Time-course of phonetic (motor speech) encoding in utterance production.

Speaking involves the preparation of the linguistic content of an utterance and of the motor programs leading to articulation. The temporal dynamics of linguistic versus motor-speech (phonetic) encoding is highly debated: phonetic encoding has been associated either to the last quarter of an utterance preparation time (∼150ms before articulation), or to virtually the entire planning time, simultaneously with linguistic encoding. We (i) review the evidence on the time-course of motor-speech encoding based on EEG/MEG event-related (ERP) studies and (ii) strive to replicate the early effects of phonological-phonetic factors in referential word production by reanalysing a large EEG/ERP dataset. The review indicates that motor-speech encoding is engaged during at least the last 300ms preceding articulation (about half of a word planning lag). By contrast, the very early involvement of phonological-phonetic factors could be replicated only partially and is not as robust as in the second half of the utterance planning time-window.

The McGurk Illusion: A Default Mechanism of the Auditory System.

Recent studies have questioned past conclusions regarding the mechanisms of the McGurk illusion, especially how McGurk susceptibility might inform our understanding of audiovisual (AV) integration. We previously proposed that the McGurk illusion is likely attributable to a default mechanism, whereby either the visual system, auditory system, or both default to specific phonemes-those implicated in the McGurk illusion. We hypothesized that the default mechanism occurs because visual stimuli with an indiscernible place of articulation (like those traditionally used in the McGurk illusion) lead to an ambiguous perceptual environment and thus a failure in AV integration. In the current study, we tested the default hypothesis as it pertains to the auditory system. Participants performed two tasks. One task was a typical McGurk illusion task, in which individuals listened to auditory-/ba/ paired with visual-/ga/ and judged what they heard. The second task was an auditory-only task, in which individuals transcribed trisyllabic words with a phoneme replaced by silence. We found that individuals' transcription of missing phonemes often defaulted to '/d/t/th/', the same phonemes often experienced during the McGurk illusion. Importantly, individuals' default rate was positively correlated with their McGurk rate. We conclude that the McGurk illusion arises when people fail to integrate visual percepts with auditory percepts, due to visual ambiguity, thus leading the auditory system to default to phonemes often implicated in the McGurk illusion.

Phonetic and Lexical Encoding of Tone in Cantonese Heritage Speakers.

Heritage speakers contend with at least two languages: the less dominant first language (L1), that is, the heritage language, and the more dominant second language (L2). In some cases, their L1 and L2 bear striking phonological differences. In the current study, we investigate Toronto-born Cantonese heritage speakers and their maintenance of Cantonese lexical tone, a linguistic feature that is absent from English, the more dominant L2. Across two experiments, Cantonese heritage speakers were tested on their phonetic/phonological and lexical encoding of tone in Cantonese. Experiment 1 was an AX discrimination task with varying inter-stimulus intervals (ISIs), which revealed that heritage speakers discriminated tone pairs with disparate pitch contours better than those with shared pitch contours. Experiment 2 was a medium-term repetition priming experiment, designed to extend the findings of Experiment 1 by examining tone representations at the lexical level. We observed a positive correlation between English dominance and priming in tone minimal pairs that shared contours. Thus, while increased English dominance does not affect heritage speakers' phonological-level representations, tasks that require lexical access suggest that heritage Cantonese speakers may not robustly and fully distinctively encode Cantonese tone in lexical memory.

Rethinking the Mechanisms Underlying the McGurk Illusion.

The McGurk illusion occurs when listeners hear an illusory percept (i.e., "da"), resulting from mismatched pairings of audiovisual (AV) speech stimuli (i.e., auditory/ba/paired with visual/ga/). Hearing a third percept-distinct from both the auditory and visual input-has been used as evidence of AV fusion. We examined whether the McGurk illusion is instead driven by visual dominance, whereby the third percept, e.g., "da," represents a default percept for visemes with an ambiguous place of articulation (POA), like/ga/. Participants watched videos of a talker uttering various consonant vowels (CVs) with (AV) and without (V-only) audios of/ba/. Individuals transcribed the CV they saw (V-only) or heard (AV). In the V-only condition, individuals predominantly saw "da"/"ta" when viewing CVs with indiscernible POAs. Likewise, in the AV condition, upon perceiving an illusion, they predominantly heard "da"/"ta" for CVs with indiscernible POAs. The illusion was stronger in individuals who exhibited weak/ba/auditory encoding (examined using a control auditory-only task). In Experiment2, we attempted to replicate these findings using stimuli recorded from a different talker. The V-only results were not replicated, but again individuals predominately heard "da"/"ta"/"tha" as an illusory percept for various AV combinations, and the illusion was stronger in individuals who exhibited weak/ba/auditory encoding. These results demonstrate that when visual CVs with indiscernible POAs are paired with a weakly encoded auditory/ba/, listeners default to hearing "da"/"ta"/"tha"-thus, tempering the AV fusion account, and favoring a default mechanism triggered when both AV stimuli are ambiguous.

Broca's Area as a Pre-articulatory Phonetic Encoder: Gating the Motor Program.

The exact nature of the role of Broca's area in control of speech and whether it is exerted at the cognitive or at the motor level is still debated. Intraoperative evidence of a lack of motor responses to direct electrical stimulation (DES) of Broca's area and the observation that its stimulation induces a "speech arrest" without an apparent effect on the ongoing activity of phono-articulatory muscles, raises the argument. Essentially, attribution of direct involvement of Broca's area in motor control of speech, requires evidence of a functional connection of this area with the phono-articulatory muscles' motoneurons. With a quantitative approach we investigated, in 20 patients undergoing surgery for brain tumors, whether DES delivered on Broca's area affects the recruitment of the phono-articulatory muscles' motor units. The electromyography (EMG) of the muscles active during two speech tasks (object picture naming and counting) was recorded during and in absence of DES on Broca's area. Offline, the EMG of each muscle was analyzed in frequency (power spectrum, PS) and time domain (root mean square, RMS) and the two conditions compared. Results show that DES on Broca's area induces an intensity-dependent "speech arrest." The intensity of DES needed to induce "speech arrest" when applied on Broca's area was higher when compared to the intensity effective on the neighboring pre-motor/motor cortices. Notably, PS and RMS measured on the EMG recorded during "speech arrest" were superimposable to those recorded at baseline. Partial interruptions of speech were not observed. Speech arrest was an "all-or-none" effect: muscle activation started only by removing DES, as if DES prevented speech onset. The same effect was observed when stimulating directly the subcortical fibers running below Broca's area. Intraoperative data point to Broca's area as a functional gate authorizing the phonetic translation to be executed by the motor areas. Given the absence of a direct effect on motor units recruitment, a direct control of Broca's area on the phono-articulatory apparatus seems unlikely. Moreover, the strict correlation between DES-intensity and speech prevention, might attribute this effect to the inactivation of the subcortical fibers rather than to Broca's cortical neurons.

Do speakers have access to a mental syllabary? ERP comparison of high frequency and novel syllable production.

The transformation of an abstract phonological code into articulation has been hypothesized to involve the retrieval of stored syllable-sized motor plans. Accordingly, gestural scores for frequently used syllables are retrieved from memory whereas gestural scores for novel and possibly low frequency syllables are assembled on-line. The present study was designed to test this hypothesis. Participants produced disyllabic pseudowords with high frequency, low frequency and non-existent (novel) initial syllables. Behavioral results revealed slower production latencies for novel than for high frequency syllables. Event-related potentials diverged in waveform amplitudes and global topographic patterns between high frequency and low frequency/novel syllables around 170 ms before the onset of articulation. These differences indicate the recruitment of different brain networks during the production of frequent and infrequent/novel syllables, in line with the hypothesis that speakers store syllabic-sized motor programs for frequent syllables and assemble these motor plans on-line for low frequency and novel syllables.

Beta oscillations reflect memory and motor aspects of spoken word production.

Two major components form the basis of spoken word production: the access of conceptual and lexical/phonological information in long-term memory, and motor preparation and execution of an articulatory program. Whereas the motor aspects of word production have been well characterized as reflected in alpha-beta desynchronization, the memory aspects have remained poorly understood. Using magnetoencephalography, we investigated the neurophysiological signature of not only motor but also memory aspects of spoken-word production. Participants named or judged pictures after reading sentences. To probe the involvement of the memory component, we manipulated sentence context. Sentence contexts were either constraining or nonconstraining toward the final word, presented as a picture. In the judgment task, participants indicated with a left-hand button press whether the picture was expected given the sentence. In the naming task, they named the picture. Naming and judgment were faster with constraining than nonconstraining contexts. Alpha-beta desynchronization was found for constraining relative to nonconstraining contexts pre-picture presentation. For the judgment task, beta desynchronization was observed in left posterior brain areas associated with conceptual processing and in right motor cortex. For the naming task, in addition to the same left posterior brain areas, beta desynchronization was found in left anterior and posterior temporal cortex (associated with memory aspects), left inferior frontal cortex, and bilateral ventral premotor cortex (associated with motor aspects). These results suggest that memory and motor components of spoken word production are reflected in overlapping brain oscillations in the beta band.