Pascale Tremblay

Imaging speech production using fMRI

Human speech is a well-learned, sensorimotor, and ecological behavior ideal for the study of neural processes and brain-behavior relations. With the advent of modern neuroimaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), the potential for investigating neural mechanisms of speech motor control, speech motor disorders, and speech motor development has increased. However, a practical issue has limited the application of fMRI to issues in spoken language production and other related behaviors (singing, swallowing). Producing these behaviors during volume acquisition introduces motion-induced signal changes that confound the activation signals of interest. A number of approaches, ranging from signal processing to using silent or covert speech, have attempted to remove or prevent the effects of motion-induced artefact. However, these approaches are flawed for a variety of reasons. An alternative approach, that has only recently been applied to study single-word production, uses pauses in volume acquisition during the production of natural speech motion. Here we present some representative data illustrating the problems associated with motion artefacts and some qualitative results acquired from subjects producing short sentences and orofacial nonspeech movements in the scanner. Using pauses or silent intervals in volume acquisition and block designs, results from individual subjects result in robust activation without motion-induced signal artefact. This approach is an efficient method for studying the neural basis of spoken language production and the effects of speech and language disorders using fMRI.

A mediating role of the premotor cortex in phoneme segmentation.

Consistent with a functional role of the motor system in speech perception, disturbing the activity of the left ventral premotor cortex by means of repetitive transcranial magnetic stimulation (rTMS) has been shown to impair auditory identification of syllables that were masked with white noise. However, whether this region is crucial for speech perception under normal listening conditions remains debated. To directly test this hypothesis, we applied rTMS to the left ventral premotor cortex and participants performed auditory speech tasks involving the same set of syllables but differing in the use of phonemic segmentation processes. Compared to sham stimulation, rTMS applied over the ventral premotor cortex resulted in slower phoneme discrimination requiring phonemic segmentation. No effect was observed in phoneme identification and syllable discrimination tasks that could be performed without need for phonemic segmentation. The findings demonstrate a mediating role of the ventral premotor cortex in speech segmentation under normal listening conditions and are interpreted in relation to theories assuming a link between perception and action in the human speech processing system.

The Language Connectome: New Pathways, New Concepts

The field of the neurobiology of language is experiencing a paradigm shift in which the predominant Broca–Wernicke–Geschwind language model is being revised in favor of models that acknowledge that language is processed within a distributed cortical and subcortical system. While it is important to identify the brain regions that are part of this system, it is equally important to establish the anatomical connectivity supporting their functional interactions. The most promising framework moving forward is one in which language is processed via two interacting “streams”—a dorsal and ventral stream—anchored by long association fiber pathways, namely the superior longitudinal fasciculus/arcuate fasciculus, uncinate fasciculus, inferior longitudinal fasciculus, inferior fronto-occipital fasciculus, and two less well-established pathways, the middle longitudinal fasciculus and extreme capsule. In this article, we review the most up-to-date literature on the anatomical connectivity and function of these pathways. We also review and emphasize the importance of the often overlooked cortico-subcortical connectivity for speech via the “motor stream” and associated fiber systems, including a recently identified cortical association tract, the frontal aslant tract. These pathways anchor the distributed cortical and subcortical systems that implement speech and language in the human brain.

Contribution of the frontal lobe to externally and internally specified verbal responses: fMRI evidence

It has been suggested that within the frontal cortex there is a lateral to medial shift in the control of action, with the lateral premotor area (PMA) involved in externally specified actions and the medial supplementary motor areas (SMA) involved in internally specified actions. Recent brain imaging studies demonstrate, however, that the control of externally and internally specified actions may involve more complex and overlapping networks involving not only the PMA and the SMA, but also the pre-SMA and the lateral prefrontal cortex (PFC). The aim of the present study was to determine whether these frontal regions are differentially involved in the production of verbal responses, when they are externally specified and when they are internally specified. Participants engaged in three overt speaking tasks in which the degree of response specification differed. The tasks involved reading aloud words (externally specified), or generating words aloud from narrow or broad semantic categories (internally specified). Using fMRI, the location and magnitude of the BOLD activity for these tasks was measured in a group of ten participants. Compared with rest, all tasks activated the primary motor area and the SMA-proper, reflecting their common role in speech production. The magnitude of the activity in the PFC (Brodmann area 45), the left PMAv and the pre-SMA increased for word generation, suggesting that each of these three regions plays a role in internally specified action selection. This confirms previous reports concerning the participation of the pre-SMA in verbal response selection. The pattern of activity in PMAv suggests participation in both externally and internally specified verbal actions.

On the selection of words and oral motor responses: evidence of a response-independent fronto-parietal network.

Several brain areas including the medial and lateral premotor areas, and the prefrontal cortex, are thought to be involved in response selection. It is unclear, however, what the specific contribution of each of these areas is. It is also unclear whether the response selection process operates independent of response modality or whether a number of specialized processes are recruited depending on the behaviour of interest. In the present study, the neural substrates for different response selection modes (volitional and stimulus-driven) were compared, using sparse-sampling functional magnetic resonance imaging, for two different response modalities: words and comparable oral motor gestures. Results demonstrate that response selection relies on a network of prefrontal, premotor and parietal areas, with the pre-supplementary motor area (pre-SMA) at the core of the process. Overall, this network is sensitive to the manner in which responses are selected, despite the absence of a medio-lateral axis, as was suggested by Goldberg (1985). In contrast, this network shows little sensitivity to the modality of the response, suggesting of a domain-general selection process. Theoretical implications of these results are discussed.

From Language Comprehension to Action Understanding and Back Again

A controversial question in cognitive neuroscience is whether comprehension of words and sentences engages brain mechanisms specific for decoding linguistic meaning or whether language comprehension occurs through more domain-general sensorimotor processes. Accumulating behavioral and neuroimaging evidence suggests a role for cortical motor and premotor areas in passive action-related language tasks, regions that are known to be involved in action execution and observation. To examine the involvement of these brain regions in language and nonlanguage tasks, we used functional magnetic resonance imaging (fMRI) on a group of 21 healthy adults. During the fMRI session, all participants 1) watched short object-related action movies, 2) looked at pictures of man-made objects, and 3) listened to and produced short sentences describing object-related actions and man-made objects. Our results are among the first to reveal, in the human brain, a functional specialization within the ventral premotor cortex (PMv) for observing actions and for observing objects, and a different organization for processing sentences describing actions and objects. These findings argue against the strongest version of the simulation theory for the processing of action-related language.

Contribution of the pre-SMA to the production of words and non-speech oral motor gestures, as revealed by repetitive transcranial magnetic stimulation (rTMS).

An emerging theoretical perspective, largely based on neuroimaging studies, suggests that the pre-SMA is involved in planning cognitive aspects of motor behavior and language, such as linguistic and non-linguistic response selection. Neuroimaging studies, however, cannot indicate whether a brain region is equally important to all tasks in which it is activated. In the present study, we tested the hypothesis that the pre-SMA is an important component of response selection, using an interference technique. High frequency repetitive TMS (10 Hz) was used to interfere with the functioning of the pre-SMA during tasks requiring selection of words and oral gestures under different selection modes (forced, volitional) and attention levels (high attention, low attention). Results show that TMS applied to the pre-SMA interferes selectively with the volitional selection condition, resulting in longer RTs. The low- and high-attention forced selection conditions were unaffected by TMS, demonstrating that the pre-SMA is sensitive to selection mode but not attentional demands. TMS similarly affected the volitional selection of words and oral gestures, reflecting the response-independent nature of the pre-SMA contribution to response selection. The implications of these results are discussed.

Processing of speech and non-speech sounds in the supratemporal plane: Auditory input preference does not predict sensitivity to statistical structure

The supratemporal plane contains several functionally heterogeneous subregions that respond strongly to speech. Much of the prior work on the issue of speech processing in the supratemporal plane has focused on neural responses to single speech vs. non-speech sounds rather than focusing on higher-level computations that are required to process more complex auditory sequences. Here we examined how information is integrated over time for speech and non-speech sounds by quantifying the BOLD fMRI response to stochastic (non-deterministic) sequences of speech and non-speech naturalistic sounds that varied in their statistical structure (from random to highly structured sequences) during passive listening. Behaviorally, the participants were accurate in segmenting speech and non-speech sequences, though they were more accurate for speech. Several supratemporal regions showed increased activation magnitude for speech sequences (preference), but, importantly, this did not predict sensitivity to statistical structure: (i) several areas showing a speech preference were sensitive to statistical structure in both speech and non-speech sequences, and (ii) several regions that responded to both speech and non-speech sounds showed distinct responses to statistical structure in speech and non-speech sequences. While the behavioral findings highlight the tight relation between statistical structure and segmentation processes, the neuroimaging results suggest that the supratemporal plane mediates complex statistical processing for both speech and non-speech sequences and emphasize the importance of studying the neurocomputations associated with auditory sequence processing. These findings identify new partitions of functionally distinct areas in the supratemporal plane that cannot be evoked by single stimuli. The findings demonstrate the importance of going beyond input preference to examine the neural computations implemented in the superior temporal plane.

Motor Response Selection in Overt Sentence Production: A Functional MRI Study

Many different cortical areas are thought to be involved in the process of selecting motor responses, from the inferior frontal gyrus, to the lateral and medial parts of the premotor cortex. The objective of the present study was to examine the neural underpinnings of motor response selection in a set of overt language production tasks. To this aim, we compared a sentence repetition task (externally constrained selection task) with a sentence generation task (volitional selection task) in a group of healthy adults. In general, the results clarify the contribution of the pre-SMA, cingulate areas, PMv, and pars triangularis to the process of selecting motor responses in the context of sentence production, and shed light on the manner in which this network is modulated by selection mode. Further, the present study suggests that response selection in sentence production engages neural resources similar to those engaged in the production of isolated words and oral motor gestures.

Regional heterogeneity in the processing and the production of speech in the human planum temporale

INTRODUCTION The role of the left planum temporale (PT) in auditory language processing has been a central theme in cognitive neuroscience since the first descriptions of its leftward neuroanatomical asymmetry. While it is clear that PT contributes to auditory language processing there is still some uncertainty about its role in spoken language production. METHODS Here we examine activation patterns of the PT for speech production, speech perception and single word reading to address potential hemispheric and regional functional specialization in the human PT. To this aim, we manually segmented the left and right PT in three non-overlapping regions (medial, lateral and caudal PT) and examined, in two complementary experiments, the contribution of exogenous and endogenous auditory input on PT activation under different speech processing and production conditions. RESULTS Our results demonstrate that different speech tasks are associated with different regional functional activation patterns of the medial, lateral and caudal PT. These patterns are similar across hemispheres, suggesting bilateral processing of the auditory signal for speech at the level of PT. CONCLUSIONS Results of the present studies stress the importance of considering the anatomical complexity of the PT in interpreting fMRI data.