Discrimination and sensorimotor adaptation of self-produced vowels in cochlear implant users.

Humans rely on auditory feedback to monitor and adjust their speech for clarity. Cochlear implants (CIs) have helped over a million people restore access to auditory feedback, which significantly improves speech production. However, there is substantial variability in outcomes. This study investigates the extent to which CI users can use their auditory feedback to detect self-produced sensory errors and make adjustments to their speech, given the coarse spectral resolution provided by their implants. First, we used an auditory discrimination task to assess the sensitivity of CI users to small differences in formant frequencies of their self-produced vowels. Then, CI users produced words with altered auditory feedback in order to assess sensorimotor adaptation to auditory error. Almost half of the CI users tested can detect small, within-channel differences in their self-produced vowels, and they can utilize this auditory feedback towards speech adaptation. An acoustic hearing control group showed better sensitivity to the shifts in vowels, even in CI-simulated speech, and elicited more robust speech adaptation behavior than the CI users. Nevertheless, this study confirms that CI users can compensate for sensory errors in their speech and supports the idea that sensitivity to these errors may relate to variability in production.

Do Not Cut Off Your Tail: A Mega-Analysis of Responses to Auditory Perturbation Experiments.

PURPOSE: The practice of removing "following" responses from speech perturbation analyses is increasingly common, despite no clear evidence as to whether these responses represent a unique response type. This study aimed to determine if the distribution of responses to auditory perturbation paradigms represents a bimodal distribution, consisting of two distinct response types, or a unimodal distribution. METHOD: This mega-analysis pooled data from 22 previous studies to examine the distribution and magnitude of responses to auditory perturbations across four tasks: adaptive pitch, adaptive formant, reflexive pitch, and reflexive formant. Data included at least 150 unique participants for each task, with studies comprising younger adult, older adult, and Parkinson's disease populations. A Silverman's unimodality test followed by a smoothed bootstrap resampling technique was performed for each task to evaluate the number of modes in each distribution. Wilcoxon signed-ranks tests were also performed for each distribution to confirm significant compensation in response to the perturbation. RESULTS: Modality analyses were not significant (p > .05) for any group or task, indicating unimodal distributions. Our analyses also confirmed compensatory reflexive responses to pitch and formant perturbations across all groups, as well as adaptive responses to sustained formant perturbations. However, analyses of sustained pitch perturbations only revealed evidence of adaptation in studies with younger adults. CONCLUSION: The demonstration of a clear unimodal distribution across all tasks suggests that following responses do not represent a distinct response pattern, but rather the tail of a unimodal distribution. SUPPLEMENTAL MATERIAL: https://doi.org/10.23641/asha.24282676.

Modulation of somatosensation by transcranial magnetic stimulation over somatosensory cortex: a systematic review.

Over the last three decades, transcranial magnetic stimulation (TMS) has gained popularity as a tool to modulate human somatosensation. However, the effects of different stimulation types on the multiple distinct subdomains of somatosensation (e.g., tactile perception, proprioception and pain) have not been systematically compared. This is especially notable in the case of newer theta-burst stimulation protocols now in widespread use. Here, we aimed to systematically and critically review the existing TMS literature and provide a complete picture of current knowledge regarding the role of TMS in modulating human somatosensation across stimulation protocols and somatosensory domains. Following the PRISMA guidelines, fifty-four studies were included in the current review and were compared based on their methodologies and results. Overall, findings from these studies provide evidence that different types of somatosensation can be both disrupted and enhanced by targeted stimulation of specific somatosensory areas. Some mixed results, however, were reported in the literature. We discussed possible reasons for these mixed results, methodological limitations of existing investigations, and potential avenues for future research.

Movement variability can be modulated in speech production.

Although movement variability is often attributed to unwanted noise in the motor system, recent work has demonstrated that variability may be actively controlled. To date, research on regulation of motor variability has relied on relatively simple, laboratory-specific reaching tasks. It is not clear how these results translate to complex, well-practiced tasks. Here, we test how variability is regulated during speech production, a complex, highly overpracticed, and natural motor behavior that relies on auditory and somatosensory feedback. Specifically, in a series of four experiments, we assessed the effects of auditory feedback manipulations that modulate perceived speech variability, shifting every production either toward (inward pushing) or away from (outward pushing) the center of the distribution for each vowel. Participants exposed to the inward-pushing perturbation (experiment 1) increased produced variability while the perturbation was applied as well as after it was removed. Unexpectedly, the outward-pushing perturbation (experiment 2) also increased produced variability during exposure, but variability returned to near-baseline levels when the perturbation was removed. Outward-pushing perturbations failed to reduce participants' produced variability both with larger perturbation magnitude (experiment 3) and after their variability had increased above baseline levels as a result of the inward-pushing perturbation (experiment 4). Simulations of the applied perturbations using a state-space model of motor behavior suggest that the increases in produced variability in response to the two types of perturbations may arise through distinct mechanisms. Together, these results suggest that motor variability is actively monitored and can be modulated even in complex and well-practiced behaviors such as speech.NEW & NOTEWORTHY By implementing a novel auditory feedback perturbation that modulates participants' perceived trial-to-trial variability without affecting their overall mean behavior, we show that variability in the speech motor system can be modulated. By assaying speech production, we expand our current understanding of variability to a well-practiced, complex behavior outside of the limb control system. Our results additionally highlight the need to incorporate the active control of variability in models of speech motor control.

A single exposure to altered auditory feedback causes observable sensorimotor adaptation in speech.

Sensory errors induce two types of behavioral changes: rapid compensation within a movement and longer-term adaptation of subsequent movements. Although adaptation is hypothesized to occur whenever a sensory error is perceived (including after a single exposure to altered feedback), adaptation of articulatory movements in speech has only been observed after repeated exposure to auditory perturbations, questioning both current theories of speech sensorimotor adaptation and the universality of more general theories of adaptation. We measured single-exposure or 'one-shot' learning in a large dataset in which participants were exposed to intermittent, unpredictable perturbations of their speech acoustics. On unperturbed trials immediately following these perturbed trials, participants adjusted their speech to oppose the preceding shift, demonstrating that learning occurs even after a single exposure to auditory error. These results provide critical support for current theories of sensorimotor adaptation in speech and align speech more closely with learning in other motor domains.

Examining the Relationship Between Speech Perception, Production Distinctness, and Production Variability.

Several studies have demonstrated that individuals' ability to perceive a speech sound contrast is related to the production of that contrast in their native language. The theoretical account for this relationship is that speech perception and production have a shared multimodal representation in relevant sensory spaces (e.g., auditory and somatosensory domains). This gives rise to a prediction that individuals with more narrowly defined targets will produce greater separation between contrasting sounds, as well as lower variability in the production of each sound. However, empirical studies that tested this hypothesis, particularly with regard to variability, have reported mixed outcomes. The current study investigates the relationship between perceptual ability and production ability, focusing on the auditory domain. We examined whether individuals' categorical labeling consistency for the American English /ε/-/æ/ contrast, measured using a perceptual identification task, is related to distance between the centroids of vowel categories in acoustic space (i.e., vowel contrast distance) and to two measures of production variability: the overall distribution of repeated tokens for the vowels (i.e., area of the ellipse) and the proportional within-trial decrease in variability as defined as the magnitude of self-correction to the initial acoustic variation of each token (i.e., centering ratio). No significant associations were found between categorical labeling consistency and vowel contrast distance, between categorical labeling consistency and area of the ellipse, or between categorical labeling consistency and centering ratio. These null results suggest that the perception-production relation may not be as robust as suggested by a widely adopted theoretical framing in terms of the size of auditory target regions. However, the present results may also be attributable to choices in implementation (e.g., the use of model talkers instead of continua derived from the participants' own productions) that should be subject to further investigation.

Responses to Auditory Feedback Manipulations in Speech May Be Affected by Previous Exposure to Auditory Errors.

Purpose Speakers use auditory feedback to guide their speech output, although individuals differ in the magnitude of their compensatory response to perceived errors in feedback. Little is known about the factors that contribute to the compensatory response or how fixed or flexible they are within an individual. Here, we test whether manipulating the perceived reliability of auditory feedback modulates speakers' compensation to auditory perturbations, as predicted by optimal models of sensorimotor control. Method Forty participants produced monosyllabic words in two separate sessions, which differed in the auditory feedback given during an initial exposure phase. In the veridical session exposure phase, feedback was normal. In the noisy session exposure phase, small, random formant perturbations were applied, reducing reliability of auditory feedback. In each session, a subsequent test phase introduced larger unpredictable formant perturbations. We assessed whether the magnitude of within-trial compensation for these larger perturbations differed across the two sessions. Results Compensatory responses to downward (though not upward) formant perturbations were larger in the veridical session than the noisy session. However, in post hoc testing, we found the magnitude of this effect is highly dependent on the choice of analysis procedures. Compensation magnitude was not predicted by other production measures, such as formant variability, and was not reliably correlated across sessions. Conclusions Our results, though mixed, provide tentative support that the feedback control system monitors the reliability of sensory feedback. These results must be interpreted cautiously given the potentially limited stability of auditory feedback compensation measures across analysis choices and across sessions. Supplemental Material https://doi.org/10.23641/asha.14167136.

How to bring peer review ghostwriters out of the dark.

Early career researchers are frequent and valuable contributors to peer review. Systemic changes that acknowledge this fact would result in ethical co-reviewing, peer reviews of greater quality, and a reduction in peer reviewer burden.

Effects of syllable stress in adaptation to altered auditory feedback in vowels.

Unstressed syllables in English most commonly contain the vowel quality [ə] (schwa), which is cross-linguistically described as having a variable target. The present study examines whether speakers are sensitive to whether their auditory feedback matches their target when producing unstressed syllables. When speakers hear themselves producing formant-altered speech, they will change their motor plans so that their altered feedback is a better match to the target. If schwa has no target, then feedback mismatches in unstressed syllables may not drive a change in production. In this experiment, participants spoke disyllabic words with initial or final stress where the auditory feedback of F1 was raised (Experiment 1) or lowered (Experiment 2) by 100 mels. Both stressed and unstressed syllables showed adaptive changes in F1. In Experiment 1, initial-stress words showed larger adaptive decreases in F1 than final-stress words, but in Experiment 2, stressed syllables overall showed greater adaptive increases in F1 than unstressed syllables in all words, regardless of which syllable contained the primary stress. These results suggest that speakers are sensitive to feedback mismatches in both stressed and unstressed syllables, but that stress and metrical foot type may mediate the corrective response.

Increased speech contrast induced by sensorimotor adaptation to a nonuniform auditory perturbation.

When auditory feedback is perturbed in a consistent way, speakers learn to adjust their speech to compensate, a process known as sensorimotor adaptation. Although this paradigm has been highly informative for our understanding of the role of sensory feedback in speech motor control, its ability to induce behaviorally relevant changes in speech that affect communication effectiveness remains unclear. Because reduced vowel contrast contributes to intelligibility deficits in many neurogenic speech disorders, we examine human speakers' ability to adapt to a nonuniform perturbation field that was designed to affect vowel distinctiveness, applying a shift that depended on the vowel being produced. Twenty-five participants were exposed to this "vowel centralization" feedback perturbation in which the first two formant frequencies were shifted toward the center of each participant's vowel space, making vowels less distinct from one another. Speakers adapted to this nonuniform shift, learning to produce corner vowels with increased vowel space area and vowel contrast to partially overcome the perceived centralization. The increase in vowel contrast occurred without a concomitant increase in duration and persisted after the feedback shift was removed, including after a 10-min silent period. These findings establish the validity of a sensorimotor adaptation paradigm to increase vowel contrast, showing that complex, nonuniform alterations to sensory feedback can successfully drive changes relevant to intelligible communication.NEW & NOTEWORTHY To date, the speech motor learning evoked in sensorimotor adaptation studies has had little ecological consequences for communication. By inducing complex, nonuniform acoustic errors, we show that adaptation can be leveraged to cause an increase in speech sound contrast, a change that has the capacity to improve intelligibility. This study is relevant for models of sensorimotor integration across motor domains, showing that complex alterations to sensory feedback can successfully drive changes relevant to ecological behavior.

Why scientific societies should involve more early-career researchers.

Early-career researchers (ECRs) make up a large portion of the academic workforce. Yet, most leadership positions in scientific societies are held by senior scientists, and ECRs have little to no say over the decisions that will shape the future of research. This article looks at the level of influence ECRs have in 20 scientific societies based in the US and UK, and provides guidelines on how societies can successfully include ECRs in leadership roles.

Assessing speech correction abilities with acoustic analyses: Evidence of preserved online correction in persons with aphasia.

Purpose: Disorders of speech production may be accompanied by abnormal processing of speech sensory feedback. Here, we introduce a semi-automated analysis designed to assess the degree to which speakers use natural online feedback to decrease acoustic variability in spoken words. Because production deficits in aphasia have been hypothesised to stem from problems with sensorimotor integration, we investigated whether persons with aphasia (PWA) can correct their speech acoustics online. Method: Eight PWA in the chronic stage produced 200 repetitions each of three monosyllabic words. Formant variability was measured for each vowel in multiple time windows within the syllable, and the reduction in formant variability from vowel onset to midpoint was quantified. Result: PWA significantly decreased acoustic variability over the course of the syllable, providing evidence of online feedback correction mechanisms. The magnitude of this corrective formant movement exceeded past measurements in control participants. Conclusion: Vowel centreing behaviour suggests that error correction abilities are at least partially spared in speakers with aphasia, and may be relied upon to compensate for feedforward deficits by bringing utterances back on track. These proof of concept data show the potential of this analysis technique to elucidate the mechanisms underlying disorders of speech production.

Online Adaptation to Altered Auditory Feedback Is Predicted by Auditory Acuity and Not by Domain-General Executive Control Resources.

When a speaker's auditory feedback is altered, he adapts for the perturbation by altering his own production, which demonstrates the role of auditory feedback in speech motor control. In the present study, we explored the role of auditory acuity and executive control in this process. Based on the DIVA model and the major cognitive control models, we expected that higher auditory acuity, and better executive control skills would predict larger adaptation to the alteration. Thirty-six Spanish native speakers performed an altered auditory feedback experiment, executive control (numerical Stroop, Simon and Flanker) tasks, and auditory acuity tasks (loudness, pitch, and melody pattern discrimination). In the altered feedback experiment, participants had to produce the pseudoword "pep" (/pep/) while perceiving their auditory feedback in real time through earphones. The auditory feedback was first unaltered and then progressively altered in F1 and F2 dimensions until maximal alteration (F1 -150 Hz; F2 +300 Hz). The normalized distance of maximal adaptation ranged from 4 to 137 Hz (median of 75 ± 36). The different measures of auditory acuity were significant predictors of adaptation, while individual measures of cognitive function skills (obtained from the executive control tasks) were not. Better auditory discriminators adapted more to the alteration. We conclude that adaptation to altered auditory feedback is very well-predicted by general auditory acuity, as suggested by the DIVA model. In line with the framework of motor-control models, no specific claim on the implication of executive resources in speech motor control can be made.

Auditory cortex processes variation in our own speech.

As we talk, we unconsciously adjust our speech to ensure it sounds the way we intend it to sound. However, because speech production involves complex motor planning and execution, no two utterances of the same sound will be exactly the same. Here, we show that auditory cortex is sensitive to natural variations in self-produced speech from utterance to utterance. We recorded event-related potentials (ERPs) from ninety-nine subjects while they uttered "ah" and while they listened to those speech sounds played back. Subjects' utterances were sorted based on their formant deviations from the previous utterance. Typically, the N1 ERP component is suppressed during talking compared to listening. By comparing ERPs to the least and most variable utterances, we found that N1 was less suppressed to utterances that differed greatly from their preceding neighbors. In contrast, an utterance's difference from the median formant values did not affect N1. Trial-to-trial pitch (f0) deviation and pitch difference from the median similarly did not affect N1. We discuss mechanisms that may underlie the change in N1 suppression resulting from trial-to-trial formant change. Deviant utterances require additional auditory cortical processing, suggesting that speaking-induced suppression mechanisms are optimally tuned for a specific production.

What does motor efference copy represent? Evidence from speech production.

How precisely does the brain predict the sensory consequences of our actions? Efference copy is thought to reflect the predicted sensation of self-produced motor acts, such as the auditory feedback heard while speaking. Here, we use magnetoencephalographic imaging (MEG-I) in human speakers to demonstrate that efference copy prediction does not track movement variability across repetitions of the same motor task. Specifically, spoken vowels were less accurately predicted when they were less similar to a speaker's median production, even though the prediction is thought to be based on the very motor commands that generate each vowel. Auditory cortical responses to less prototypical speech productions were less suppressed, resembling responses to speech errors, and were correlated with later corrective movement, suggesting that the suppression may be functionally significant for error correction. The failure of the motor system to accurately predict less prototypical speech productions suggests that the efferent-driven suppression does not reflect a sensory prediction, but a sensory goal.

Vowel category boundaries enhance cortical and behavioral responses to speech feedback alterations.

Auditory feedback is instrumental in the online control of speech, allowing speakers to compare their self-produced speech signal with a desired auditory target and correct for errors. However, there is little account of the representation of "target" and "error": does error depend purely on acoustic distance from a target, or is error enhanced by phoneme category changes? Here, we show an effect of vowel boundaries on compensatory responses to a real-time auditory perturbation. While human subjects spoke monosyllabic words, event-triggered functional magnetic resonance imaging was used to characterize neural responses to unexpected changes in auditory feedback. Capitalizing on speakers' natural variability, we contrasted the responses to feedback perturbations applied to two classes of utterances: (1) those that fell nearer to the category boundary, for which perturbations were designed to change the phonemic identity of the heard speech; and (2) those that fell farther from the boundary, for which perturbations resulted in only sub-phonemic auditory differences. Subjects' behavioral compensation was more than three times greater when feedback shifts were applied nearer to a category boundary. Furthermore, a near-boundary shift resulted in stronger cortical responses, most notably in right posterior superior temporal gyrus, than an identical shift that occurred far from the boundary. Across participants, a correlation was found between the amount of compensation to the perturbation and the amount of activity in a network of superior temporal and inferior frontal brain regions. Together, these results demonstrate that auditory feedback control of speech is sensitive to linguistic categories learned through auditory experience.

Human cortical sensorimotor network underlying feedback control of vocal pitch.

The control of vocalization is critically dependent on auditory feedback. Here, we determined the human peri-Sylvian speech network that mediates feedback control of pitch using direct cortical recordings. Subjects phonated while a real-time signal processor briefly perturbed their output pitch (speak condition). Subjects later heard the same recordings of their auditory feedback (listen condition). In posterior superior temporal gyrus, a proportion of sites had suppressed responses to normal feedback, whereas other spatially independent sites had enhanced responses to altered feedback. Behaviorally, speakers compensated for perturbations by changing their pitch. Single-trial analyses revealed that compensatory vocal changes were predicted by the magnitude of both auditory and subsequent ventral premotor responses to perturbations. Furthermore, sites whose responses to perturbation were enhanced in the speaking condition exhibited stronger correlations with behavior. This sensorimotor cortical network appears to underlie auditory feedback-based control of vocal pitch in humans.