A little more conversation, a little less action: Candidate roles for motor cortex in speech perception

The motor theory of speech perception assumes that activation of the motor system is essential in the perception of speech. However, deficits in speech perception and comprehension do not arise from damage that is restricted to the motor cortex, few functional imaging studies reveal activity in motor cortex during speech perception, and the motor cortex is strongly activated by many different sound categories. Here, we evaluate alternative roles for the motor cortex in spoken communication and suggest a specific role in sensorimotor processing in conversation. We argue that motor-cortex activation it is essential in joint speech, particularly for the timing of turn-taking

Quest for an open MRI scanner

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On the context-dependent nature of the contribution of the ventral premotor cortex to speech perception

What is the nature of the interface between speech perception and production, where auditory and motor representations converge? One set of explanations suggests that during perception, the motor circuits involved in producing a perceived action are in some way enacting the action without actually causing movement (covert simulation) or sending along the motor information to be used to predict its sensory consequences (i.e., efference copy). Other accounts either reject entirely the involvement of motor representations in perception, or explain their role as being more supportive than integral, and not employing the identical circuits used in production. Using fMRI, we investigated whether there are brain regions that are conjointly active for both speech perception and production, and whether these regions are sensitive to articulatory (syllabic) complexity during both processes, which is predicted by a covert simulation account. A group of healthy young adults (1) observed a female speaker produce a set of familiar words (perception), and (2) observed and then repeated the words (production). There were two types of words, varying in articulatory complexity, as measured by the presence or absence of consonant clusters. The simple words contained no consonant cluster (e.g. “palace”), while the complex words contained one to three consonant clusters (e.g. “planet”). Results indicate that the left ventral premotor cortex (PMv) was significantly active during speech perception and speech production but that activation in this region was scaled to articulatory complexity only during speech production, revealing an incompletely specified efferent motor signal during speech perception. The right planum temporal (PT) was also active during speech perception and speech production, and activation in this region was scaled to articulatory complexity during both production and perception. These findings are discussed in the context of current theories of speech perception, with particular attention to accounts that include an explanatory role for mirror neurons

Corpus Callosum and Word Reading in Adult Survivors of Childhood Posterior Fossa Tumors

Adult survivors of childhood posterior fossa tumors can experience reading difficulties related to white matter integrity. Previously, reading was shown to be related to cortical white matter tracts, however information transfer across the corpus callosum (CC) may also play a role in reading. The current study used both macro- and microstructural measures of the WM structure of the corpus callosum. The current study examined how white matter volume and fractional anisotropy (FA) in five divisions of the CC was related to degree of neurological risk and reading skill, and tested two mediation models predicting reading. Participants included 20 adult survivors of childhood posterior fossa tumor and 23 healthy controls. Volume and FA were measured in five divisions of the mid-sagittal corpus callosum. Total intracranial vault was used as a covariate in volume analyses. FA was reduced in CC1 and volume was reduced in each subregion in survivors. Volume but not FA was related to degree of neurological risk. Results identified that reduced volume in CC1 and CC5, and FA in CC5 appear to be specifically related to reading skill in line with the cortical reading regions that connect in these subregions of the CC. Mediation models indicate that processing speed is the mechanism by which volume is related to reading skill. These findings have implications for addressing processing speed in reading interventions in survivors and provide insight into the interhemispheric connections in the reading network

Comprehension in-situ: how multimodal information shapes language processing

The human brain supports communication in dynamic face-to-face environments where spoken words are embedded in linguistic discourse and accompanied by multimodal cues, such as prosody, gestures and mouth movements. However, we only have limited knowledge of how these multimodal cues jointly modulate language comprehension. In a series of behavioural and EEG studies, we investigated the joint impact of these cues when processing naturalistic-style materials. First, we built a mouth informativeness corpus of English words, to quantify mouth informativeness of a large number of words used in the following experiments. Then, across two EEG studies, we found and replicated that native English speakers use multimodal cues and that their interactions dynamically modulate N400 amplitude elicited by words that are less predictable in the discourse context (indexed by surprisal values per word). We then extended the findings to second language comprehenders, finding that multimodal cues modulate L2 comprehension, just like in L1, but to a lesser extent; although L2 comprehenders benefit more from meaningful gestures and mouth movements. Finally, in two behavioural experiments investigating whether multimodal cues jointly modulate the learning of new concepts, we found some evidence that presence of iconic gestures improves memory, and that the effect may be larger if information is presented also with prosodic accentuation. Overall, these findings suggest that real-world comprehension uses all cues present and weights cues differently in a dynamic manner. Therefore, multimodal cues should not be neglected for language studies. Investigating communication in naturalistic contexts containing more than one cue can provide new insight into our understanding of language comprehension in the real world

Audiovisual Integration in Apraxia of Speech: EEG Evidence for Processing Differences

Speech perception is a unique audiovisual experience in part because timing of the speech signal is influenced by simultaneous overlapping gestures in coarticulation. Apraxia of speech (AOS) is a motor planning disorder that impairs coarticulation. Imaging studies show that brain regions damaged in AOS are critical to audiovisual speech perception. Although AOS is a motor planning disorder, individuals with AOS may have a disruption to the perceptual system for speech gestures. To evaluate this hypothesis we investigated audiovisual mismatch negativity (MMN) brain responses in adults with damage to Broca’s area (n =5) compared to a healthy age-matched comparison group (n = 5). We utilized the McGurk effect, in which incongruent auditory and visual information alters perception. Participants viewed videos of a speaker articulating the syllable /ba/ (standard) for 80% trials and /ga/ (deviant) for 20% of the trials while the auditory stimulus /ba/ remained consistent throughout. Responses to this McGurk audiovisual condition were compared to an inverse McGurk audiovisual condition in which the visual stimulus remained constant while the auditory stimulus changed, and a visual-only condition without sound to control for evoked activity from changes to the visual stimulus. Incongruent McGurk deviants elicited an MMN over left hemisphere electrodes in the comparison group, while the AOS group exhibited a later, attention-based response, a P300. The AOS group similarly responded to inverse McGurk deviants, which do not require fusion of the percept, with a P300 response, indicating that auditory and visual aspects of the incongruent McGurk deviants were not integrated. In the visual-only control condition, the AOS group showed a left-lateralized MMN, suggesting greater influence of visual processing when confronted with conflicting multisensory information compared to the comparison group. Overall, the comparison group’s responses were indicative of early and automatic audiovisual integration of incongruent McGurk percepts while the responses of the AOS group showed contributions of both attentional and visual processing. The timing of the response in the AOS group was correlated with speech production characteristics of apraxia, as well as performance on taxing motor speech tasks. Results of this study support the hypothesis that AOS is a disorder beyond motor planning, with implications for higher-level linguistic and cognitive systems