Formant Variability Research Articles

Does pre-speech auditory modulation reflect processes related to feedback monitoring or speech movement planning?

Previous studies have revealed that auditory processing is modulated during the planning phase immediately prior to speech onset. To date, the functional relevance of this pre-speech auditory modulation (PSAM) remains unknown. Here, we investigated whether PSAM reflects neuronal processes that are associated with preparing auditory cortex for optimized feedback monitoring as reflected in online speech corrections. Combining electroencephalographic PSAM data from a previous data set with new acoustic measures of the same participants’ speech, we asked whether individual speakers’ extent of PSAM is correlated with the implementation of within-vowel articulatory adjustments during /b/-vowel-/d/ word productions. Online articulatory adjustments were quantified as the extent of change in inter-trial formant variability from vowel onset to vowel midpoint (a phenomenon known as centering). This approach allowed us to also consider inter-trial variability in formant production, and its possible relation to PSAM, at vowel onset and midpoint separately. Results showed that inter-trial formant variability was significantly smaller at vowel midpoint than at vowel onset. PSAM was not significantly correlated with this amount of change in variability as an index of within-vowel adjustments. Surprisingly, PSAM was negatively correlated with inter-trial formant variability not only in the middle but also at the very onset of the vowels. Thus, speakers with more PSAM produced formants that were already less variable at vowel onset. Findings suggest that PSAM may reflect processes that influence speech acoustics as early as vowel onset and, thus, that are directly involved in motor command preparation (feedforward control) rather than output monitoring (feedback control).

Does pre-speech auditory modulation reflect processes related to feedback monitoring or speech movement planning?

Previous studies have revealed that auditory processing is modulated during the planning phase immediately prior to speech onset. To date, the functional relevance of this pre-speech auditory modulation (PSAM) remains unknown. Here, we investigated whether PSAM reflects neuronal processes that are associated with preparing auditory cortex for optimized feedback monitoring as reflected in online speech corrections. Combining electroencephalographic PSAM data from a previous data set with new acoustic measures of the same participants' speech, we asked whether individual speakers' extent of PSAM is correlated with the implementation of within-vowel articulatory adjustments during /b/-vowel-/d/ word productions. Online articulatory adjustments were quantified as the extent of change in inter-trial formant variability from vowel onset to vowel midpoint (a phenomenon known as centering). This approach allowed us to also consider inter-trial variability in formant production and its possible relation to PSAM at vowel onset and midpoint separately. Results showed that inter-trial formant variability was significantly smaller at vowel midpoint than at vowel onset. PSAM was not significantly correlated with this amount of change in variability as an index of within-vowel adjustments. Surprisingly, PSAM was negatively correlated with inter-trial formant variability not only in the middle but also at the very onset of the vowels. Thus, speakers with more PSAM produced formants that were already less variable at vowel onset. Findings suggest that PSAM may reflect processes that influence speech acoustics as early as vowel onset and, thus, that are directly involved in motor command preparation (feedforward control) rather than output monitoring (feedback control).

Levodopa may modulate specific speech impairment in Parkinson's disease: an fMRI study.

Hypokinetic dysarthria (HD) is a difficult-to-treat symptom affecting quality of life in patients with Parkinson's disease (PD). Levodopa may partially alleviate some symptoms of HD in PD, but the neural correlates of these effects are not fully understood. The aim of our study was to identify neural mechanisms by which levodopa affects articulation and prosody in patients with PD. Altogether 20 PD patients participated in a task fMRI study (overt sentence reading). Using a single dose of levodopa after an overnight withdrawal of dopaminergic medication, levodopa-induced BOLD signal changes within the articulatory pathway (in regions of interest; ROIs) were studied. We also correlated levodopa-induced BOLD signal changes with the changes in acoustic parameters of speech. We observed no significant changes in acoustic parameters due to acute levodopa administration. After levodopa administration as compared to the OFF dopaminergic condition, patients showed task-induced BOLD signal decreases in the left ventral thalamus (p = 0.0033). The changes in thalamic activation were associated with changes in pitch variation (R = 0.67, p = 0.006), while the changes in caudate nucleus activation were related to changes in the second formant variability which evaluates precise articulation (R = 0.70, p = 0.003). The results are in line with the notion that levodopa does not have a major impact on HD in PD, but it may induce neural changes within the basal ganglia circuitries that are related to changes in speech prosody and articulation.

Inter-Trial Formant Variability in Speech Production Is Actively Controlled but Does Not Affect Subsequent Adaptation to a Predictable Formant Perturbation.

Despite ample evidence that speech production is associated with extensive trial-to-trial variability, it remains unclear whether this variability represents merely unwanted system noise or an actively regulated mechanism that is fundamental for maintaining and adapting accurate speech movements. Recent work on upper limb movements suggest that inter-trial variability may be not only actively regulated based on sensory feedback, but also provide a type of workspace exploration that facilitates sensorimotor learning. We therefore investigated whether experimentally reducing or magnifying inter-trial formant variability in the real-time auditory feedback during speech production (a) leads to adjustments in formant production variability that compensate for the manipulation, (b) changes the temporal structure of formant adjustments across productions, and (c) enhances learning in a subsequent adaptation task in which a predictable formant-shift perturbation is applied to the feedback signal. Results show that subjects gradually increased formant variability in their productions when hearing auditory feedback with reduced variability, but subsequent formant-shift adaptation was not affected by either reducing or magnifying the perceived variability. Thus, findings provide evidence for speakers’ active control of inter-trial formant variability based on auditory feedback from previous trials, but–at least for the current short-term experimental manipulation of feedback variability–not for a role of this variability regulation mechanism in subsequent auditory-motor learning.

Intact Correction for Self-Produced Vowel Formant Variability in Individuals With Cerebellar Ataxia Regardless of Auditory Feedback Availability.

Purpose Individuals with cerebellar ataxia (CA) caused by cerebellar degeneration exhibit larger reactive compensatory responses to unexpected auditory feedback perturbations than neurobiologically typical speakers, suggesting they may rely more on feedback control during speech. We test this hypothesis by examining variability in unaltered speech. Previous studies of typical speakers have demonstrated a reduction in formant variability (centering) observed during the initial phase of vowel production from vowel onset to vowel midpoint. Centering is hypothesized to reflect feedback-based corrections for self-produced variability and thus may provide a behavioral assay of feedback control in unperturbed speech in the same manner as the compensatory response does for feedback perturbations. Method To comprehensively compare centering in individuals with CA and controls, we examine centering in two vowels (/i/ and /ɛ/) under two contexts (isolated words and connected speech). As a control, we examine speech produced both with and without noise to mask auditory feedback. Results Individuals with CA do not show increased centering compared to age-matched controls, regardless of vowel, context, or masking. Contrary to previous results in neurobiologically typical speakers, centering was not affected by the presence of masking noise in either group. Conclusions The similar magnitude of centering seen with and without masking noise questions whether centering is driven by auditory feedback. However, if centering is at least partially driven by auditory/somatosensory feedback, these results indicate that the larger compensatory response to altered auditory feedback observed in individuals with CA may not reflect typical motor control processes during normal, unaltered speech production.

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.

Formant variability is actively regulated in vowel production

Although movement variability is typically attributed to unwanted noise in the sensorimotor system, recent work in reaching has demonstrated that variability may be at least in part actively controlled. However, it is unknown if this is true for speech production, a complex, well-practiced task controlled via non-visual sensory feedback. Here, we test how variability in formant production for vowels may be actively regulated through real-time perturbations to auditory feedback that push vowel formants toward or away from the center of each speaker’s distribution for that vowel in F1/F2 space. These inward and outward perturbations effectively decrease and increase perceived variability, respectively. Participants exposed to the inward-pushing perturbation increased their produced variability both during and after exposure, indicating that lower perceived variability “frees” the motor system to be less precise. However, the outward-pushing perturbation had no consistent effect on produced variability. This suggests that at least some skilled movements are already produced at the lower limits of possible variability. While overall variability did not change, vowel “centering,” a measure of within-trial correction for variability, did increase, suggesting participants became more responsive to errors. Together, these results suggest that variability in speech production is actively regulated.

F0-induced formant measurement errors result in biased variabilities.

Many developmental studies attribute reduction of acoustic variability to increasing motor control. However, linear prediction-based formant measurements are known to be biased toward the nearest harmonic of F0, especially at high F0s. Thus, the amount of reported formant variability generated by changes in F0 is unknown. Here, 470 000 vowels were synthesized, mimicking statistics reported in four developmental studies, to estimate the proportion of formant variability that can be attributed to F0 bias, as well as other formant measurement errors. Results showed that the F0-induced formant measurements errors are large and systematic, and cannot be eliminated by a large sample size.

Vowel formant dispersion in apraxia of speech and aphasia: repeated elicitation versus spontaneous speech.

Vowel formant dispersion in apraxia of speech and aphasia: repeated elicitation versus spontaneous speech.

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.

How much formant variability in linear prediction-based measurements can be explained by F0 variability?

How much formant variability in linear prediction-based measurements can be explained by F0 variability?

Asymmetrical patterns of formant variability in English vowels

Previous studies have claimed that lower formants should be weighted more than higher formants in a perceptual model of vowel perception (e.g., Schwartz et al., 1997). Given this formant weighting hypothesis, and if vowels have acoustic targets, vowels should be more variable in higher formant frequencies. Here, we examined within-speaker variability for five English vowels /æ, ʌ, ɔ, ɛ, ɪ/ in various contexts as produced by 32 speakers in the x-ray microbeam database (Westbury, 1994). For variabilities of the first three formants, only /ɔ/ follows this prediction (i.e., variability: F3 > F2 > F1) while /æ/ exhibits the opposite pattern; if we ignore F3 (as being less reliably measured), most vowels conform to the prediction (i.e., variability: F2 > F1), except for /æ/. Although the F2 variability is generally consistent with the perceptual model of vowel perception, it is also consistent with a possibly greater effect of consonant coarticulation (which is extensive here) on F2 relative to F1; this requires more testing. Further, while these results do not fully conform to the prediction made by the hypotheses, coproduction effects arising from the diverse contexts likely interact with the expected tendency. Correlation with observed kinematic variabilities will also be discussed. [Work supported by NIH grant DC-002717.]

Functional connectivity associated with acoustic stability during vowel production: implications for vocal-motor control.

Vowels provide the acoustic foundation of communication through speech and song, but little is known about how the brain orchestrates their production. Positron emission tomography was used to study regional cerebral blood flow (rCBF) during sustained production of the vowel /a/. Acoustic and blood flow data from 13, normal, right-handed, native speakers of American English were analyzed to identify CBF patterns that predicted the stability of the first and second formants of this vowel. Formants are bands of resonance frequencies that provide vowel identity and contribute to voice quality. The results indicated that formant stability was directly associated with blood flow increases and decreases in both left- and right-sided brain regions. Secondary brain regions (those associated with the regions predicting formant stability) were more likely to have an indirect negative relationship with first formant variability, but an indirect positive relationship with second formant variability. These results are not definitive maps of vowel production, but they do suggest that the level of motor control necessary to produce stable vowels is reflected in the complexity of an underlying neural system. These results also extend a systems approach to functional image analysis, previously applied to normal and ataxic speech rate that is solely based on identifying patterns of brain activity associated with specific performance measures. Understanding the complex relationships between multiple brain regions and the acoustic characteristics of vocal stability may provide insight into the pathophysiology of the dysarthrias, vocal disorders, and other speech changes in neurological and psychiatric disorders.

Precursors of dancing and singing to music in three- to four-months-old infants.

Dancing and singing to music involve auditory-motor coordination and have been essential to our human culture since ancient times. Although scholars have been trying to understand the evolutionary and developmental origin of music, early human developmental manifestations of auditory-motor interactions in music have not been fully investigated. Here we report limb movements and vocalizations in three- to four-months-old infants while they listened to music and were in silence. In the group analysis, we found no significant increase in the amount of movement or in the relative power spectrum density around the musical tempo in the music condition compared to the silent condition. Intriguingly, however, there were two infants who demonstrated striking increases in the rhythmic movements via kicking or arm-waving around the musical tempo during listening to music. Monte-Carlo statistics with phase-randomized surrogate data revealed that the limb movements of these individuals were significantly synchronized to the musical beat. Moreover, we found a clear increase in the formant variability of vocalizations in the group during music perception. These results suggest that infants at this age are already primed with their bodies to interact with music via limb movements and vocalizations.

Detecting voice disguise from speech variability: Analysis of three glottal and vocal tract measures

The deliberate attempt by speakers to conceal their identity (voice disguise) presents a challenge for forensics and for automated speaker identification systems. Using a database of natural and disguised voices of three professional voice impersonators, we build on earlier findings (Amin et al., 2012) by exploring how certain glottal and vocal tract measures, including fundamental frequency (f0), glottal timing (Open Quotient), and vowel formants, are exploited to create novel voice identities. Specifically, we explored whether the amount and type of variation exhibited by impersonators can be used to develop a metric for distinguishing natural from disguised voices. As expected, variation in f0 and Open Quotient was speaker-dependent, and corresponded closely to social attributes (i.e., gender/age) of the voice identities involved. In a novel finding, the effects of voice identity on vowel formants were highly dependent on vowel category, and could not be readily characterized as global modifications to the vowel space (Bradlow et al.,1996). We therefore developed a no-reference objective metric for voice disguise that treats formant variability on a vowel-by-vowel basis. This metric consistently assigned high rankings to natural voices (3.3/27 on average). This correlated closely with the subjective disguisedness ratings of 18 naïve listeners, even outperforming them slightly.

A case of foreign accent syndrome: Acoustic analyses and an empirical test of accent perception

Foreign Accent Syndrome (FAS) is a rare acquired syndrome following stroke, manifesting as a perceived change in the speaker’s accent. We present acoustic-phonetic analyses of the speech of a patient, RD, with FAS presenting as an apparent accent shift from Southern Ontarian to Atlantic Canadian who was first described in Naidoo, Warriner, Oczkowski, Sévigny, and Humphreys (2008). As well as more fully documenting this case, this paper also seeks to examine whether the accompanying articulatory deficits constitute a mild form of some other motor speech disorder such as apraxia of speech (AOS) or dysarthria. Acoustic-phonetic analyses showed increased vowel formant variability, overlap of voice onset times for phonemically contrastive stops, inconsistent consonant distortions, and global prosodic attenuation. These, combined with the patient’s fluent speech, can account for the perception of a foreign accent as opposed to disordered speech. The observed inconsistency of consonant distortions coupled with other articulatory deficits suggest that at least in this case, FAS can be considered to be a mild form of AOS.

Vowel Formant Values in Hearing and Hearing-Impaired Children: A Discriminant Analysis

Hearing-impaired speakers show changes in vowel production and formant pitch and variability, as well as more cases of overlapping between vowels and more restricted formant space, than hearing speakers; consequently their speech is less intelligible. The purposes of this paper were to determine the differences in vowel formant values between 32 hearing speakers, 14 severely hearing-impaired speakers, and 25 profoundly hearing-impaired speakers, and to investigate the influence of perceptual constraints on the contrastiveness of spoken vowels in speakers with hearing loss, as these underline the importance of good phonation, articulation, and resonance in speech production. Several differences in formant values were confirmed with Anova-Welch tests, except for the F1 of open /e/ and /a/, the most open and loud vowels in Slovene. In addition, discriminant analysis showed real differences in vowel production (97.1% classification success), thus making it possible to differentiate between groups of hearing and hearing-impaired speakers based on this one function of vowel production. The hypothesis was confirmed that vowel production in hearing-impaired individuals is different from that in hearing individuals.The results suggest that speech rehabilitation should consider specific tasks and training for vowel production, including neuromuscular oral control and self-monitoring, to improve speech intelligibility. The results are also useful for those who are in contact with people with hearing impairment, such as teachers, audiologists, speech and language pathologists, nurses, and researchers in speech and hearing sciences.

Vowel Acoustics in Adults With Apraxia of Speech

To investigate the hypothesis that vowel production is more variable in adults with acquired apraxia of speech (AOS) relative to healthy individuals with unimpaired speech. Vowel formant frequency measures were selected as the specific target of focus. Seven adults with AOS and aphasia produced 15 repetitions of 6 American English vowels in /hVC/ context (hid, head, hat, hot, hub, hoot). Vowel formant frequency measures (F1, F2) were Bark transformed and compared with data from archival sources. Measures of vowel acoustics in speakers with AOS did not differ from those of unimpaired speakers, including absolute Bark formant values, vowel space area, intervowel distance, and individual trial-to-trial formant variability. Comparison with normative acoustic measures suggested that vowel production at the word level is unimpaired in the current speakers with AOS, supporting previous studies that have shown vowel production is relatively intact in AOS.

An algorithm for token-by-token linear predictive coding analysis of oral monophthongs.

Linear Predictive Coding (LPC) is the most reliable vowel formant estimation method. LPC trajectories closely approximate formants and are preferable as a means of avoiding human bias. Nevertheless, formant analysis by LPC is not error free, as it can be affected by fundamental frequency, noise and vocal tract morphology. Error minimization is usually achieved through manual checking of LPC results with spectral contour. An algorithm to find optimal LPC parameters (Burg method) on a token by token basis was developed and implemented as a Praat script. The algorithm minimizes inter- and intra-vowel formant variability without knowledge of vowel identity. Algorithm results were compared with results of manual adjustment of LPC parameters on a trial and error basis on a corpus of 180 stressed and unstressed oral monophthongs of Brazilian Portuguese from a male speaker. First and second formants linear correlations are r2 = 0.98 and 0.99 respectively. The areas of /i, a, u/ vocalic triangles (mean values as vertices) of manual and automatic measurements showed only 4.5% difference. These results suggest the algorithm can be generalized to any speaker and speed up the measurement of vowel formants in large corpora. [Work supported by CNPq and Fapesp.]

Variability in the production of quantal vowels revisited

Articulatory and acoustic variability in the production of five American English vowels was examined. The data were movement records for selected fleshpoints on the midsagittal tongue surface, recorded using the x-ray microbeam. An algorithm for nonlinearly transforming fleshpoint positions to a new Cartesian space in which the x and y axes represent, respectively, the distance of the fleshpoint along the opposing vocal tract wall and the distance perpendicular to the tract wall, is described. The transformation facilitates a test of Quantal Theory in which variability in the two dimensions is compared over many productions of a given vowel type. The data provide some support for the theory. For fleshpoints near ‘‘quantal’’ constriction sites, the primary variability was in the x dimension (constriction location). The y-dimension values were more tightly constrained, and the formant frequencies were more significantly correlated with the y values than with the x values. The greater variability in constriction location than in degree was not an artifact of the greater distances traversed in the x dimension between the vowel and constrictions in neighboring consonants, since the pattern was preserved when pellet values were translated to take into account a ‘‘context-free’’ vowel target (the average values in the context of preceding and following labial consonants). Moreover, the observed correlations between formant values and pellet positions in the two dimensions for [i] and [u] were duplicated in an articulatory-to-acoustic modeling test using values for constriction length and cross-sectional area estimated from the data. The model showed smaller second formant variability in the x dimension than in the y dimension for equal-sized excursions near the constriction sites for these close vowels, in keeping with the interpretation that speakers exercise less precise control in just the dimensions and regions where quantal stability is available. However, the articulatory pattern was seen not just in vowels which clearly have consonantlike constrictions (the quantal vowels [i], [u], and [ɑ ]), but also in nonquantal vowels such as [æ].