Large group differences in binaural sensitivity are represented in preattentive responses from auditory cortex.

Correlated sounds presented to two ears are perceived as compact and centrally lateralized, whereas decorrelation between ears leads to intracranial image widening. Though most listeners have fine resolution for perceptual changes in interaural correlation (IAC), some investigators have reported large variability in IAC thresholds, and some normal-hearing listeners even exhibit seemingly debilitating IAC thresholds. It is unknown whether or not this variability across individuals and outlier manifestations are a product of task difficulty, poor training, or a neural deficit in the binaural auditory system. The purpose of this study was first to identify listeners with normal and abnormal IAC resolution, second to evaluate the neural responses elicited by IAC changes, and third to use a well-established model of binaural processing to determine a potential explanation for observed individual variability. Nineteen subjects were enrolled in the study, eight of whom were identified as poor performers in the IAC-threshold task. Global scalp responses (N1 and P2 amplitudes of an auditory change complex) in the individuals with poor IAC behavioral thresholds were significantly smaller than for listeners with better IAC resolution. Source-localized evoked responses confirmed this group effect in multiple subdivisions of the auditory cortex, including Heschl's gyrus, planum temporale, and the temporal sulcus. In combination with binaural modeling results, this study provides objective electrophysiological evidence of a binaural processing deficit linked to internal noise, that corresponds to very poor IAC thresholds in listeners that otherwise have normal audiometric profiles and lack spatial hearing complaints.NEW & NOTEWORTHY Group differences in the perception of interaural correlation (IAC) were observed in human adults with normal audiometric sensitivity. These differences were reflected in cortical-evoked activity measured via electroencephalography (EEG). For some participants, weak representation of the binaural cue at the cortical level in preattentive N1-P2 cortical responses may be indicative of a potential processing deficit. Such a deficit may be related to a poorly understood condition known as hidden hearing loss.

Remote auditory assessment using Portable Automated Rapid Testing (PART) and participant-owned devices.

Remote testing of auditory function can be transformative to both basic research and hearing healthcare; however, historically, many obstacles have limited remote collection of reliable and valid auditory psychometric data. Here, we report performance on a battery of auditory processing tests using a remotely administered system, Portable Automatic Rapid Testing. We compare a previously reported dataset collected in a laboratory setting with the same measures using uncalibrated, participant-owned devices in remote settings (experiment 1, n = 40) remote with and without calibrated hardware (experiment 2, n = 36) and laboratory with and without calibrated hardware (experiment 3, n = 58). Results were well-matched across datasets and had similar reliability, but overall performance was slightly worse than published norms. Analyses of potential nuisance factors such as environmental noise, distraction, or lack of calibration failed to provide reliable evidence that these factors contributed to the observed variance in performance. These data indicate feasibility of remote testing of suprathreshold auditory processing using participants' own devices. Although the current investigation was limited to young participants without hearing difficulties, its outcomes demonstrate the potential for large-scale, remote hearing testing of more hearing-diverse populations both to advance basic science and to establish the clinical viability of auditory remote testing.

Selective auditory attention modulates cortical responses to sound location change in younger and older adults.

The present study measured scalp potentials in response to low-frequency, narrowband noise bursts changing location in the front, azimuthal plane. At question was whether selective auditory attention has a modulatory effect on the cortical encoding of spatial change and whether older listeners with normal-hearing thresholds would show depressed cortical representation for spatial changes relative to younger listeners. Young and older normal-hearing listeners were instructed to either passively listen to the stimulus presentation or actively attend to a single location (either 30° left or right of midline) and detect when a noise stream moved to the attended location. Prominent peaks of the electroencephalographic scalp waveforms were compared across groups, locations, and attention conditions. In addition, an opponent-channel model of spatial coding was performed to capture the effect of attention on spatial-change tuning. Younger listeners showed not only larger responses overall but a greater dynamic range in their response to location changes. Results suggest that younger listeners were acquiring and encoding key spatial cues at early cortical processing areas. On the other hand, each group exhibited modulatory effects of attention to spatial-change tuning, indicating that both younger and older listeners selectively attend to space in a manner that amplifies the available signal.NEW & NOTEWORTHY In complex acoustic scenes, listeners take advantage of spatial cues to selectively attend to sounds that are deemed immediately relevant. At the neural level, selective attention amplifies electrical responses to spatial changes. We tested whether older and younger listeners have comparable modulatory effects of attention to stimuli moving in the free field. Results indicate that although older listeners do have depressed overall responses, selective attention enhances spatial-change tuning in younger and older listeners alike.

Temporal integration of monaural and dichotic frequency modulation.

Frequency modulation (FM) detection at low modulation frequencies is commonly used as an index of temporal fine-structure processing. The present study evaluated the rate of improvement in monaural and dichotic FM across a range of test parameters. In experiment I, dichotic and monaural FM detection was measured as a function of duration and modulator starting phase. Dichotic FM thresholds were lower than monaural FM thresholds and the modulator starting phase had no effect on detection. Experiment II measured monaural FM detection for signals that differed in modulation rate and duration such that the improvement with duration in seconds (carrier) or cycles (modulator) was compared. Monaural FM detection improved monotonically with the number of modulation cycles, suggesting that the modulator is extracted prior to detection. Experiment III measured dichotic FM detection for shorter signal durations to test the hypothesis that dichotic FM relies primarily on the signal onset. The rate of improvement decreased as duration increased, which is consistent with the use of primarily onset cues for the detection of dichotic FM. These results establish that improvement with duration occurs as a function of the modulation cycles at a rate consistent with the independent-samples model for monaural FM, but later cycles contribute less to detection in dichotic FM.

Exponential spectro-temporal modulation generation.

Traditionally, real-time generation of spectro-temporally modulated noise has been performed on a linear amplitude scale, partially due to computational constraints. Experiments often require modulation that is sinusoidal on a logarithmic amplitude scale as a result of the many perceptual and physiological measures which scale linearly with exponential changes in the signal magnitude. A method is presented for computing exponential spectro-temporal modulation, showing that it can be expressed analytically as a sum over linearly offset sidebands with component amplitudes equal to the values of the modified Bessel function of the first kind. This approach greatly improves the efficiency and precision of stimulus generation over current methods, facilitating real-time generation for a broad range of carrier and envelope signals.

Portable Automated Rapid Testing (PART) for auditory assessment: Validation in a young adult normal-hearing population.

This study aims to determine the degree to which Portable Automated Rapid Testing (PART), a freely available program running on a tablet computer, is capable of reproducing standard laboratory results. Undergraduate students were assigned to one of three within-subject conditions that examined repeatability of performance on a battery of psychoacoustical tests of temporal fine structure processing, spectro-temporal amplitude modulation, and targets in competition. The repeatability condition examined test/retest with the same system, the headphones condition examined the effects of varying headphones (passive and active noise-attenuating), and the noise condition examined repeatability in the presence of recorded cafeteria noise. In general, performance on the test battery showed high repeatability, even across manipulated conditions, and was similar to that reported in the literature. These data serve as validation that suprathreshold psychoacoustical tests can be made accessible to run on consumer-grade hardware and perform in less controlled settings. This dataset also provides a distribution of thresholds that can be used as a normative baseline against which auditory dysfunction can be identified in future work.

Development of the Continuous Number Identification Test (CNIT): feasibility of dynamic assessment of speech intelligibility.

Objective: The present study was motivated by a need for a speech intelligibility test capable of indexing dynamic changes in the environment and adaptive processing in hearing aids. The Continuous Number Identification Test (CNIT) was developed to meet these aims.Design: From one location in the free field, speech was presented in noise (∼2 words/s) with a 100-ms inter-word interval. On average, every fourth word was a target digit and all other words were monosyllabic words. Non-numeric words had a fixed presentation level such that the dominant signal-to-noise-ratio (SNR) was held at +6 dB SNR relative to background maskers. To prevent ceiling effects, however, targets were presented at a user-specific SNR, determined by an initial adaptive-tracking procedure that estimated the 79.4% speech reception threshold.Study sample: Ten normal-hearing listeners participated.Results: The CNIT showed comparable psychometric qualities of other established speech tests for long time scales (Exp. 1). Target-location changes did not affect performance on the CNIT (Exp. 2), but the test did show high temporal resolution in assessing sudden changes to SNR (Exp. 3).Conclusions: The CNIT is highly customisable, and the initial experiments tested feasibility of its primary features which set it apart from currently available speech-in-noise tests.

Electrophysiological responses to lateral shifts are not consistent with opponent-channel processing of interaural level differences.

Cortical encoding of auditory space relies on two major peripheral cues, interaural time difference (ITD) and interaural level difference (ILD) of the sounds arriving at a listener's ears. In much of the precortical auditory pathway, ITD and ILD cues are processed independently, and it is assumed that cue integration is a higher order process. However, there remains debate on how ITDs and ILDs are encoded in the cortex and whether they share a common mechanism. The present study used electroencephalography (EEG) to measure evoked cortical potentials from narrowband noise stimuli with imposed binaural cue changes. Previous studies have similarly tested ITD shifts to demonstrate that neural populations broadly favor one spatial hemifield over the other, which is consistent with an opponent-channel model that computes the relative activity between broadly tuned neural populations. However, it is still a matter of debate whether the same coding scheme applies to ILDs and, if so, whether processing the two binaural cues is distributed across similar regions of the cortex. The results indicate that ITD and ILD cues have similar neural signatures with respect to the monotonic responses to shift magnitude; however, the direction of the shift did not elicit responses equally across cues. Specifically, ITD shifts evoked greater responses for outward than inward shifts, independently of the spatial hemifield of the shift, whereas ILD-shift responses were dependent on the hemifield in which the shift occurred. Active cortical structures showed only minor overlap between responses to cues, suggesting the two are not represented by the same pathway.NEW & NOTEWORTHY Interaural time differences (ITDs) and interaural level differences (ILDs) are critical to locating auditory sources in the horizontal plane. The higher order perceptual feature of auditory space is thought to be encoded together by these binaural differences, yet evidence of their integration in cortex remains elusive. Although present results show some common effects between the two cues, key differences were observed that are not consistent with an ITD-like opponent-channel process for ILD encoding.

Cortical Correlates of Binaural Temporal Processing Deficits in Older Adults.

OBJECTIVES: This study was designed to evaluate binaural temporal processing in young and older adults using a binaural masking level difference (BMLD) paradigm. Using behavioral and electrophysiological measures within the same listeners, a series of stimulus manipulations was used to evaluate the relative contribution of binaural temporal fine-structure and temporal envelope cues. We evaluated the hypotheses that age-related declines in the BMLD task would be more strongly associated with temporal fine-structure than envelope cues and that age-related declines in behavioral measures would be correlated with cortical auditory evoked potential (CAEP) measures. DESIGN: Thirty adults participated in the study, including 10 young normal-hearing, 10 older normal-hearing, and 10 older hearing-impaired adults with bilaterally symmetric, mild-to-moderate sensorineural hearing loss. Behavioral and CAEP thresholds were measured for diotic (So) and dichotic (Sπ) tonal signals presented in continuous diotic (No) narrowband noise (50-Hz wide) maskers. Temporal envelope cues were manipulated by using two different narrowband maskers; Gaussian noise (GN) with robust envelope fluctuations and low-noise noise (LNN) with minimal envelope fluctuations. The potential to use temporal fine-structure cues was controlled by varying the signal frequency (500 or 4000 Hz), thereby relying on the natural decline in phase-locking with increasing frequency. RESULTS: Behavioral and CAEP thresholds were similar across groups for diotic conditions, while the masking release in dichotic conditions was larger for younger than for older participants. Across all participants, BMLDs were larger for GN than LNN and for 500-Hz than for 4000-Hz conditions, where envelope and fine-structure cues were most salient, respectively. Specific age-related differences were demonstrated for 500-Hz dichotic conditions in GN and LNN, reflecting reduced binaural temporal fine-structure coding. No significant age effects were observed for 4000-Hz dichotic conditions, consistent with similar use of binaural temporal envelope cues across age in these conditions. For all groups, thresholds and derived BMLD values obtained using the behavioral and CAEP methods were strongly correlated, supporting the notion that CAEP measures may be useful as an objective index of age-related changes in binaural temporal processing. CONCLUSIONS: These results demonstrate an age-related decline in the processing of binaural temporal fine-structure cues with preserved temporal envelope coding that was similar with and without mild-to-moderate peripheral hearing loss. Such age-related changes can be reliably indexed by both behavioral and CAEP measures in young and older adults.

Distribution of spectral modulation transfer functions in a young, normal-hearing population.

Spectral modulation transfer functions (SMTFs) were measured in 49 young (18-35 years of age) normal-hearing listeners. Noise carriers spanned six octaves from 200 to 12 800 Hz. Sinusoidal (on a log-amplitude scale) spectral modulation with random starting phase was superimposed on the carrier at spectral modulation frequencies of 0.25, 0.5, 1.0, 2.0, 4.0, and 8.0 cycles/octave. Modulation detection thresholds (in dB) yielded SMTFs that were bandpass in nature, consistent with previous investigations reporting data for only a few subjects. Thresholds were notably consistent across subjects despite minimal practice. Population statistics are reported that may serve as reference data for future studies.

Reduced temporal processing in older, normal-hearing listeners evident from electrophysiological responses to shifts in interaural time difference.

Previous electrophysiological studies of interaural time difference (ITD) processing have demonstrated that ITDs are represented by a nontopographic population rate code. Rather than narrow tuning to ITDs, neural channels have broad tuning to ITDs in either the left or right auditory hemifield, and the relative activity between the channels determines the perceived lateralization of the sound. With advancing age, spatial perception weakens and poor temporal processing contributes to declining spatial acuity. At present, it is unclear whether age-related temporal processing deficits are due to poor inhibitory controls in the auditory system or degraded neural synchrony at the periphery. Cortical processing of spatial cues based on a hemifield code are susceptible to potential age-related physiological changes. We consider two distinct predictions of age-related changes to ITD sensitivity: declines in inhibitory mechanisms would lead to increased excitation and medial shifts to rate-azimuth functions, whereas a general reduction in neural synchrony would lead to reduced excitation and shallower slopes in the rate-azimuth function. The current study tested these possibilities by measuring an evoked response to ITD shifts in a narrow-band noise. Results were more in line with the latter outcome, both from measured latencies and amplitudes of the global field potentials and source-localized waveforms in the left and right auditory cortices. The measured responses for older listeners also tended to have reduced asymmetric distribution of activity in response to ITD shifts, which is consistent with other sensory and cognitive processing models of aging.

The psychophysics of roughness applied to dysphonic voice.

Roughness is a sound quality that has been related to the amplitude modulation characteristics of the acoustic stimulus. Roughness also is considered one of the primary elements of voice quality associated with natural variations across normal voices and is a salient feature of many dysphonic voices. It is known that the roughness of tonal stimuli is dependent on the frequency and depth of amplitude modulation and on the carrier frequency. Here, it is determined if similar dependencies exist for voiced speech stimuli. Knowledge of such dependencies can lead to a better understanding of the acoustic characteristics of vocal roughness along the continuum of normal to dysphonic and may facilitate computational estimates of vocal roughness. Synthetic vowel stimuli were modeled after talkers selected from the Satloff/Heman-Ackah disordered voice database. To parametrically control amplitude modulation frequency and depth, synthesized stimuli had minimal amplitude fluctuations, and amplitude modulation was superimposed with the desired frequency and depth. Perceptual roughness judgments depended on amplitude modulation frequency and depth in a manner that closely matched data from tonal carriers. The dependence of perceived roughness on amplitude modulation frequency and depth closely matched the roughness of sinusoidal carriers as reported by Fastl and Zwicker [(2007) Psychoacoustics: Facts and Models, 3rd ed. (Springer, New York)].

The focal index as a singular metric for beamforming effectiveness.

Hearing-impaired individuals frequently cite intelligibility problems in multi-talker environments. Microphone arrays performing time-delay beamforming address conditions of poor signal-to-noise ratio by spatially filtering incoming sound. Existing beam pattern metrics including peak side lobe level, integrated side lobe level, beamwidth, and planar directivity index fail to quantitatively capture all elements essential for improving speech intelligibility in multi-talker situations. The focal index (FI) was developed to address these deficiencies. Simulations were performed to exemplify the robust nature of the FI and to demonstrate the utility of this metric for driving array parameter selection. Beam patterns were generated and the metrics were calculated and evaluated against the strict unidirectional requirements for the array. Array performance was assessed by human subjects in a speech recognition task that incorporated competing speech from multiple locations. Simulations of array output were presented under conditions differing in array sparsity. The resulting human subject data were used to demonstrate the linear relationship (R(2) > 0.975) between speech-intelligibility-weighted FI (SII-FI) and the signal-to-noise ratio thresholds for 20% and 80% correct responses. Data indicate that the FI and SII-FI are robust singular metrics for determining the effectiveness of the array.

A Step Toward Precision Audiology: Individual Differences and Characteristic Profiles From Auditory Perceptual and Cognitive Abilities.

Older adults with normal hearing or with age-related hearing loss face challenges when listening to speech in noisy environments. To better serve individuals with communication difficulties, precision diagnostics are needed to characterize individuals' auditory perceptual and cognitive abilities beyond pure tone thresholds. These abilities can be heterogenous across individuals within the same population. The goal of the present study is to consider the suprathreshold variability and develop characteristic profiles for older adults with normal hearing (ONH) and with hearing loss (OHL). Auditory perceptual and cognitive abilities were tested on ONH (n = 20) and OHL (n = 20) on an abbreviated test battery using portable automated rapid testing. Using cluster analyses, three main profiles were revealed for each group, showing differences in auditory perceptual and cognitive abilities despite similar audiometric thresholds. Analysis of variance showed that ONH profiles differed in spatial release from masking, speech-in-babble testing, cognition, tone-in-noise, and binaural temporal processing abilities. The OHL profiles differed in spatial release from masking, speech-in-babble testing, cognition, and tolerance to background noise performance. Correlation analyses showed significant relationships between auditory and cognitive abilities in both groups. This study showed that auditory perceptual and cognitive deficits can be present to varying degrees in the presence of audiometrically normal hearing and among listeners with similar degrees of hearing loss. The results of this study inform the need for taking individual differences into consideration and developing targeted intervention options beyond pure tone thresholds and speech testing.

Counseling Protocol for a Transitional Intervention for Debilitating Hyperacusis.

INTRODUCTION: This clinical focus article describes a structured counseling protocol for use with protected sound management and therapeutic sound in a transitional intervention for debilitating hyperacusis. The counseling protocol and its associated visual aids are crafted as a teaching tool to educate affected individuals about hyperacusis and encourage their acceptance of a transitional intervention. DESCRIPTION OF COUNSELING COMPONENTS: The counseling protocol includes five components. First, the patient's audiometric results are reviewed with the patient, and the transitional intervention is introduced. An overview of peripheral auditory structures and central neural pathways and the concept of central gain are covered in the second and third components. Maladaptive hyper-gain processes within the auditory neural pathways, which underlie the hyperacusis condition, and associated connections with nonauditory processes responsible for negative reactions to hyperacusis are covered in the fourth component. Detrimental effects from misused hearing protection devices (HPDs) and the necessity to wean the patient from overuse of HPDs are also discussed. In the fifth component, the importance of therapeutic sound is introduced as a tool to downregulate hyper-gain activity within the auditory pathways; its implementation in uncontrolled and controlled sound environments is described. It is explained that, over the course of the transitional intervention, recalibration of the hyper-gain processes will be ongoing, leading to restoration of normal homeostasis within the auditory pathways. In turn, associated activation of reactive nonauditory processes, which contribute to hyperacusis-related distress, will be reduced or eliminated. As recalibration progresses, there will be less need for protected sound management and sound therapy. Sound tolerance will improve, hyperacusis will subside, and daily activities in typical healthy sound environments will again become routine. RESULTS AND CONCLUSION: The combination of counseling with protected sound management and therapeutic sound is highlighted in companion reports, including a summary of the outcomes of a successful trial of the transitional intervention.

Background and Rationale for a Transitional Intervention for Debilitating Hyperacusis.

PURPOSE: This report provides the experimental, clinical, theoretical, and historical background that motivated a patented transitional intervention and its implementation and evaluation in a field trial for mitigation of debilitating loudness-based hyperacusis (LH). BACKGROUND AND RATIONALE: Barriers for ameliorating LH, which is differentiated here from other forms of hyperacusis, are delineated, including counterproductive management and treatment strategies that may exacerbate the condition. Evidence for hyper-gain central auditory processes as the bases for LH and the associated LH-induced distress and stress responses are presented. This presentation is followed by an overview of prior efforts to use counseling and therapeutic sound as interventional tools for recalibrating the hyper-gain LH response. We also consider previous efforts to use output-limiting sound-protection devices in the management of LH. This historical background lays the foundation for our transitional intervention protocol and its implementation and evaluation in a field trial. CONCLUSIONS: The successful implementation and evaluation of a transitional intervention, which we document in the outcomes of a companion proof-of-concept field trial in this issue, build on our prior efforts and those of others to understand, manage, and treat hyperacusis. These efforts to overcome significant barriers and vexing long-standing challenges in the management and treatment of LH, as reviewed here, are the pillars of the transitional intervention and its primary components, namely, counseling combined with protective sound management and therapeutic sound, which we detail in separate reports in this issue.

Results of a 6-Month Field Trial of a Transitional Intervention for Debilitating Hyperacusis.

PURPOSE: We present results from a 6-month field trial of a transitional intervention for debilitating primary hyperacusis, including a combination of structured counseling; promotion of safe, comfortable, and healthy sound exposure; and therapeutic broadband sound from sound generators. This intervention is designed to overcome barriers to successful delivery of therapeutic sound as a tool to downregulate neural hyperactivity in the central auditory pathways (i.e., the maladaptive mechanism believed to account for primary hyperacusis) and, together with the counseling, reduce the associated negative emotional and physiological reactions to debilitating hyperacusis. METHOD: Twelve adults with normal or near-normal audiometric thresholds, complaints consistent with their pretreatment loudness discomfort levels ≤ 75 dB HL at multiple frequencies, and hearing questionnaire scores ≥ 24 completed the sound therapy-based intervention. The low-level broadband therapeutic sound was delivered by ear-level devices fitted bilaterally with either occluding earpieces and output-limiting loudness suppression (LS; to limit exposure to offensive sound levels) or open domes to maximize comfort and exposure to sound therapy. Thresholds for LS (primary outcome) were incrementally adjusted across six monthly visits based on treatment-driven change in loudness judgments for running speech in sound field. Secondary outcomes included categorical loudness judgments, speech understanding, and questionnaires to assess the hyperacusis problem, quality of life, and depression. An exit survey assessed satisfaction with and benefit from the intervention and the counseling, therapeutic sound, and LS components. RESULTS: The mean change in LS (34.8 dB) was highly significant (effect size = 2.045). Eleven of 12 participants achieved ≥ 16-dB change in LS, consistent with highly significant change in sound-based questionnaire scores. Exit surveys indicated satisfaction with and benefit from the intervention. CONCLUSION: The transitional intervention was successful in improving the hyperacusis conditions of 11 of 12 study participants while reducing their sound avoidance behaviors and reliance on sound protection.

Device and Fitting Protocol for a Transitional Intervention for Debilitating Hyperacusis.

PURPOSE: This report describes a hearing device and corresponding fitting protocol designed for use in a transitional intervention for debilitating loudness-based hyperacusis. METHOD: The intervention goal is to transition patients with hyperacusis from their typical counterproductive sound avoidance behaviors (i.e., sound attenuation and limited exposure to healthy low-level sounds) into beneficial sound therapy treatment that can expand their dynamic range to the point where they can tolerate everyday sounds and experience an improved quality of life. This requires a combination of counseling and sound therapy, the latter of which is provided via the hearing device technology, signal processing, and precision fitting approach described in this report. The device combines a miniature behind-the-ear sound processor and a custom earpiece designed to maximize the attenuation of external sounds. Output-limiting loudness suppression is used to restrict exposure to offending high-level sounds while unity gain amplification maximizes exposure to healthy and tolerable lower level sounds. The fitting process includes measurement of the real-ear unaided response, the real-ear measurement (REM) system noise floor, the real-ear occluded response, real-ear insertion gain, and the output limit. With these measurements, the device can achieve the prescribed unity gain needed to provide transparent access to comfortable sound levels. It also supports individualized configuration of the therapeutic noise from an on-board sound generator and adaptive output limiting based on treatment-induced increases in dynamic range. RESULTS AND CONCLUSION: The utility of this device and fitting protocol, in combination with structured counseling, is highlighted in the outcomes of a successful 6-month trial of the transitional intervention described in a companion report in this issue.

Perceptual learning evidence for tuning to spectrotemporal modulation in the human auditory system.

Natural sounds are characterized by complex patterns of sound intensity distributed across both frequency (spectral modulation) and time (temporal modulation). Perception of these patterns has been proposed to depend on a bank of modulation filters, each tuned to a unique combination of a spectral and a temporal modulation frequency. There is considerable physiological evidence for such combined spectrotemporal tuning. However, direct behavioral evidence is lacking. Here we examined the processing of spectrotemporal modulation behaviorally using a perceptual-learning paradigm. We trained human listeners for ∼1 h/d for 7 d to discriminate the depth of spectral (0.5 cyc/oct; 0 Hz), temporal (0 cyc/oct; 32 Hz), or upward spectrotemporal (0.5 cyc/oct; 32 Hz) modulation. Each trained group learned more on their respective trained condition than did controls who received no training. Critically, this depth-discrimination learning did not generalize to the trained stimuli of the other groups or to downward spectrotemporal (0.5 cyc/oct; -32 Hz) modulation. Learning on discrimination also led to worsening on modulation detection, but only when the same spectrotemporal modulation was used for both tasks. Thus, these influences of training were specific to the trained combination of spectral and temporal modulation frequencies, even when the trained and untrained stimuli had one modulation frequency in common. This specificity indicates that training modified circuitry that had combined spectrotemporal tuning, and therefore that circuits with such tuning can influence perception. These results are consistent with the possibility that the auditory system analyzes sounds through filters tuned to combined spectrotemporal modulation.

Psychometric properties associated with perceived vocal roughness using a matching task.

A psychophysical matching paradigm has been used to better quantify voice quality under laboratory conditions. The goals of this study were to establish which of two candidate comparison stimuli would best ensure that the range of perceived vocal roughness could be adequately bracketed using a matching task and to provide a general solution to the problem of estimating vocal roughness. Psychometric functions for roughness matching indicated that a speech-like sawtooth-plus-noise complex (20 dB signal-to-noise ratio) amplitude modulated by a sinusoidal function raised to the 4th power yielded a comparison stimulus with a perceptual dynamic range well suited for roughness matching.