ABA- tone patterns varying in tone duration and stimulus onset asynchrony were presented to evaluate the effect of forward suppression. Subjects indicated their 1- or 2-stream percept on a touch screen at the end of each ABA- sequence (presentation time 5 or 15 s). Tone pattern, fmodA, Δfmod, carrier frequency, modulation depth and presentation time significantly affected the percentage of a 2-stream percept. The human psychophysical results are compared to responses of avian forebrain neurons evoked by different ABA- SAM tone conditions [1] that were broadly overlapping those of the present study. The neurons also showed significant effects of tone pattern and Δfmod that were comparable to effects observed in the present psychophysical study. Depending on the carrier frequency, modulation frequency, modulation depth and the width of the auditory filters, SAM tones may provide mainly temporal cues (sidebands fall within the range of the filter), spectral cues (sidebands fall outside the range of the filter) or possibly both. A computational model based on excitation pattern differences was used to predict the 50% threshold of 2-stream responses. In conditions for which the model predicts a considerably larger 50% threshold of 2-stream responses (i.e., larger Δfmod at threshold) than was observed, it is unlikely that spectral cues can provide an explanation of stream segregation by SAM.

Temporal modulation transfer functions measured from auditory-nerve responses following sensorineural hearing loss

The ability of auditory-nerve (AN) fibers to encode modulation frequencies, as characterized by temporal modulation transfer functions (TMTFs), generally shows a low-pass shape with a cut-off frequency that increases with fiber characteristic frequency (CF). Because AN-fiber bandwidth increases with CF, this result has been interpreted to suggest that peripheral filtering has a significant effect on limiting the encoding of higher modulation frequencies. Sensorineural hearing loss (SNHL), which is typically associated with broadened tuning, is thus predicted to increase the range of modulation frequencies encoded; however, perceptual studies have generally not supported this prediction. The present study sought to determine whether the range of modulation frequencies encoded by AN fibers is affected by SNHL, and whether the effects of SNHL on envelope coding are similar at all modulation frequencies within the TMTF passband. Modulation response gain for sinusoidally amplitude modulated (SAM) tones was measured as a function of modulation frequency, with the carrier frequency placed at fiber CF. TMTFs were compared between normal-hearing chinchillas and chinchillas with a noise-induced hearing loss for which AN fibers had significantly broadened tuning. Synchrony and phase responses for individual SAM tone components were quantified to explore a variety of factors that can influence modulation coding. Modulation gain was found to be higher than normal in noise-exposed fibers across the entire range of modulation frequencies encoded by AN fibers. The range of modulation frequencies encoded by noise-exposed AN fibers was not affected by SNHL, as quantified by TMTF 3- and 10-dB cut-off frequencies. These results suggest that physiological factors other than peripheral filtering may have a significant role in determining the range of modulation frequencies encoded in AN fibers. Furthermore, these neural data may help to explain the lack of a consistent association between perceptual measures of temporal resolution and degraded frequency selectivity.

The auditory steady state response: Far-field recordings from the chinchilla

Objective: Previous studies in our lab have found that the presentation of multiple ASSR-generating stimuli results in a decrease in ASSR amplitude when recorded from an electrode implanted in the chinchilla inferior colliculus. The purpose of the present experiment was to determine whether this same effect occurs in far-field recordings, i.e. recordings similar to those made in human subjects. The effect of inhalant anesthesia on ASSR amplitude in response to multiple stimuli was also investigated. Design: Stimuli consisted of three sinusoidally-amplitude modulated tones with carrier/modulation frequencies of (1/.095 kHz), (2/.1 kHz), or (4/.107 kHz). The modulated carriers were presented to the right ear either alone or in combination, while recordings were made from subdermal needle electrodes placed on the head. Study sample: Nine adult chinchillas. Results: A 20%–70% decrease in the response amplitude with the presentation of multiple ASSR-generating stimuli was found, which depended on both carrier frequency as well as stimulus pairing. In general, both the ASSR and the noise floor were reduced under anesthesia. Conclusions: The time savings obtained from presenting multiple stimuli simultaneously may not be as great as initially predicted, as the time saving is at least partially offset by the observed amplitude reduction.

A Modeling Study of the Responses of the Lateral Superior Olive to Ipsilateral Sinusoidally Amplitude-Modulated Tones

The lateral superior olive (LSO) is a brainstem nucleus that is classically understood to encode binaural information in high-frequency sounds. Previous studies have shown that LSO cells are sensitive to envelope interaural time difference in sinusoidally amplitude-modulated (SAM) tones (Joris and Yin, J Neurophysiol 73:1043–1062, 1995; Joris, J Neurophysiol 76:2137–2156, 1996) and that a subpopulation of LSO neurons exhibit low-threshold potassium currents mediated by Kv1 channels (Barnes-Davies et al., Eur J Neurosci 19:325–333, 2004). It has also been shown that in many LSO cells the average response rate to ipsilateral SAM tones decreases with modulation frequency above a few hundred Hertz (Joris and Yin, J Neurophysiol 79:253–269, 1998). This low-pass feature is not directly inherited from the inputs to the LSO since the response rate of these input neurons changes little with increasing modulation frequency. In the current study, an LSO cell model is developed to investigate mechanisms consistent with the responses described above, notably the emergent rate decrease with increasing frequency. The mechanisms explored included the effects of after-hyperpolarization (AHP) channels, the dynamics of low-threshold potassium channels (KLT), and the effects of background inhibition. In the model, AHP channels alone were not sufficient to induce the observed rate decrease at high modulation frequencies. The model also suggests that the background inhibition alone, possibly from the medial nucleus of the trapezoid body, can account for the small rate decrease seen in some LSO neurons, but could not explain the large rate decrease seen in other LSO neurons at high modulation frequencies. In contrast, both the small and large rate decreases were replicated when KLT channels were included in the LSO neuron model. These results support the conclusion that KLT channels may play a major role in the large rate decreases seen in some units and that background inhibition may be a contributing factor, a factor that could be adequate for small decreases.