Perturbing the consistency of auditory feedback in speech.

Sensory information, including auditory feedback, is used by talkers to maintain fluent speech articulation. Current models of speech motor control posit that speakers continually adjust their motor commands based on discrepancies between the sensory predictions made by a forward model and the sensory consequences of their speech movements. Here, in two within-subject design experiments, we used a real-time formant manipulation system to explore how reliant speech articulation is on the accuracy or predictability of auditory feedback information. This involved introducing random formant perturbations during vowel production that varied systematically in their spatial location in formant space (Experiment 1) and temporal consistency (Experiment 2). Our results indicate that, on average, speakers' responses to auditory feedback manipulations varied based on the relevance and degree of the error that was introduced in the various feedback conditions. In Experiment 1, speakers' average production was not reliably influenced by random perturbations that were introduced every utterance to the first (F1) and second (F2) formants in various locations of formant space that had an overall average of 0 Hz. However, when perturbations were applied that had a mean of +100 Hz in F1 and -125 Hz in F2, speakers demonstrated reliable compensatory responses that reflected the average magnitude of the applied perturbations. In Experiment 2, speakers did not significantly compensate for perturbations of varying magnitudes that were held constant for one and three trials at a time. Speakers' average productions did, however, significantly deviate from a control condition when perturbations were held constant for six trials. Within the context of these conditions, our findings provide evidence that the control of speech movements is, at least in part, dependent upon the reliability and stability of the sensory information that it receives over time.

Exposure to Auditory Feedback Delay while Speaking Induces Perceptual Habituation but does not Mitigate the Disruptive Effect of Delay on Speech Auditory-motor Learning.

Perceiving the sensory consequences of our actions with a delay alters the interpretation of these afferent signals and impacts motor learning. For reaching movements, delayed visual feedback of hand position reduces the rate and extent of visuomotor adaptation, but substantial adaptation still occurs. Moreover, the detrimental effect of visual feedback delay on reach motor learning-selectively affecting its implicit component-can be mitigated by prior habituation to the delay. Auditory-motor learning for speech has been reported to be more sensitive to feedback delay, and it remains unknown whether habituation to auditory delay reduces its negative impact on learning. We investigated whether 30 min of exposure to auditory delay during speaking (a) affects the subjective perception of delay, and (b) mitigates its disruptive effect on speech auditory-motor learning. During a speech adaptation task with real-time perturbation of vowel spectral properties, participants heard this frequency-shifted feedback with no delay, 75 ms delay, or 115 ms delay. In the delay groups, 50% of participants had been exposed to the delay throughout a preceding 30-minute block of speaking whereas the remaining participants completed this block without delay. Although habituation minimized awareness of the delay, no improvement in adaptation to the spectral perturbation was observed. Thus, short-term habituation to auditory feedback delays is not effective in reducing the negative impact of delay on speech auditory-motor adaptation. Combined with previous findings, the strong negative effect of delay and the absence of an influence of delay awareness suggest the involvement of predominantly implicit learning mechanisms in speech.

The influence of bistable auditory feedback on speech motor control.

The precision of speech production is strongly influenced by the auditory feedback of our voice. Studies have demonstrated that when speakers receive perturbed auditory feedback, they spontaneously change their articulation to reduce the difference between the intended sound and what was heard. For controlling the accuracy of vowel and consonant production, this corrective behavior reflects the intended sound's category represented in the mind. This in turn suggests that sounds that are not represented as a category would be controlled differently. The current study investigated the effect of linguistic status on controlling the production of a sound. Participants used an apparatus to produce a bistable sound, which could be heard either as a vowel or an acoustic buzz depending on the instructions. During the production of the target sound, their auditory feedback was perturbed and the change in acoustics in response to the perturbation was measured. The results showed that the group producing a linguistic target exhibited an error-reducing behavior similar to what was reported in natural vowel production, whereas the group producing a non-linguistic acoustic target exhibited an error-amplifying behavior, even when the majority in this group had noticed the perturbation and consciously tried to counteract it. Our findings indicate that access to a stable representation differentiates the perceptual organization of auditory feedback, and consequently changes how the production of the sound is controlled.

Similar within-utterance loci of dysfluency in acquired neurogenic and persistent developmental stuttering.

Although the underlying neural mechanisms remain unknown for both persistent developmental stuttering (PSD) and acquired neurogenic stuttering (ANS), few studies have examined similarities/differences between these two disorders. We evaluated in both PDS (n = 35) and ANS (n = 5) phonetic, word class, word length, and word position variables that are widely believed to influence at which loci within utterances PDS speakers' stuttering is most likely to occur. For both groups, (a) word weights based on the combination of variables were greater for stuttered vs. fluent words, and (b) stuttered words were loaded more by individual variables. However, contrary to long-standing views regarding PDS, greater loading for stuttered words was not found for the position variable. Findings suggest similar loci of stuttering in adults with PDS and ANS, and, for both groups, the probability of stuttering on a given word was more influenced by motor production variables than language variables.

Linguistic initiation signals increase auditory feedback error correction.

Previous research has shown that speakers can adapt their speech in a flexible manner as a function of a variety of contextual and task factors. While it is known that speech tasks may play a role in speech motor behavior, it remains to be explored if the manner in which the speaking action is initiated can modify low-level, automatic control of vocal motor action. In this study, the nature (linguistic vs non-linguistic) and modality (auditory vs visual) of the go signal (i.e., the prompts) was manipulated in an otherwise identical vocal production task. Participants were instructed to produce the word "head" when prompted, and the auditory feedback they were receiving was altered by systematically changing the first formants of the vowel /ε/ in real time using a custom signal processing system. Linguistic prompts induced greater corrective behaviors to the acoustic perturbations than non-linguistic prompts. This suggests that the accepted variance for the intended speech sound decreases when external linguistic templates are provided to the speaker. Overall, this result shows that the automatic correction of vocal errors is influenced by flexible, context-dependant mechanisms.

Modulation of auditory-motor learning in response to formant perturbation as a function of delayed auditory feedback.

The interaction of language production and perception has been substantiated by empirical studies where speakers compensate their speech articulation in response to the manipulated sound of their voice heard in real-time as auditory feedback. A recent study by Max and Maffett [(2015). Neurosci. Lett. 591, 25-29] reported an absence of compensation (i.e., auditory-motor learning) for frequency-shifted formants when auditory feedback was delayed by 100 ms. In the present study, the effect of auditory feedback delay was studied when only the first formant was manipulated while delaying auditory feedback systematically. In experiment 1, a small yet significant compensation was observed even with 100 ms of auditory delay unlike the past report. This result suggests that the tolerance of feedback delay depends on different types of auditory errors being processed. In experiment 2, it was revealed that the amount of formant compensation had an inverse linear relationship with the amount of auditory delay. One of the speculated mechanisms to account for these results is that as auditory delay increases, undelayed (and unperturbed) somatosensory feedback is given more preference for accuracy control of vowel formants.

Occlusion effect on compensatory formant production and voice amplitude in response to real-time perturbation.

The importance of auditory feedback for controlling speech articulation has been substantiated by the use of the real-time auditory perturbation paradigm. With this paradigm, speakers receive their own manipulated voice signal in real-time while they produce a simple speech segment. In response, they spontaneously compensate for the manipulation. In the case of vowel formant control, various studies have reported behavioral and neural mechanisms of how auditory feedback is processed for compensatory behavior. However, due to technical limitations such as avoiding an electromagnetic artifact or metal transducers near a scanner, some studies require foam tip insert earphones. These earphones occlude the ear canal, and may cause more energy of the unmanipulated first formant to reach the cochlea through bone conduction and thus confound the effect of formant manipulation. Moreover, amplification of lower frequencies due to occluded ear canals may influence speakers' voice amplitude. The current study examined whether using circumaural headphones and insert earphones would elicit different compensatory speech production when speakers' first formant was manipulated in real-time. The results of the current study showed that different headphones did not elicit different compensatory formant production. Voice amplitude results were varied across different vowels examined; however, voice amplitude tended to decrease with the introduction of F1 perturbation.

Formant compensation for auditory feedback with English vowels.

Past studies have shown that speakers spontaneously adjust their speech acoustics in response to their auditory feedback perturbed in real time. In the case of formant perturbation, the majority of studies have examined speaker's compensatory production using the English vowel /ɛ/ as in the word "head." Consistent behavioral observations have been reported, and there is lively discussion as to how the production system integrates auditory versus somatosensory feedback to control vowel production. However, different vowels have different oral sensation and proprioceptive information due to differences in the degree of lingual contact or jaw openness. This may in turn influence the ways in which speakers compensate for auditory feedback. The aim of the current study was to examine speakers' compensatory behavior with six English monophthongs. Specifically, the current study tested to see if "closed vowels" would show less compensatory production than "open vowels" because closed vowels' strong lingual sensation may richly specify production via somatosensory feedback. Results showed that, indeed, speakers exhibited less compensatory production with the closed vowels. Thus sensorimotor control of vowels is not fixed across all vowels; instead it exerts different influences across different vowels.

Temporal control and compensation for perturbed voicing feedback.

Previous research employing a real-time auditory perturbation paradigm has shown that talkers monitor their own speech attributes such as fundamental frequency, vowel intensity, vowel formants, and fricative noise as part of speech motor control. In the case of vowel formants or fricative noise, what was manipulated is spectral information about the filter function of the vocal tract. However, segments can be contrasted by parameters other than spectral configuration. It is possible that the feedback system monitors phonation timing in the way it does spectral information. This study examined whether talkers exhibit a compensatory behavior when manipulating information about voicing. When talkers received feedback of the cognate of the intended voicing category (saying "tipper" while hearing "dipper" or vice versa), they changed the voice onset time and in some cases the following vowel.

Language dependent vowel representation in speech production.

The representation of speech goals was explored using an auditory feedback paradigm. When talkers produce vowels the formant structure of which is perturbed in real time, they compensate to preserve the intended goal. When vowel formants are shifted up or down in frequency, participants change the formant frequencies in the opposite direction to the feedback perturbation. In this experiment, the specificity of vowel representation was explored by examining the magnitude of vowel compensation when the second formant frequency of a vowel was perturbed for speakers of two different languages (English and French). Even though the target vowel was the same for both language groups, the pattern of compensation differed. French speakers compensated to smaller perturbations and made larger compensations overall. Moreover, French speakers modified the third formant in their vowels to strengthen the compensation even though the third formant was not perturbed. English speakers did not alter their third formant. Changes in the perceptual goodness ratings by the two groups of participants were consistent with the threshold to initiate vowel compensation in production. These results suggest that vowel goals not only specify the quality of the vowel but also the relationship of the vowel to the vowel space of the spoken language.

A cross-language study of compensation in response to real-time formant perturbation.

Past studies have shown that when formants are perturbed in real time, speakers spontaneously compensate for the perturbation by changing their formant frequencies in the opposite direction to the perturbation. Further, the pattern of these results suggests that the processing of auditory feedback error operates at a purely acoustic level. This hypothesis was tested by comparing the response of three language groups to real-time formant perturbations, (1) native English speakers producing an English vowel /ε/, (2) native Japanese speakers producing a Japanese vowel (/e([inverted perpendicular])/), and (3) native Japanese speakers learning English, producing /ε/. All three groups showed similar production patterns when F1 was decreased; however, when F1 was increased, the Japanese groups did not compensate as much as the native English speakers. Due to this asymmetry, the hypothesis that the compensatory production for formant perturbation operates at a purely acoustic level was rejected. Rather, some level of phonological processing influences the feedback processing behavior.