Auditory enhancement in younger and older listeners with normal and impaired hearinga).

Auditory enhancement is a spectral contrast aftereffect that can facilitate the detection of novel events in an ongoing background. A single-interval paradigm combined with roved frequency content between trials can yield as much as 20 dB enhancement in young normal-hearing listeners. This study compared such enhancement in 15 listeners with sensorineural hearing loss with that in 15 age-matched adults and 15 young adults with normal audiograms. All groups were presented with stimulus levels of 70 dB sound pressure level (SPL) per component. The two groups with normal hearing were also tested at 45 dB SPL per component. The hearing-impaired listeners showed very little enhancement overall. However, when tested at the same high (70-dB) level, both young and age-matched normal-hearing listeners also showed substantially reduced enhancement, relative to that found at 45 dB SPL. Some differences in enhancement emerged between young and older normal-hearing listeners at the lower sound level. The results suggest that enhancement is highly level-dependent and may also decrease somewhat with age or slight hearing loss. Implications for hearing-impaired listeners may include a poorer ability to adapt to real-world acoustic variability, due in part to the higher levels at which sound must be presented to be audible.

Distinct Representations of Tonotopy and Pitch in Human Auditory Cortex.

Frequency-to-place mapping, or tonotopy, is a fundamental organizing principle throughout the auditory system, from the earliest stages of auditory processing in the cochlea to subcortical and cortical regions. Although cortical maps are referred to as tonotopic, it is unclear whether they simply reflect a mapping of physical frequency inherited from the cochlea, a computation of pitch based on the fundamental frequency, or a mixture of these two features. We used high-resolution functional magnetic resonance imaging (fMRI) to measure BOLD responses as male and female human participants listened to pure tones that varied in frequency or complex tones that varied in either spectral content (brightness) or fundamental frequency (pitch). Our results reveal evidence for pitch tuning in bilateral regions that partially overlap with the traditional tonotopic maps of spectral content. In general, primary regions within Heschl's gyri (HGs) exhibited more tuning to spectral content, whereas areas surrounding HGs exhibited more tuning to pitch.SIGNIFICANCE STATEMENT Tonotopy, an orderly mapping of frequency, is observed throughout the auditory system. However, it is not known whether the tonotopy observed in the cortex simply reflects the frequency spectrum (as in the ear) or instead represents the higher-level feature of fundamental frequency, or pitch. Using carefully controlled stimuli and high-resolution functional magnetic resonance imaging (fMRI), we separated these features to study their cortical representations. Our results suggest that tonotopy in primary cortical regions is driven predominantly by frequency, but also reveal evidence for tuning to pitch in regions that partially overlap with the tonotopic gradients but extend into nonprimary cortical areas. In addition to resolving ambiguities surrounding cortical tonotopy, our findings provide evidence that selectivity for pitch is distributed bilaterally throughout auditory cortex.

Neural auditory contrast enhancement in humans.

The perception of sensory events can be enhanced or suppressed by the surrounding spatial and temporal context in ways that facilitate the detection of novel objects and contribute to the perceptual constancy of those objects under variable conditions. In the auditory system, the phenomenon known as auditory enhancement reflects a general principle of contrast enhancement, in which a target sound embedded within a background sound becomes perceptually more salient if the background is presented first by itself. This effect is highly robust, producing an effective enhancement of the target of up to 25 dB (more than two orders of magnitude in intensity), depending on the task. Despite the importance of the effect, neural correlates of auditory contrast enhancement have yet to be identified in humans. Here, we used the auditory steady-state response to probe the neural representation of a target sound under conditions of enhancement. The probe was simultaneously modulated in amplitude with two modulation frequencies to distinguish cortical from subcortical responses. We found robust correlates for neural enhancement in the auditory cortical, but not subcortical, responses. Our findings provide empirical support for a previously unverified theory of auditory enhancement based on neural adaptation of inhibition and point to approaches for improving sensory prostheses for hearing loss, such as hearing aids and cochlear implants.

An online headphone screening test based on dichotic pitch.

Online experimental platforms can be used as an alternative to, or complement, lab-based research. However, when conducting auditory experiments via online methods, the researcher has limited control over the participants' listening environment. We offer a new method to probe one aspect of that environment, headphone use. Headphones not only provide better control of sound presentation but can also "shield" the listener from background noise. Here we present a rapid (< 3 min) headphone screening test based on Huggins Pitch (HP), a perceptual phenomenon that can only be detected when stimuli are presented dichotically. We validate this test using a cohort of "Trusted" online participants who completed the test using both headphones and loudspeakers. The same participants were also used to test an existing headphone test (AP test; Woods et al., 2017, Attention Perception Psychophysics). We demonstrate that compared to the AP test, the HP test has a higher selectivity for headphone users, rendering it as a compelling alternative to existing methods. Overall, the new HP test correctly detects 80% of headphone users and has a false-positive rate of 20%. Moreover, we demonstrate that combining the HP test with an additional test-either the AP test or an alternative based on a beat test (BT)-can lower the false-positive rate to ~ 7%. This should be useful in situations where headphone use is particularly critical (e.g., dichotic or spatial manipulations). Code for implementing the new tests is publicly available in JavaScript and through Gorilla (gorilla.sc).

Sensitivity to binaural temporal-envelope beats with single-sided deafness and a cochlear implant as a measure of tonotopic match (L).

For cochlear-implant users with near-normal contralateral hearing, a mismatch between the frequency-to-place mapping in the two ears could produce a suboptimal performance. This study assesses tonotopic matches via binaural interactions. Dynamic interaural time-difference sensitivity was measured using bandpass-filtered pulse trains at different rates in the acoustic and implanted ear, creating binaural envelope beats. Sensitivity to beats should peak when the same tonotopic region is stimulated in both ears. All nine participants detected dynamic interaural timing differences and demonstrated some frequency selectivity. This method provides a guide to frequency-to-place mapping without compensation for inherent latency differences between the acoustic and implanted ears.

Speech perception is similar for musicians and non-musicians across a wide range of conditions.

It remains unclear whether musical training is associated with improved speech understanding in a noisy environment, with different studies reaching differing conclusions. Even in those studies that have reported an advantage for highly trained musicians, it is not known whether the benefits measured in laboratory tests extend to more ecologically valid situations. This study aimed to establish whether musicians are better than non-musicians at understanding speech in a background of competing speakers or speech-shaped noise under more realistic conditions, involving sounds presented in space via a spherical array of 64 loudspeakers, rather than over headphones, with and without simulated room reverberation. The study also included experiments testing fundamental frequency discrimination limens (F0DLs), interaural time differences limens (ITDLs), and attentive tracking. Sixty-four participants (32 non-musicians and 32 musicians) were tested, with the two groups matched in age, sex, and IQ as assessed with Raven's Advanced Progressive matrices. There was a significant benefit of musicianship for F0DLs, ITDLs, and attentive tracking. However, speech scores were not significantly different between the two groups. The results suggest no musician advantage for understanding speech in background noise or talkers under a variety of conditions.

Pitch discrimination with mixtures of three concurrent harmonic complexes.

In natural listening contexts, especially in music, it is common to hear three or more simultaneous pitches, but few empirical or theoretical studies have addressed how this is achieved. Place and pattern-recognition theories of pitch require at least some harmonics to be spectrally resolved for pitch to be extracted, but it is unclear how often such conditions exist when multiple complex tones are presented together. In three behavioral experiments, mixtures of three concurrent complexes were filtered into a single bandpass spectral region, and the relationship between the fundamental frequencies and spectral region was varied in order to manipulate the extent to which harmonics were resolved either before or after mixing. In experiment 1, listeners discriminated major from minor triads (a difference of 1 semitone in one note of the triad). In experiments 2 and 3, listeners compared the pitch of a probe tone with that of a subsequent target, embedded within two other tones. All three experiments demonstrated above-chance performance, even in conditions where the combinations of harmonic components were unlikely to be resolved after mixing, suggesting that fully resolved harmonics may not be necessary to extract the pitch from multiple simultaneous complexes.

The upper frequency limit for the use of phase locking to code temporal fine structure in humans: A compilation of viewpoints.

The relative importance of neural temporal and place coding in auditory perception is still a matter of much debate. The current article is a compilation of viewpoints from leading auditory psychophysicists and physiologists regarding the upper frequency limit for the use of neural phase locking to code temporal fine structure in humans. While phase locking is used for binaural processing up to about 1500 Hz, there is disagreement regarding the use of monaural phase-locking information at higher frequencies. Estimates of the general upper limit proposed by the contributors range from 1500 to 10000 Hz. The arguments depend on whether or not phase locking is needed to explain psychophysical discrimination performance at frequencies above 1500 Hz, and whether or not the phase-locked neural representation is sufficiently robust at these frequencies to provide useable information. The contributors suggest key experiments that may help to resolve this issue, and experimental findings that may cause them to change their minds. This issue is of crucial importance to our understanding of the neural basis of auditory perception in general, and of pitch perception in particular.

Cortical Correlates of Attention to Auditory Features.

Pitch and timbre are two primary features of auditory perception that are generally considered independent. However, an increase in pitch (produced by a change in fundamental frequency) can be confused with an increase in brightness (an attribute of timbre related to spectral centroid) and vice versa. Previous work indicates that pitch and timbre are processed in overlapping regions of the auditory cortex, but are separable to some extent via multivoxel pattern analysis. Here, we tested whether attention to one or other feature increases the spatial separation of their cortical representations and if attention can enhance the cortical representation of these features in the absence of any physical change in the stimulus. Ten human subjects (four female, six male) listened to pairs of tone triplets varying in pitch, timbre, or both and judged which tone triplet had the higher pitch or brighter timbre. Variations in each feature engaged common auditory regions with no clear distinctions at a univariate level. Attending to one did not improve the separability of the neural representations of pitch and timbre at the univariate level. At the multivariate level, the classifier performed above chance in distinguishing between conditions in which pitch or timbre was discriminated. The results confirm that the computations underlying pitch and timbre perception are subserved by strongly overlapping cortical regions, but reveal that attention to one or other feature leads to distinguishable activation patterns even in the absence of physical differences in the stimuli.SIGNIFICANCE STATEMENT Although pitch and timbre are generally thought of as independent auditory features of a sound, pitch height and timbral brightness can be confused for one another. This study shows that pitch and timbre variations are represented in overlapping regions of auditory cortex, but that they produce distinguishable patterns of activation. Most importantly, the patterns of activation can be distinguished based on whether subjects attended to pitch or timbre even when the stimuli remained physically identical. The results therefore show that variations in pitch and timbre are represented by overlapping neural networks, but that attention to different features of the same sound can lead to distinguishable patterns of activation.

Examining replicability of an otoacoustic measure of cochlear function during selective attention.

Attention to a target stimulus within a complex scene often results in enhanced cortical representations of the target relative to the background. It remains unclear where along the auditory pathways attentional effects can first be measured. Anatomy suggests that attentional modulation could occur through corticofugal connections extending as far as the cochlea itself. Earlier attempts to investigate the effects of attention on human cochlear processing have revealed small and inconsistent effects. In this study, stimulus-frequency otoacoustic emissions were recorded from a total of 30 human participants as they performed tasks that required sustained selective attention to auditory or visual stimuli. In the first sample of 15 participants, emission magnitudes were significantly weaker when participants attended to the visual stimuli than when they attended to the auditory stimuli, by an average of 5.4 dB. However, no such effect was found in the second sample of 15 participants. When the data were pooled across samples, the average attentional effect was significant, but small (2.48 dB), with 12 of 30 listeners showing a significant effect, based on bootstrap analysis of the individual data. The results highlight the need for considering sources of individual differences and using large sample sizes in future investigations.

Effect of age and hearing loss on auditory stream segregation of speech sounds.

Segregating and understanding speech in complex environments is a major challenge for hearing-impaired (HI) listeners. It remains unclear to what extent these difficulties are dominated by direct interference, such as simultaneous masking, or by a failure of the mechanisms of stream segregation. This study compared older HI listeners' performance with that of young and older normal-hearing (NH) listeners in stream segregation tasks involving speech sounds. Listeners were presented with sequences of speech tokens, each consisting of a fricative consonant and a voiced vowel (CV). The CV tokens were concatenated into interleaved sequences that alternated in fundamental frequency (F0) and/or simulated vocal tract length (VTL). Each pair of interleaved sequences was preceded by a "word" consisting of two random tokens. The listeners were asked to indicate whether the word was present in the following interleaved sequences. The word, if present, occurred within one of the interleaved sequences, so that performance improved if the listeners were able to perceptually segregate the two sequences. Although HI listeners' identification of the speech tokens in isolation was poorer than that of the NH listeners, HI listeners were generally able to use both F0 and VTL cues to segregate the interleaved sequences. The results suggest that the difficulties experienced by HI listeners in complex acoustic environments cannot be explained by a loss of basic stream segregation abilities.

Learning for pitch and melody discrimination in congenital amusia.

Congenital amusia is currently thought to be a life-long neurogenetic disorder in music perception, impervious to training in pitch or melody discrimination. This study provides an explicit test of whether amusic deficits can be reduced with training. Twenty amusics and 20 matched controls participated in four sessions of psychophysical training involving either pure-tone (500 Hz) pitch discrimination or a control task of lateralization (interaural level differences for bandpass white noise). Pure-tone pitch discrimination at low, medium, and high frequencies (500, 2000, and 8000 Hz) was measured before and after training (pretest and posttest) to determine the specificity of learning. Melody discrimination was also assessed before and after training using the full Montreal Battery of Evaluation of Amusia, the most widely used standardized test to diagnose amusia. Amusics performed more poorly than controls in pitch but not localization discrimination, but both groups improved with practice on the trained stimuli. Learning was broad, occurring across all three frequencies and melody discrimination for all groups, including those who trained on the non-pitch control task. Following training, 11 of 20 amusics no longer met the global diagnostic criteria for amusia. A separate group of untrained controls (n = 20), who also completed melody discrimination and pretest, improved by an equal amount as trained controls on all measures, suggesting that the bulk of learning for the control group occurred very rapidly from the pretest. Thirty-one trained participants (13 amusics) returned one year later to assess long-term maintenance of pitch and melody discrimination. On average, there was no change in performance between posttest and one-year follow-up, demonstrating that improvements on pitch- and melody-related tasks in amusics and controls can be maintained. The findings indicate that amusia is not always a life-long deficit when using the current standard diagnostic criteria.

Auditory enhancement under simultaneous masking in normal-hearing and hearing-impaired listeners.

Auditory enhancement, where a target sound within a masker is rendered more audible by the prior presentation of the masker alone, may play an important role in auditory perception under variable everyday acoustic conditions. Cochlear hearing loss may reduce enhancement effects, potentially contributing to the difficulties experienced by hearing-impaired (HI) individuals in noisy and reverberant environments. However, it remains unknown whether, and by how much, enhancement under simultaneous masking is reduced in HI listeners. Enhancement of a pure tone under simultaneous masking with a multi-tone masker was measured in HI listeners and age-matched normal-hearing (NH) listeners as function of the spectral notch width of the masker, using stimuli at equal sensation levels as well as at equal sound pressure levels, but with the stimuli presented in noise to the NH listeners to maintain the equal sensation level between listener groups. The results showed that HI listeners exhibited some enhancement in all conditions. However, even when conditions were made as comparable as possible, in terms of effective spectral notch width and presentation level, the enhancement effect in HI listeners under simultaneous masking was reduced relative to that observed in NH listeners.

Weak Middle-Ear-Muscle Reflex in Humans with Noise-Induced Tinnitus and Normal Hearing May Reflect Cochlear Synaptopathy.

Chronic tinnitus is a prevalent hearing disorder, and yet no successful treatments or objective diagnostic tests are currently available. The aim of this study was to investigate the relationship between the presence of tinnitus and the strength of the middle-ear-muscle reflex (MEMR) in humans with normal and near-normal hearing. Clicks were used as test stimuli to obtain a wideband measure of the effect of reflex activation on ear-canal sound pressure. The reflex was elicited using a contralateral broadband noise. The results show that the reflex strength is significantly reduced in individuals with noise-induced continuous tinnitus and normal or near-normal audiometric thresholds compared with no-tinnitus controls. Due to a shallower growth of the reflex strength in the tinnitus group, the difference between the two groups increased with increasing elicitor level. No significant difference in the effect of tinnitus on the strength of the middle-ear muscle reflex was found between males and females. The weaker reflex could not be accounted for by differences in audiometric hearing thresholds between the tinnitus and control groups. Similarity between our findings in humans and the findings of a reduced middle-ear muscle reflex in noise-exposed animals suggests that noise-induced tinnitus in individuals with clinically normal hearing may be a consequence of cochlear synaptopathy, a loss of synaptic connections between inner hair cells (IHCs) in the cochlea and auditory-nerve (AN) fibers that has been termed hidden hearing loss.

An auditory illusion reveals the role of streaming in the temporal misallocation of perceptual objects.

This study investigates the neural correlates and processes underlying the ambiguous percept produced by a stimulus similar to Deutsch's 'octave illusion', in which each ear is presented with a sequence of alternating pure tones of low and high frequencies. The same sequence is presented to each ear, but in opposite phase, such that the left and right ears receive a high-low-high … and a low-high-low … pattern, respectively. Listeners generally report hearing the illusion of an alternating pattern of low and high tones, with all the low tones lateralized to one side and all the high tones lateralized to the other side. The current explanation of the illusion is that it reflects an illusory feature conjunction of pitch and perceived location. Using psychophysics and electroencephalogram measures, we test this and an alternative hypothesis involving synchronous and sequential stream segregation, and investigate potential neural correlates of the illusion. We find that the illusion of alternating tones arises from the synchronous tone pairs across ears rather than sequential tones in one ear, suggesting that the illusion involves a misattribution of time across perceptual streams, rather than a misattribution of location within a stream. The results provide new insights into the mechanisms of binaural streaming and synchronous sound segregation.This article is part of the themed issue 'Auditory and visual scene analysis'.

Sustained Cortical and Subcortical Measures of Auditory and Visual Plasticity following Short-Term Perceptual Learning.

Short-term training can lead to improvements in behavioral discrimination of auditory and visual stimuli, as well as enhanced EEG responses to those stimuli. In the auditory domain, fluency with tonal languages and musical training has been associated with long-term cortical and subcortical plasticity, but less is known about the effects of shorter-term training. This study combined electroencephalography (EEG) and behavioral measures to investigate short-term learning and neural plasticity in both auditory and visual domains. Forty adult participants were divided into four groups. Three groups trained on one of three tasks, involving discrimination of auditory fundamental frequency (F0), auditory amplitude modulation rate (AM), or visual orientation (VIS). The fourth (control) group received no training. Pre- and post-training tests, as well as retention tests 30 days after training, involved behavioral discrimination thresholds, steady-state visually evoked potentials (SSVEP) to the flicker frequencies of visual stimuli, and auditory envelope-following responses simultaneously evoked and measured in response to rapid stimulus F0 (EFR), thought to reflect subcortical generators, and slow amplitude modulation (ASSR), thought to reflect cortical generators. Enhancement of the ASSR was observed in both auditory-trained groups, not specific to the AM-trained group, whereas enhancement of the SSVEP was found only in the visually-trained group. No evidence was found for changes in the EFR. The results suggest that some aspects of neural plasticity can develop rapidly and may generalize across tasks but not across modalities. Behaviorally, the pattern of learning was complex, with significant cross-task and cross-modal learning effects.

Sequential stream segregation of voiced and unvoiced speech sounds based on fundamental frequency.

Differences in fundamental frequency (F0) between voiced sounds are known to be a strong cue for stream segregation. However, speech consists of both voiced and unvoiced sounds, and less is known about whether and how the unvoiced portions are segregated. This study measured listeners' ability to integrate or segregate sequences of consonant-vowel tokens, comprising a voiceless fricative and a vowel, as a function of the F0 difference between interleaved sequences of tokens. A performance-based measure was used, in which listeners detected the presence of a repeated token either within one sequence or between the two sequences (measures of voluntary and obligatory streaming, respectively). The results showed a systematic increase of voluntary stream segregation as the F0 difference between the two interleaved sequences increased from 0 to 13 semitones, suggesting that F0 differences allowed listeners to segregate speech sounds, including the unvoiced portions. In contrast to the consistent effects of voluntary streaming, the trend towards obligatory stream segregation at large F0 differences failed to reach significance. Listeners were no longer able to perform the voluntary-streaming task reliably when the unvoiced portions were removed from the stimuli, suggesting that the unvoiced portions were used and correctly segregated in the original task. The results demonstrate that streaming based on F0 differences occurs for natural speech sounds, and that the unvoiced portions are correctly assigned to the corresponding voiced portions.

Representations of Pitch and Timbre Variation in Human Auditory Cortex.

Pitch and timbre are two primary dimensions of auditory perception, but how they are represented in the human brain remains a matter of contention. Some animal studies of auditory cortical processing have suggested modular processing, with different brain regions preferentially coding for pitch or timbre, whereas other studies have suggested a distributed code for different attributes across the same population of neurons. This study tested whether variations in pitch and timbre elicit activity in distinct regions of the human temporal lobes. Listeners were presented with sequences of sounds that varied in either fundamental frequency (eliciting changes in pitch) or spectral centroid (eliciting changes in brightness, an important attribute of timbre), with the degree of pitch or timbre variation in each sequence parametrically manipulated. The BOLD responses from auditory cortex increased with increasing sequence variance along each perceptual dimension. The spatial extent, region, and laterality of the cortical regions most responsive to variations in pitch or timbre at the univariate level of analysis were largely overlapping. However, patterns of activation in response to pitch or timbre variations were discriminable in most subjects at an individual level using multivoxel pattern analysis, suggesting a distributed coding of the two dimensions bilaterally in human auditory cortex. SIGNIFICANCE STATEMENT: Pitch and timbre are two crucial aspects of auditory perception. Pitch governs our perception of musical melodies and harmonies, and conveys both prosodic and (in tone languages) lexical information in speech. Brightness-an aspect of timbre or sound quality-allows us to distinguish different musical instruments and speech sounds. Frequency-mapping studies have revealed tonotopic organization in primary auditory cortex, but the use of pure tones or noise bands has precluded the possibility of dissociating pitch from brightness. Our results suggest a distributed code, with no clear anatomical distinctions between auditory cortical regions responsive to changes in either pitch or timbre, but also reveal a population code that can differentiate between changes in either dimension within the same cortical regions.

Effects of auditory enhancement on the loudness of masker and target components.

Auditory enhancement refers to the observation that the salience of one spectral region (the "signal") of a broadband sound can be enhanced and can "pop out" from the remainder of the sound (the "masker") if it is preceded by the broadband sound without the signal. The present study investigated auditory enhancement as an effective change in loudness, to determine whether it reflects a change in the loudness of the signal, the masker, or both. In the first experiment, the 500-ms precursor, an inharmonic complex with logarithmically spaced components, was followed after a 50-ms gap by the 100-ms signal or masker alone, the loudness of which was compared with that of the same signal or masker presented 2 s later. In the second experiment, the loudness of the signal embedded in the masker was assessed with and without a precursor using the same method, as was the loudness of the entire signal-plus-masker complex. The results suggest that the precursor does not affect the loudness of the signal or the masker alone, but enhances the loudness of the signal in the presence of the masker, while leaving the loudness of the surrounding masker unaffected. The results are consistent with an explanation based on "adaptation of inhibition" [Viemeister and Bacon (1982). J. Acoust. Soc. Am. 71, 1502-1507].

Using individual differences to test the role of temporal and place cues in coding frequency modulation.

The question of how frequency is coded in the peripheral auditory system remains unresolved. Previous research has suggested that slow rates of frequency modulation (FM) of a low carrier frequency may be coded via phase-locked temporal information in the auditory nerve, whereas FM at higher rates and/or high carrier frequencies may be coded via a rate-place (tonotopic) code. This hypothesis was tested in a cohort of 100 young normal-hearing listeners by comparing individual sensitivity to slow-rate (1-Hz) and fast-rate (20-Hz) FM at a carrier frequency of 500 Hz with independent measures of phase-locking (using dynamic interaural time difference, ITD, discrimination), level coding (using amplitude modulation, AM, detection), and frequency selectivity (using forward-masking patterns). All FM and AM thresholds were highly correlated with each other. However, no evidence was obtained for stronger correlations between measures thought to reflect phase-locking (e.g., slow-rate FM and ITD sensitivity), or between measures thought to reflect tonotopic coding (fast-rate FM and forward-masking patterns). The results suggest that either psychoacoustic performance in young normal-hearing listeners is not limited by peripheral coding, or that similar peripheral mechanisms limit both high- and low-rate FM coding.