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Speech processing relies on interactions between auditory and motor systems and is asymmetrically organized in the human brain. The left auditory system is specialized for processing of phonemes, whereas the right is specialized for processing of pitch changes in speech affecting prosody. In speakers of tonal languages, however, processing of pitch (i.e., tone) changes that alter word meaning is left-lateralized indicating that linguistic function and language experience shape speech processing asymmetries. Here, we investigated the asymmetry of motor contributions to auditory speech processing in male and female speakers of tonal and non-tonal languages. We temporarily disrupted the right or left speech motor cortex using transcranial magnetic stimulation (TMS) and measured the impact of these disruptions on auditory discrimination (mismatch negativity; MMN) responses to phoneme and tone changes in sequences of syllables using electroencephalography (EEG). We found that the effect of motor disruptions on processing of tone changes differed between language groups: disruption of the right speech motor cortex suppressed responses to tone changes in non-tonal language speakers, whereas disruption of the left speech motor cortex suppressed responses to tone changes in tonal language speakers. In non-tonal language speakers, the effects of disruption of left speech motor cortex on responses to tone changes were inconclusive. For phoneme changes, disruption of left but not right speech motor cortex suppressed responses in both language groups. We conclude that the contributions of the right and left speech motor cortex to auditory speech processing are determined by the functional roles of acoustic cues in the listener's native language.SIGNIFICANCE STATEMENTThe principles underlying hemispheric asymmetries of auditory speech processing remain debated. The asymmetry of processing of speech sounds is affected by low-level acoustic cues, but also by their linguistic function. By combining transcranial magnetic stimulation (TMS) and electroencephalography (EEG), we investigated the asymmetry of motor contributions to auditory speech processing in tonal and non-tonal language speakers. We provide causal evidence that the functional role of the acoustic cues in the listener's native language affects the asymmetry of motor influences on auditory speech discrimination ability [indexed by mismatch negativity (MMN) responses]. Lateralized top-down motor influences can affect asymmetry of speech processing in the auditory system.

Significance Statistical learning mechanisms enable extraction of patterns in the environment from infancy to adulthood. For example, they enable segmentation of continuous speech streams into novel words. Adults typically become aware of the hidden words even when passively listening to speech streams. It remains poorly understood how cognitive development and brain maturation affect implicit statistical learning (i.e., infant-like learning without awareness). Here, we show that the depletion of the cognitive control system by noninvasive brain stimulation or by demanding cognitive tasks boosts adults’ implicit but not explicit word-segmentation abilities. These findings suggest that the adult cognitive architecture constrains statistical learning mechanisms that are likely to contribute to early language acquisition and opens avenues to enhance language-learning abilities in adults., Human learning is supported by multiple neural mechanisms that maturate at different rates and interact in mostly cooperative but also sometimes competitive ways. We tested the hypothesis that mature cognitive mechanisms constrain implicit statistical learning mechanisms that contribute to early language acquisition. Specifically, we tested the prediction that depleting cognitive control mechanisms in adults enhances their implicit, auditory word-segmentation abilities. Young adults were exposed to continuous streams of syllables that repeated into hidden novel words while watching a silent film. Afterward, learning was measured in a forced-choice test that contrasted hidden words with nonwords. The participants also had to indicate whether they explicitly recalled the word or not in order to dissociate explicit versus implicit knowledge. We additionally measured electroencephalography during exposure to measure neural entrainment to the repeating words. Engagement of the cognitive mechanisms was manipulated by using two methods. In experiment 1 (n = 36), inhibitory theta-burst stimulation (TBS) was applied to the left dorsolateral prefrontal cortex or to a control region. In experiment 2 (n = 60), participants performed a dual working-memory task that induced high or low levels of cognitive fatigue. In both experiments, cognitive depletion enhanced word recognition, especially when participants reported low confidence in remembering the words (i.e., when their knowledge was implicit). TBS additionally modulated neural entrainment to the words and syllables. These findings suggest that cognitive depletion improves the acquisition of linguistic knowledge in adults by unlocking implicit statistical learning mechanisms and support the hypothesis that adult language learning is antagonized by higher cognitive mechanisms.

In a randomised controlled trial, we showed that a five-day intervention combining anodal transcranial direct current stimulation over the left inferior frontal cortex with temporary speech fluency enhancing techniques reduces stuttering. Speech fluency was unchanged by the fluency training alone, as predicted. Here, we report the neural changes associated with the intervention, measured using functional MRI during sentence reading before the training and one-week later. We obtained imaging data in 25 adult men who stutter (median age = 32 y, inter-quartile range = 11) at the pre-intervention baseline and again one-week post-intervention. A control group of 15 adult men who do not stutter (median age = 30 y, inter-quartile range = 10) and did not complete the intervention were scanned on one occasion. In a whole-brain analysis of perceptibly fluent sentence reading, we compared the change in task-evoked neural activity in the sub-group of men who stutter who had received active stimulation during the intervention (N=13) with those who had sham stimulation (N=12). We hypothesised that the combination of anodal stimulation over the left inferior frontal cortex and fluency-enhancing training would result in lasting change to the brain networks supporting fluent speech production. An additional region-of-interest analysis explored effects on basal ganglia nuclei, which are thought to have a key role in the casual mechanism of stuttering, and which we hypothesised would be engaged by the behavioural approach used during training (choral and metronome-timed speaking). One week after the intervention, the group who had received active transcranial stimulation showed increased activity in speech-related brain regions, relative to the group who had received sham stimulation. Cortically, these changes were evident in left inferior frontal cortex (pars opercularis and orbitalis), anterior insula, anterior superior temporal gyrus, anterior cingulate cortex, and supplementary motor area. Subcortically, activation increased in the caudate nuclei and putamen bilaterally, and in right globus pallidus and thalamus. Together these regions form cortico-striatal-thalamo-cortical loops involved in the planning and initiation and control of speech movements.Our findings reveal that the mechanism of action of the tDCS intervention involved increasing activity across the network involved in the production of fluent speech, indicating that tDCS can be used to promote neural plasticity to strengthen networks supporting natural fluency. This study advances the potential of using non-invasive brain stimulation to improve therapy efficacy for those people who stutter who choose to work on increasing fluency.

Comprehending speech can be particularly challenging in a noisy environment and in the absence of semantic context. It has been proposed that the articulatory motor system would be recruited especially in difficult listening conditions. However, it remains unknown how signal-to-noise ratio (SNR) and semantic context affect the recruitment of the articulatory motor system when listening to continuous speech. The aim of the present study was to address the hypothesis that involvement of the articulatory motor cortex increases when the intelligibility and clarity of the spoken sentences decreases, because of noise and the lack of semantic context. We applied Transcranial Magnetic Stimulation (TMS) to the lip and hand representations in the primary motor cortex and measured motor evoked potentials from the lip and hand muscles, respectively, to evaluate motor excitability when young adults listened to sentences. In Experiment 1, we found that the excitability of the lip motor cortex was facilitated during listening to both semantically anomalous and coherent sentences in noise, but neither SNR nor semantic context modulated the facilitation. In Experiment 2, we replicated these findings and found no difference in the excitability of the lip motor cortex between sentences in noise and clear sentences without noise. Thus, our results show that the articulatory motor cortex is involved in speech processing even in optimal and ecologically valid listening conditions and that its involvement is not modulated by the intelligibility and clarity of speech.

See Crinion (doi:10.1093/brain/awy075) for a scientific commentary on this article. Chesters et al. present the first randomized controlled trial of transcranial direct current stimulation (tDCS) as an adjunctive therapy for stuttering. Thirty adult males underwent a five-day intervention using choral and metronome-timed speech to induce fluency while receiving anodal or sham stimulation. tDCS reduced disfluency for at least 6 weeks., See Crinion (doi:10.1093/brain/awy075) for a scientific commentary on this article. Stuttering is a neurodevelopmental condition affecting 5% of children, and persisting in 1% of adults. Promoting lasting fluency improvement in adults who stutter is a particular challenge. Novel interventions to improve outcomes are of value, therefore. Previous work in patients with acquired motor and language disorders reported enhanced benefits of behavioural therapies when paired with transcranial direct current stimulation. Here, we report the results of the first trial investigating whether transcranial direct current stimulation can improve speech fluency in adults who stutter. We predicted that applying anodal stimulation to the left inferior frontal cortex during speech production with temporary fluency inducers would result in longer-lasting fluency improvements. Thirty male adults who stutter completed a randomized, double-blind, controlled trial of anodal transcranial direct current stimulation over left inferior frontal cortex. Fifteen participants received 20 min of 1-mA stimulation on five consecutive days while speech fluency was temporarily induced using choral and metronome-timed speech. The other 15 participants received the same speech fluency intervention with sham stimulation. Speech fluency during reading and conversation was assessed at baseline, before and after the stimulation on each day of the 5-day intervention, and at 1 and 6 weeks after the end of the intervention. Anodal stimulation combined with speech fluency training significantly reduced the percentage of disfluent speech measured 1 week after the intervention compared with fluency intervention alone. At 6 weeks after the intervention, this improvement was maintained during reading but not during conversation. Outcome scores at both post-intervention time points on a clinical assessment tool (the Stuttering Severity Instrument, version 4) also showed significant improvement in the group receiving transcranial direct current stimulation compared with the sham group, in whom fluency was unchanged from baseline. We conclude that transcranial direct current stimulation combined with behavioural fluency intervention can improve fluency in adults who stutter. Transcranial direct current stimulation thereby offers a potentially useful adjunct to future speech therapy interventions for this population, for whom fluency therapy outcomes are currently limited.

Monitoring the sensory consequences of articulatory movements supports speaking. For example, delaying auditory feedback of a speaker's voice disrupts speech production. Also, there is evidence that this disruption may be decreased by immediate visual feedback, i.e., seeing one's own articulatory movements. It is, however, unknown whether delayed visual feedback affects speech production in fluent speakers. Here, the effects of delayed auditory and visual feedback on speech fluency (i.e., speech rate and errors), vocal control (i.e., intensity and pitch), and speech rhythm were investigated. Participants received delayed (by 200 ms) or immediate auditory feedback, while repeating sentences. Moreover, they received either no visual feedback, immediate visual feedback, or delayed visual feedback (by 200, 400, and 600 ms). Delayed auditory feedback affected fluency, vocal control, and rhythm. Immediate visual feedback had no effect on any of the speech measures when it was combined with delayed auditory feedback. Delayed visual feedback did, however, affect speech fluency when it was combined with delayed auditory feedback. In sum, the findings show that delayed auditory feedback disrupts fluency, vocal control, and rhythm and that delayed visual feedback can strengthen the disruptive effect of delayed auditory feedback on fluency.

According to the Intuitive Belief Hypothesis, supernatural belief relies heavily on intuitive thinking—and decreases when analytic thinking is engaged. After pointing out various limitations in prior attempts to support this Intuitive Belief Hypothesis, we test it across three new studies using a variety of paradigms, ranging from a pilgrimage field study to a neurostimulation experiment. In all three studies, we found no relationship between intuitive or analytical thinking and supernatural belief. We conclude that it is premature to explain belief in gods as ‘intuitive’, and that other factors, such as socio-cultural upbringing, are likely to play a greater role in the emergence and maintenance of supernatural belief than cognitive style.

Older adults often experience difficulties in understanding speech, partly because of age-related hearing loss. In young adults, activity of the left articulatory motor cortex is enhanced and it interacts with the auditory cortex via the left-hemispheric dorsal stream during speech processing. Little is known about the effect of ageing and age-related hearing loss on this auditory-motor interaction and speech processing in the articulatory motor cortex. It has been proposed that up-regulation of the motor system during speech processing could compensate for hearing loss and auditory processing deficits in older adults. Alternatively, age-related auditory deficits could reduce and distort the input from the auditory cortex to the articulatory motor cortex, suppressing recruitment of the motor system during listening to speech. The aim of the present study was to investigate the effects of ageing and age-related hearing loss on the excitability of the tongue motor cortex during listening to spoken sentences using transcranial magnetic stimulation and electromyography. Our results show that the excitability of the tongue motor cortex was facilitated during listening to speech in young and older adults with normal hearing. This facilitation was significantly reduced in older adults with hearing loss. These findings suggest a decline of auditory-motor processing of speech in adults with age-related hearing loss.

Mounting evidence indicates a role in perceptual decoding of speech for the dorsal auditory stream connecting between temporal auditory and frontal-parietal articulatory areas. The activation time course in auditory, somatosensory and motor regions during speech processing is seldom taken into account in models of speech perception. We critically review the literature with a focus on temporal information, and contrast between three alternative models of auditory-motor speech processing: parallel, hierarchical, and interactive. We argue that electrophysiological and transcranial magnetic stimulation studies support the interactive model. The findings reveal that auditory and somatomotor areas are engaged almost simultaneously, before 100 msec. There is also evidence of early interactions between auditory and motor areas. We propose a new interactive model of auditory-motor speech perception in which auditory and articulatory somatomotor areas are connected from early stages of speech processing. We also discuss how attention and other factors can affect the timing and strength of auditory-motor interactions and propose directions for future research.

Recent studies using repetitive transcranial magnetic stimulation (TMS) have demonstrated that disruptions of the articulatory motor cortex impair performance in demanding speech perception tasks. These findings have been interpreted as support for the idea that the motor cortex is critically involved in speech perception. However, the validity of this interpretation has been called into question, because it is unknown whether the TMS-induced disruptions in the motor cortex affect speech perception or rather response bias. In the present TMS study, we addressed this question by using signal detection theory to calculate sensitivity (i.e., d′) and response bias (i.e., criterion c). We used repetitive TMS to temporarily disrupt the lip or hand representation in the left motor cortex. Participants discriminated pairs of sounds from a “ba”–“da” continuum before TMS, immediately after TMS (i.e., during the period of motor disruption), and after a 30-min break. We found that the sensitivity for between-category pairs was reduced during the disruption of the lip representation. In contrast, disruption of the hand representation temporarily reduced response bias. This double dissociation indicates that the hand motor cortex contributes to response bias during demanding discrimination tasks, whereas the articulatory motor cortex contributes to perception of speech sounds.

We investigated cerebral processing of audiovisual speech stimuli in humans using functional magnetic resonance imaging (fMRI). Ten healthy volunteers were scanned with a 'clustered volume acquisition' paradigm at 3 T during observation of phonetically matching (e.g., visual and acoustic /y/) and conflicting (e.g., visual /a/ and acoustic /y/) audiovisual vowels. Both stimuli activated the sensory-specific auditory and visual cortices, along with the superior temporal, inferior frontal (Broca's area), premotor, and visual-parietal regions bilaterally. Phonetically conflicting vowels, contrasted with matching ones, specifically increased activity in Broca's area. Activity during phonetically matching stimuli, contrasted with conflicting ones, was not enhanced in any brain region. We suggest that the increased activity in Broca's area reflects processing of conflicting visual and acoustic phonetic inputs in partly disparate neuron populations. On the other hand, matching acoustic and visual inputs would converge on the same neurons.

Highlights • Native and non-native English speakers can visually discriminate English from an unknown language. • Viewing known speech excites the articulatory motor cortex more than unknown speech. • Viewing known speech excites the articulatory motor cortex more than non-speech mouth movements. • Motor excitability is high during observation of a face not speaking. • Motor excitability does not differ between native and non-native speakers., It is possible to comprehend speech and discriminate languages by viewing a speaker’s articulatory movements. Transcranial magnetic stimulation studies have shown that viewing speech enhances excitability in the articulatory motor cortex. Here, we investigated the specificity of this enhanced motor excitability in native and non-native speakers of English. Both groups were able to discriminate between speech movements related to a known (i.e., English) and unknown (i.e., Hebrew) language. The motor excitability was higher during observation of a known language than an unknown language or non-speech mouth movements, suggesting that motor resonance is enhanced specifically during observation of mouth movements that convey linguistic information. Surprisingly, however, the excitability was equally high during observation of a static face. Moreover, the motor excitability did not differ between native and non-native speakers. These findings suggest that the articulatory motor cortex processes several kinds of visual cues during speech communication.

Observing a speaker's articulatory gestures can contribute considerably to auditory speech perception. At the level of neural events, seen articulatory gestures can modify auditory cortex responses to speech sounds and modulate auditory cortex activity also in the absence of heard speech. However, possible effects of attention on this modulation have remained unclear. To investigate the effect of attention on visual speech‐induced auditory cortex activity, we scanned 10 healthy volunteers with functional magnetic resonance imaging (fMRI) at 3 T during simultaneous presentation of visual speech gestures and moving geometrical forms, with the instruction to either focus on or ignore the seen articulations. Secondary auditory cortex areas in the bilateral posterior superior temporal gyrus and planum temporale were active both when the articulatory gestures were ignored and when they were attended to. However, attention to visual speech gestures enhanced activity in the left planum temporale compared to the situation when the subjects saw identical stimuli but engaged in a nonspeech motion discrimination task. These findings suggest that attention to visually perceived speech gestures modulates auditory cortex function and that this modulation takes place at a hierarchically relatively early processing level. Hum Brain Mapp, 2005. © 2005 Wiley‐Liss, Inc.

During social interaction speech is perceived simultaneously by audition and vision. We studied interactions in the processing of auditory (A) and visual (V) speech signals in the human brain by comparing neuromagnetic responses to phonetically congruent audiovisual (AV) syllables with the arithmetic sum of responses to A and V syllables. Differences between AV and A+V responses were found bilaterally in the auditory cortices 150–200 ms and in the right superior temporal sulcus (STS) 250–600 ms after stimulus onset, showing that both sensory-specific and multisensory regions of the human temporal cortices are involved in AV speech processing. Importantly, our results suggest that AV interaction in the auditory cortex precedes that in the multisensory STS region.

Seeing a talker's articulatory gestures may affect the observer's auditory speech percept. Observing congruent articulatory gestures may enhance the recognition of speech sounds [J. Acoust. Soc. Am. 26 (1954) 212], whereas observing incongruent gestures may change the auditory percept phonetically as occurs in the McGurk effect [Nature 264 (1976) 746]. For example, simultaneous acoustic /ba/ and visual /ga/ are usually heard as /da/. We studied cortical processing of occasional changes in audiovisual and visual speech stimuli with magnetoencephalography. In the audiovisual experiment congruent (acoustic /iti/, visual /iti/) and incongruent (acoustic /ipi/, visual /iti/) audiovisual stimuli, which were both perceived as /iti/, were presented among congruent /ipi/ (acoustic /ipi/, visual /ipi/) stimuli. In the visual experiment only the visual components of these stimuli were presented. A visual change both in audiovisual and visual experiments activated supratemporal auditory cortices bilaterally. The auditory cortex activation to a visual change occurred later in the visual than in the audiovisual experiment, suggesting that interaction between modalities accelerates the detection of visual change in speech.

The motor regions that control movements of the articulators activate during listening to speech and contribute to performance in demanding speech recognition and discrimination tasks. Whether the articulatory motor cortex modulates auditory processing of speech sounds is unknown. Here, we aimed to determine whether the articulatory motor cortex affects the auditory mechanisms underlying discrimination of speech sounds in the absence of demanding speech tasks. Using electroencephalography, we recorded responses to changes in sound sequences, while participants watched a silent video. We also disrupted the lip or the hand representation in left motor cortex using transcranial magnetic stimulation. Disruption of the lip representation suppressed responses to changes in speech sounds, but not piano tones. In contrast, disruption of the hand representation had no effect on responses to changes in speech sounds. These findings show that disruptions within, but not outside, the articulatory motor cortex impair automatic auditory discrimination of speech sounds. The findings provide evidence for the importance of auditory-motor processes in efficient neural analysis of speech sounds.

Viewing another person's hand actions enhances excitability in an observer's left and right primary motor (M1) cortex. We aimed to determine whether viewing communicative hand actions alters this bilateral sensorimotor resonance. Using single-pulse transcranial magnetic stimulation (TMS), we measured excitability in the left and right M1 while right-handed non-signing participants observed bimanual communicative hand actions, i.e., meaningful signs in British Sign Language. TMS-induced motor evoked potentials were recorded from hand muscles during sign observation before and after teaching the participants to associate meanings with half of the signs. Before this teaching, when participants did not know that the presented hand actions were signs, excitability of left and right M1 was modulated equally. After learning the meanings of half the signs, excitability of the left, but not right, M1 was significantly enhanced. This left-lateralized enhancement of M1 excitability occurred during observation of signs with known and unknown meanings. The findings suggest that awareness of the communicative nature of another person's hand actions strengthens sensorimotor resonance in the left M1 cortex and alters hemispheric balance during action observation.

Publication in the conference proceedings of EUSIPCO, Tampere, Finland, 2000

International audience; This research topic presents speech as a natural, well-learned, multisensory communication signal, processed by multiple mechanisms. Reflecting the general status of the field, most articles focus on audiovisual speech perception and many utilize the McGurk effect, which arises when discrepant visual and auditory speech stimuli are presented (McGurk and MacDonald, 1976). Tiippana (2014) argues that the McGurk effect can be used as a proxy for multisensory integration provided it is not interpreted too narrowly. Several articles shed new light on audiovisual speech perception in special populations. It is known that individuals with autism spectrum disorder (ASD, e.g., Saalasti et al., 2012) or language impairment (e.g., Meronen et al., 2013) are generally less influenced by the talking face than peers with typical development. Here Stevenson et al. (2014) propose that a deficit in multisensory integration could be a marker of ASD, and a component of the associated deficit in communication. However, three studies suggest that integration is not deficient in some communication disorders. Irwin and Brancazio (2014) show that children with ASD looked less at the mouth region, resulting in poorer visual speech perception and consequently weaker visual influence. Leybaert et al. (2014) report that children with specific language impairment recognized visual and auditory speech less accurately than their controls, affecting audiovisual speech perception, while audiovisual integration per se seemed unimpaired. In a similar vein, adult patients with aphasia showed unisensory deficits but still integrated audiovisual speech information (Andersen and Starrfelt, 2015). Multisensory information can influence response accuracy and processing speed (e.g., Molholm et al., 2002; Klucharev et al., 2003). Scarbel et al. (2014) show that oral responses to speech in noise were faster but less accurate than manual responses, suggesting that oral responses are planned at an earlier stage than manual responses. Sekiyama et al. (2014) show that older adults were more influenced by visual speech than younger adults and correlated this fact to their slower reaction times to auditory stimuli. Altieri and Hudock (2014) report variation in reaction time and accuracy benefits for audiovisual speech in hearing-impaired observers, emphasizing the importance of individual differences in integration. Finally, Heald and Nusbaum (2014) show that when there were two possible talkers instead of just one, audiovisual information appeared to distract the observer from the task of word recognition and slowed down their performance. This finding demonstrates that multisensory stimulation does not always facilitate performance. While multisensory stimulation is thought to be beneficial for learning (Shams and Seitz, 2008), evidence for this is still scarce. In the current research topic, the overall utility of multisensory learning is brought under question. In a paradigm training to associate novel words and pictures , Bernstein et al. (2014) show no benefit of audiovisual presentation compared with auditory presentation for normal hearing individuals, and even a degradation for adults with hearing impairment. In a study of cued speech, i.e., specific hand-signs for different speech sounds, Bayard et al. (2014) demonstrate that individuals with hearing impairment used the visual cues differently from their controls, even though both groups were experts in cued speech. Kelly et al. (2014)

People can communicate by using hand actions, e.g., signs. Understanding communicative actions requires that the observer knows that the actor has an intention to communicate and the meanings of the actions. Here, we investigated how this prior knowledge affects processing of observed actions. We used functional MRI to determine changes in action processing when non-signers were told that the observed actions are communicative (i.e., signs) and learned the meanings of half of the actions. Processing of hand actions activated the left and right inferior frontal gyrus (IFG, BA 44 and 45) when the communicative intention of the actor was known, even when the meanings of the actions remained unknown. These regions were not active when the observers did not know about the communicative nature of the hand actions. These findings suggest that the left and right IFG play a role in understanding the intention of the actor, but do not process visuospatial features of the communicative actions. Knowing the meanings of the hand actions further enhanced activity in the anterior part of the IFG (BA 45), the inferior parietal lobule and posterior inferior and middle temporal gyri in the left hemisphere. These left-hemisphere language regions could provide a link between meanings and observed actions. In sum, the findings provide evidence for the segregation of the networks involved in the neural processing of visuospatial features of communicative hand actions and those involved in understanding the actor’s intention and the meanings of the actions., Highlights • Participants observed hand actions before and after learning that they are signs. • Learning-induced changes in brain activity measured using fMRI. • No activity in mirror neuron system when actions were not known to be communicative. • Knowing the actor’s intention to communicate activated IFG and IPL. • Knowing meanings of the actions increased activity in left IFG (BA 45), IPL and MTG.

Perceiving speech engages parts of the motor system involved in speech production. The role of the motor cortex in speech perception has been demonstrated using low-frequency repetitive transcranial magnetic stimulation (rTMS) to suppress motor excitability in the lip representation and disrupt discrimination of lip-articulated speech sounds (Möttönen & Watkins, 2009). Another form of rTMS, continuous theta-burst stimulation (cTBS), can produce longer-lasting disruptive effects following a brief train of stimulation. We investigated the effects of cTBS on motor excitability and discrimination of speech and non-speech sounds. cTBS was applied for 40 seconds over either the hand or the lip representation of motor cortex. Motor-evoked potentials recorded from the lip and hand muscles in response to single pulses of TMS revealed no measurable change in motor excitability due to cTBS. This failure to replicate previous findings may reflect the unreliability of measurements of motor excitability related to inter-individual variability. We also measured the effects of cTBS on a listener’s ability to discriminate:(1) lip-articulated speech sounds from sounds not articulated by the lips (‘ba’vs.‘da’); (2) two speech sounds not articulated by the lips (‘ga’vs.‘da’); and (3) non-speech sounds produced by the hands (‘claps’vs.‘clicks’). Discrimination of lip-articulated speech sounds was impaired between 20 and 35 minutes after cTBS over the lip motor representation. Specifically, discrimination of across-category ba–da sounds presented with an 800-ms inter-stimulus interval was reduced to chance level performance. This effect was absent for speech sounds that do not require the lips for articulation and non-speech sounds. Stimulation over the hand motor representation did not affect discrimination of speech or non-speech sounds. These findings show that stimulation of the lip motor representation disrupts discrimination of speech sounds in an articulatory feature-specific way.

Transcranial magnetic stimulation (TMS) has proven to be a useful tool in investigating the role of the articulatory motor cortex in speech perception. Researchers have used single-pulse and repetitive TMS to stimulate the lip representation in the motor cortex. The excitability of the lip motor representation can be investigated by applying single TMS pulses over this cortical area and recording TMS-induced motor evoked potentials (MEPs) via electrodes attached to the lip muscles (electromyography; EMG). Larger MEPs reflect increased cortical excitability. Studies have shown that excitability increases during listening to speech as well as during viewing speech-related movements. TMS can be used also to disrupt the lip motor representation. A 15-min train of low-frequency sub-threshold repetitive stimulation has been shown to suppress motor excitability for a further 15-20 min. This TMS-induced disruption of the motor lip representation impairs subsequent performance in demanding speech perception tasks and modulates auditory-cortex responses to speech sounds. These findings are consistent with the suggestion that the motor cortex contributes to speech perception. This article describes how to localize the lip representation in the motor cortex and how to define the appropriate stimulation intensity for carrying out both single-pulse and repetitive TMS experiments.

The earliest stages of cortical processing of speech sounds take place in the auditory cortex. Transcranial magnetic stimulation (TMS) studies have provided evidence that the human articulatory motor cortex contributes also to speech processing. For example, stimulation of the motor lip representation influences specifically discrimination of lip-articulated speech sounds. However, the timing of the neural mechanisms underlying these articulator-specific motor contributions to speech processing is unknown. Furthermore, it is unclear whether they depend on attention. Here, we used magnetoencephalography and TMS to investigate the effect of attention on specificity and timing of interactions between the auditory and motor cortex during processing of speech sounds. We found that TMS-induced disruption of the motor lip representation modulated specifically the early auditory-cortex responses to lip-articulated speech sounds when they were attended. These articulator-specific modulations were left-lateralized and remarkably early, occurring 60-100 ms after sound onset. When speech sounds were ignored, the effect of this motor disruption on auditory-cortex responses was nonspecific and bilateral, and it started later, 170 ms after sound onset. The findings indicate that articulatory motor cortex can contribute to auditory processing of speech sounds even in the absence of behavioral tasks and when the sounds are not in the focus of attention. Importantly, the findings also show that attention can selectively facilitate the interaction of the auditory cortex with specific articulator representations during speech processing. ispartof: JOURNAL OF NEUROSCIENCE vol:34 issue:11 pages:4064-4069 ispartof: location:United States status: published

Background: The ability to communicate using speech is a remarkable skill, which requires precise coordination of articulatory movements and decoding of complex acoustic signals. According to the traditional view, speech production and perception rely on motor and auditory brain areas, respectively. However, there is growing evidence that auditory-motor circuits support both speech production and perception.Aims: In this article we provide a review of how transcranial magnetic stimulation (TMS) has been used to investigate the excitability of the motor system during listening to speech and the contribution of the motor system to performance in various speech perception tasks. We also discuss how TMS can be used in combination with brain-imaging techniques to study interactions between motor and auditory systems during speech perception.Main contribution: TMS has proven to be a powerful tool to investigate the role of the articulatory motor system in speech perception.Conclusions: TMS studies have provided support for the view that the motor structures that control the movements of the articulators contribute not only to speech production but also to speech perception. © 2012 Copyright 2012 Psychology Press, an imprint of the Taylor and Francis Group, an Informa business.

Audiovisual speech perception has been considered to operate independent of sound location, since the McGurk effect (altered auditory speech perception caused by conflicting visual speech) has been shown to be unaffected by whether speech sounds are presented in the same or different location as a talking face. Here we show that sound location effects arise with manipulation of spatial attention. Sounds were presented from loudspeakers in five locations: the centre (location of the talking face) and 45°/90° to the left/right. Auditory spatial attention was focused on a location by presenting the majority (90%) of sounds from this location. In Experiment 1, the majority of sounds emanated from the centre, and the McGurk effect was enhanced there. In Experiment 2, the major location was 90° to the left, causing the McGurk effect to be stronger on the left and centre than on the right. Under control conditions, when sounds were presented with equal probability from all locations, the McGurk effect tended to be stronger for sounds emanating from the centre, but this tendency was not reliable. Additionally, reaction times were the shortest for a congruent audiovisual stimulus, and this was the case independent of location. Our main finding is that sound location can modulate audiovisual speech perception, and that spatial attention plays a role in this modulation.

Watching the lips of a speaker enhances speech perception. At the same time, the 100 ms response to speech sounds is suppressed in the observer's auditory cortex. Here, we used whole-scalp 306-channel magnetoencephalography (MEG) to study whether lipreading modulates human auditory processing already at the level of the most elementary sound features, i.e., pure tones. We further envisioned the temporal dynamics of the suppression to tell whether the effect is driven by top-down influences. Nineteen subjects were presented with 50 ms tones spanning six octaves (125–8000 Hz) (1) during “lipreading,” i.e., when they watched video clips of silent articulations of Finnish vowels /a/, /i/, /o/, and /y/, and reacted to vowels presented twice in a row; (2) during a visual control task; (3) during a still-face passive control condition; and (4) in a separate experiment with a subset of nine subjects, during covert production of the same vowels. Auditory-cortex 100 ms responses (N100m) were equally suppressed in the lipreading and covert-speech-production tasks compared with the visual control and baseline tasks; the effects involved all frequencies and were most prominent in the left hemisphere. Responses to tones presented at different times with respect to the onset of the visual articulation showed significantly increased N100m suppression immediately after the articulatory gesture. These findings suggest that the lipreading-related suppression in the auditory cortex is caused by top-down influences, possibly by an efference copy from the speech-production system, generated during both own speech and lipreading.

Listening to speech modulates activity in human motor cortex. It is unclear, however, whether the motor cortex has an essential role in speech perception. Here, we aimed to determine whether the motor representations of articulators contribute to categorical perception of speech sounds. Categorization of continuously variable acoustic signals into discrete phonemes is a fundamental feature of speech communication. We used repetitive transcranial magnetic stimulation (rTMS) to temporarily disrupt the lip representation in the left primary motor cortex. This disruption impaired categorical perception of artificial acoustic continua ranging between two speech sounds that differed in place of articulation, in that the vocal tract is opened and closed rapidly either with the lips or the tip of the tongue (/ba/–/da/ and /pa/–/ta/). In contrast, it did not impair categorical perception of continua ranging between speech sounds that do not involve the lips in their articulation (/ka/–/ga/ and /da/–/ga/). Furthermore, an rTMS-induced disruption of the hand representation had no effect on categorical perception of either of the tested continua (/ba/–da/ and /ka/–/ga/). These findings indicate that motor circuits controlling production of speech sounds also contribute to their perception. Mapping acoustically highly variable speech sounds onto less variable motor representations may facilitate their phonemic categorization and be important for robust speech perception.

Recent studies have yielded contradictory evidence on whether visual speech perception (watching articulatory gestures) can activate the human primary auditory cortex. To circumvent confounds due to inter-individual anatomical variation, we defined our subjects' Heschl's gyri and assessed blood oxygenation-dependent signal changes at 3 T within this confined region during visual speech perception and observation of moving circles. Visual speech perception activated Heschl's gyri in nine subjects, with activation in seven of them extending to the area of primary auditory cortex. Activation was significantly stronger during visual speech perception than during observation of the moving circles. Further, a significant hemisphere by stimulus interaction occurred, suggesting left Heschl's gyrus specialization for visual speech processing.

The technique of 306-channel magnetoencephalogaphy (MEG) was used in eight healthy volunteers to test whether silent lip-reading modulates auditory-cortex processing of phonetic sounds. Auditory test stimuli (either Finnish vowel /ae/ or /ø/) were preceded by a 500 ms lag by either another auditory stimulus (/ae/, /ø/ or the second-formant midpoint between /ae/ and /ø/), or silent movie of a person articulating /ae/ or /ø/. Compared with N1 responses to auditory /ae/ and /ø/ when presented without a preceding stimulus, the amplitudes of left-hemisphere N1 responses to the test stimuli were significantly suppressed both when preceded by auditory and visual stimuli, this effect being significantly stronger with preceding auditory stimuli. This suggests that seeing articulatory gestures of a speaker influences auditory speech perception by modulating the responsiveness of auditory-cortex neurons.

We studied the interactions in neural processing of auditory and visual speech by recording event-related brain potentials (ERPs). Unisensory (auditory - A and visual - V) and audiovisual (AV) vowels were presented to 11 subjects. AV vowels were phonetically either congruent (e.g., acoustic /a/ and visual /a/) or incongruent (e.g., acoustic /a/ and visual /y/). ERPs to AV stimuli and the sum of the ERPs to A and V stimuli (A+V) were compared. Similar ERPs to AV and A+V were hypothesized to indicate independent processing of A and V stimuli. Differences on the other hand would suggest AV interactions. Three deflections, the first peaking at about 85 ms after the A stimulus onset, were significantly larger in the ERPs to A+V than in the ERPs to both congruent and incongruent AV stimuli. We suggest that these differences reflect AV interactions in the processing of general, non-phonetic, features shared by the acoustic and visual stimulus (spatial location, coincidence in time). The first difference in the ERPs to incongruent and congruent AV vowels peaked at 155 ms from the A stimuli onset. This and two later differences are suggested to reflect interactions at phonetic level. The early general AV interactions probably reflect modified activity in the sensory-specific cortices, whereas the later phonetic AV interactions are likely generated in the heteromodal cortices. Thus, our results suggest that sensory-specific and heteromodal brain regions participate in AV speech integration at separate latencies and are sensitive to different features of A and V speech stimuli.

We studied the modification of auditory perception in three different conditions in twenty subjects. Observing other person's discordant articulatory gestures deteriorated identification of acoustic speech stimuli and modified the auditory percept, causing a strong McGurk effect. A similar effect was found when the subjects watched their own silent articulation in a mirror and acoustic stimuli were simultaneously presented to their ears. Interestingly, a smaller but significant effect was even obtained when the subjects just silently articulated the syllables without visual feedback. On the other hand, observing other person's or one's own concordant articulation and silently articulating a concordant syllable improved identification of the acoustic stimuli. The modification of auditory percepts caused by visual observation of speech and silently articulating it are both suggested to be due to the alteration of activity in the auditory cortex. Our findings support the idea of a close relationship between speech perception and production.

The sensory systems have an important role in speech production. Monitoring sensory consequences of articulatory movements supports fluent speaking. It is well known that delayed auditory feedback disrupts fluency of speech. Also, there is some evidence that immediate visual feedback, i.e., seeing one’s own articulatory movements in a mirror, decreases the disruptive effect of delayed auditory feedback (Jones and Striemer, 2007). It is unknown whether delayed visual feedback affects fluency of speech. Here, we aimed to investigate the effects of delayed auditory, visual and audiovisual feedback on speech fluency. 20 native English speakers (with no history of speech and language problems) participated in the experiment. Participants received delayed (200 ms) or immediate auditory feedback, whilst repeating sentences. Moreover, they received either no visual feedback, immediate visual feedback or delayed visual feedback (200, 400, 600 ms). Under delayed auditory feedback, the duration of sentences was longer and number of speech errors was greater than under immediate auditory feedback, confirming that delayed auditory feedback disrupts speech. Immediate visual feedback had no effect on speech fluency. Importantly, fluency of speech was most disrupted when both auditory and visual feedback was delayed, suggesting that delayed visual feedback strengthened the disruptive effect of delayed auditory feedback. However, delayed visual feedback combined with immediate auditory feedback had no effect on speech fluency. Our findings demonstrate that although visual feedback is not available during speaking in every-day life, it can be integrated with auditory feedback and influence fluency of speech.

The left superior temporal cortex shows greater responsiveness to speech than to non-speech sounds according to previous neuroimaging studies, suggesting that this brain region has a special role in speech processing. However, since speech sounds differ acoustically from the non-speech sounds, it is possible that this region is not involved in speech perception per se, but rather in processing of some complex acoustic features. "Sine wave speech" (SWS) provides a tool to study neural speech specificity using identical acoustic stimuli, which can be perceived either as speech or non-speech, depending on previous experience of the stimuli. We scanned 21 subjects using 3T functional MRI in two sessions, both including SWS and control stimuli. In the pre-training session, all subjects perceived the SWS stimuli as non-speech. In the post-training session, the identical stimuli were perceived as speech by 16 subjects. In these subjects, SWS stimuli elicited significantly stronger activity within the left posterior superior temporal sulcus (STSp) in the post- vs. pre-training session. In contrast, activity in this region was not enhanced after training in 5 subjects who did not perceive SWS stimuli as speech. Moreover, the control stimuli, which were always perceived as non-speech, elicited similar activity in this region in both sessions. Altogether, the present findings suggest that activation of the neural speech representations in the left STSp might be a pre-requisite for hearing sounds as speech.