Motor response selection in overt sentence production: a functional MRI study

Decoding words during sentence production: Syntactic role encoding and structure-dependent dynamics revealed by ECoG

Sentence production is the uniquely human ability to transform complex thoughts into strings of words. Despite the importance of this process, language production research has primarily focused on single words. However, it remains a largely untested assumption that the principles of word production generalize to more naturalistic utterances like sentences. Here, we investigate this using high-resolution neurosurgical recordings (ECoG) and an overt production experiment where patients produced six words in isolation (picture naming) and in sentences (scene description). We trained machine learning classifiers to identify the unique brain activity patterns for each word during picture naming, and used these patterns to decode which words patients were processing while they produced sentences. Our findings confirm that words share cortical representations across tasks, but reveal a division of labor within the language network. In sensorimotor cortex, words were consistently activated in the order in which they were said in the sentence. However, in inferior and middle frontal gyri (IFG and MFG), the order in which words were processed depended on the syntactic structure of the sentence. Deeper analysis of this pattern revealed a spatial code for representing a word's position in the sentence, with subjects selectively encoded in IFG and objects in MFG. Finally, we argue that the processes we observe in prefrontal cortex may impose a subtle pressure on language evolution, explaining why nearly all the world's languages position subjects before objects.

Functional Roles of Sensorimotor Alpha and Beta Oscillations in Overt Speech Production

Power decreases, or desynchronization, of sensorimotor alpha and beta oscillations (i.e., alpha and beta ERD) have long been considered as indices of sensorimotor control in overt speech production. However, their specific functional roles are not well understood. Hence, we first conducted a systematic review to investigate how these two oscillations are modulated by speech motor tasks in typically fluent speakers (TFS) and in persons who stutter (PWS). Eleven EEG/MEG papers with source localization were included in our systematic review. The results revealed consistent alpha and beta ERD in the sensorimotor cortex of TFS and PWS. Furthermore, the results suggested that sensorimotor alpha and beta ERD may be functionally dissociable, with alpha related to (somato-)sensory feedback processing during articulation and beta related to motor processes throughout planning and articulation. To (partly) test this hypothesis of a potential functional dissociation between alpha and beta ERD, we then analyzed existing intracranial electro-encephalography (iEEG) data from the primary somatosensory cortex (S1) of picture naming. We found moderate evidence for alpha, but not beta, ERD's sensitivity to speech movements in S1, lending supporting evidence for the functional dissociation hypothesis identified by the systematic review.

Engagement of the speech motor system in challenging speech perception: Activation likelihood estimation meta-analyses

The relationship between speech production and perception is a topic of ongoing debate. Some argue that there is little interaction between the two, while others claim they share representations and processes. One perspective suggests increased recruitment of the speech motor system in demanding listening situations to facilitate perception. However, uncertainties persist regarding the specific regions involved and the listening conditions influencing its engagement. This study used activation likelihood estimation in coordinate-based meta-analyses to investigate the neural overlap between speech production and three speech perception conditions: speech-in-noise, spectrally degraded speech and linguistically complex speech. Neural overlap was observed in the left frontal, insular and temporal regions. Key nodes included the left frontal operculum (FOC), left posterior lateral part of the inferior frontal gyrus (IFG), left planum temporale (PT), and left pre-supplementary motor area (pre-SMA). The left IFG activation was consistently observed during linguistic processing, suggesting sensitivity to the linguistic content of speech. In comparison, the left pre-SMA activation was observed when processing degraded and noisy signals, indicating sensitivity to signal quality. Activations of the left PT and FOC activation were noted in all conditions, with the posterior FOC area overlapping in all conditions. Our meta-analysis reveals context-independent (FOC, PT) and context-dependent (pre-SMA, posterior lateral IFG) regions within the speech motor system during challenging speech perception. These regions could contribute to sensorimotor integration and executive cognitive control for perception and production.

Parallel or sequential? Decoding conceptual and phonological/phonetic information from MEG signals during language production

Speaking requires the temporally coordinated planning of core linguistic information, from conceptual meaning to articulation. Recent neurophysiological results suggested that these operations involve a cascade of neural events with subsequent onset times, whilst competing evidence suggests early parallel neural activation. To test these hypotheses, we examined the sources of neuromagnetic activity recorded from 34 participants overtly naming 134 images from 4 object categories (animals, tools, foods and clothes). Within each category, word length and phonological neighbourhood density were co-varied to target phonological/phonetic processes. Multivariate pattern analyses (MVPA) searchlights in source space decoded object categories in occipitotemporal and middle temporal cortex, and phonological/phonetic variables in left inferior frontal (BA 44) and motor cortex early on. The findings suggest early activation of multiple variables due to intercorrelated properties and interactivity of processing, thus raising important questions about the representational properties of target words during the preparatory time enabling overt speaking.

A narrative review of the anatomy and function of the white matter tracts in language production and comprehension

Much is known about the role of cortical areas in language processing. The shift towards network approaches in recent years has highlighted the importance of uncovering the role of white matter in connecting these areas. However, despite a large body of research, many of these tracts' functions are not well-understood. We present a comprehensive review of the empirical evidence on the role of eight major tracts that are hypothesized to be involved in language processing (inferior longitudinal fasciculus, inferior fronto-occipital fasciculus, uncinate fasciculus, extreme capsule, middle longitudinal fasciculus, superior longitudinal fasciculus, arcuate fasciculus, and frontal aslant tract). For each tract, we hypothesize its role based on the function of the cortical regions it connects. We then evaluate these hypotheses with data from three sources: studies in neurotypical individuals, neuropsychological data, and intraoperative stimulation studies. Finally, we summarize the conclusions supported by the data and highlight the areas needing further investigation.

Relationship among Connectivity of the Frontal Aslant Tract, Executive Functions, and Speech and Language Impairment in Children with Childhood Apraxia of Speech

Childhood apraxia of speech (CAS) is a subtype of motor speech disorder usually co-occurring with language impairment. A supramodal processing difficulty, involving executive functions (EFs), might contribute to the cognitive endophenotypes and behavioral manifestations. The present study aimed to profile the EFs in CAS, investigating the relationship between EFs, speech and language severity, and the connectivity of the frontal aslant tract (FAT), a white matter tract involved in both speech and EFs. A total of 30 preschool children with CAS underwent speech, language, and EF assessments and brain MRIs. Their FAT connectivity metrics were compared to those of 30 children without other neurodevelopmental disorders (NoNDs), who also underwent brain MRIs. Alterations in some basic EF components were found. Inhibition and working memory correlated with speech and language severity. Compared to NoND children, a weak, significant reduction in fractional anisotropy (FA) in the left presupplementary motor area (preSMA) FAT component was found. Only speech severity correlated and predicted FA values along with the FAT in both of its components, and visual-spatial working memory moderated the relationship between speech severity and FA in the left SMA. Our study supports the conceptualization of a composite and complex picture of CAS, not limited to the speech core deficit, but also involving high-order cognitive skills.

Neural bases of elements of syntax during speech production in patients with aphasia

The ability to string together words into a structured arrangement capable of conveying nuanced information is key to speech production. The assessment of the neural bases for structuring sentences has been challenged by the need of experts to delineate the aberrant morphosyntactic structures in aphasic speech. Most studies have relied on focused tasks with limited ecological validity. We characterized syntactic complexity during connected speech produced by patients with chronic post-stroke aphasia. We automated this process by employing Natural Language Processing (NLP). We conducted voxel-based and connectome-based lesion-symptom mapping to identify brain regions crucially associated with sentence production and syntactic complexity. Posterior-inferior aspects of left frontal and parietal lobes, as well as white matter tracts connecting these areas, were essential for syntactic complexity, particularly the posterior inferior frontal gyrus. These findings suggest that sentence structuring during word production depends on the integrity of Broca's area and the dorsal stream of language processing.

First Language Attrition: What It Is, What It Isn't, and What It Can Be

This review aims at clarifying the concept of first language attrition by tracing its limits, identifying its phenomenological and contextual constraints, discussing controversies associated with its definition, and suggesting potential directions for future research. We start by reviewing different definitions of attrition as well as associated inconsistencies. We then discuss the underlying mechanisms of first language attrition and review available evidence supporting different background hypotheses. Finally, we attempt to provide the groundwork to build a unified theoretical framework allowing for generalizable results. To this end, we suggest the deployment of a rigorous neuroscientific approach, in search of neural markers of first language attrition in different linguistic domains, putting forward hypothetical experimental ways to identify attrition's neural traces and formulating predictions for each of the proposed experimental paradigms.

Verbal Adynamia and Conceptualization in Partial Rhombencephalosynapsis and Corpus Callosum Dysgenesis

Verbal adynamia is characterized by markedly reduced spontaneous speech that is not attributable to a core language deficit such as impaired naming, reading, repetition, or comprehension. In some cases, verbal adynamia is severe enough to be considered dynamic aphasia. We report the case of a 40-year-old, left-handed, male native English speaker who presented with partial rhombencephalosynapsis, corpus callosum dysgenesis, and a language profile that is consistent with verbal adynamia, or subclinical dynamic aphasia, possibly underpinned by difficulties selecting and generating ideas for expression. This case is only the second investigation of dynamic aphasia in an individual with a congenital brain malformation. It is also the first detailed neuropsychological report of an adult with partial rhombencephalosynapsis and corpus callosum dysgenesis, and the only known case of superior intellectual abilities in this context.

Electrophysiology of the Human Superior Temporal Sulcus during Speech Processing

The superior temporal sulcus (STS) is a crucial hub for speech perception and can be studied with high spatiotemporal resolution using electrodes targeting mesial temporal structures in epilepsy patients. Goals of the current study were to clarify functional distinctions between the upper (STSU) and the lower (STSL) bank, hemispheric asymmetries, and activity during self-initiated speech. Electrophysiologic properties were characterized using semantic categorization and dialog-based tasks. Gamma-band activity and alpha-band suppression were used as complementary measures of STS activation. Gamma responses to auditory stimuli were weaker in STSL compared with STSU and had longer onset latencies. Activity in anterior STS was larger during speaking than listening; the opposite pattern was observed more posteriorly. Opposite hemispheric asymmetries were found for alpha suppression in STSU and STSL. Alpha suppression in the STS emerged earlier than in core auditory cortex, suggesting feedback signaling within the auditory cortical hierarchy. STSL was the only region where gamma responses to words presented in the semantic categorization tasks were larger in subjects with superior task performance. More pronounced alpha suppression was associated with better task performance in Heschl's gyrus, superior temporal gyrus, and STS. Functional differences between STSU and STSL warrant their separate assessment in future studies.

Speech rhythms and their neural foundations

The recognition of spoken language has typically been studied by focusing on either words or their constituent elements (for example, low-level features or phonemes). More recently, the 'temporal mesoscale' of speech has been explored, specifically regularities in the envelope of the acoustic signal that correlate with syllabic information and that play a central role in production and perception processes. The temporal structure of speech at this scale is remarkably stable across languages, with a preferred range of rhythmicity of 2- 8 Hz. Importantly, this rhythmicity is required by the processes underlying the construction of intelligible speech. A lot of current work focuses on audio-motor interactions in speech, highlighting behavioural and neural evidence that demonstrates how properties of perceptual and motor systems, and their relation, can underlie the mesoscale speech rhythms. The data invite the hypothesis that the speech motor cortex is best modelled as a neural oscillator, a conjecture that aligns well with current proposals highlighting the fundamental role of neural oscillations in perception and cognition. The findings also show motor theories (of speech) in a different light, placing new mechanistic constraints on accounts of the action-perception interface.

Idea Formulation for Spoken Language Production: The Interface of Cognition and Language

BACKGROUND

Language and communication are fundamental to the human experience, and, traditionally, spoken language is studied as an isolated skill. However, before propositional language (i.e., spontaneous, voluntary, novel speech) can be produced, propositional content or 'ideas' must be formulated.

OBJECTIVE

This review highlights the role of broader cognitive processes, particularly 'executive attention', in the formulation of propositional content (i.e., 'ideas') for propositional language production.

CONCLUSIONS

Several key lines of evidence converge to suggest that the formulation of ideas for propositional language production draws on executive attentional processes. Larger-scale clinical research has demonstrated a link between attentional processes and language, while detailed case studies of neurological patients have elucidated specific idea formulation mechanisms relating to the generation, selection and sequencing of ideas for expression. Furthermore, executive attentional processes have been implicated in the generation of ideas for propositional language production. Finally, neuroimaging studies suggest that a widely distributed network of brain regions, including parts of the prefrontal and parietal cortices, supports propositional language production.

IMPLICATIONS

Theoretically driven experimental research studies investigating mechanisms involved in the formulation of ideas are lacking. We suggest that novel experimental approaches are needed to define the contribution of executive attentional processes to idea formulation, from which comprehensive models of spoken language production can be developed. Clinically, propositional language impairments should be considered in the context of broader executive attentional deficits.

Neural networks for sentence comprehension and production: An ALE-based meta-analysis of neuroimaging studies

Comprehending and producing sentences is a complex endeavor requiring the coordinated activity of multiple brain regions. We examined three issues related to the brain networks underlying sentence comprehension and production in healthy individuals: First, which regions are recruited for sentence comprehension and sentence production? Second, are there differences for auditory sentence comprehension vs. visual sentence comprehension? Third, which regions are specifically recruited for the comprehension of syntactically complex sentences? Results from activation likelihood estimation (ALE) analyses (from 45 studies) implicated a sentence comprehension network occupying bilateral frontal and temporal lobe regions. Regions implicated in production (from 15 studies) overlapped with the set of regions associated with sentence comprehension in the left hemisphere, but did not include inferior frontal cortex, and did not extend to the right hemisphere. Modality differences between auditory and visual sentence comprehension were found principally in the temporal lobes. Results from the analysis of complex syntax (from 37 studies) showed engagement of left inferior frontal and posterior temporal regions, as well as the right insula. The involvement of the right hemisphere in the comprehension of these structures has potentially important implications for language treatment and recovery in individuals with agrammatic aphasia following left hemisphere brain damage.

The frontal aslant tract (FAT) and its role in speech, language and executive function

In this review, we examine the structural connectivity of a recently-identified fiber pathway, the frontal aslant tract (FAT), and explore its function. We first review structural connectivity studies using tract-tracing methods in non-human primates, and diffusion-weighted imaging and electrostimulation in humans. These studies suggest a monosynaptic connection exists between the lateral inferior frontal gyrus and the pre-supplementary and supplementary motor areas of the medial superior frontal gyrus. This connection is termed the FAT. We then review research on the left FAT's putative role in supporting speech and language function, with particular focus on speech initiation, stuttering and verbal fluency. Next, we review research on the right FAT's putative role supporting executive function, namely inhibitory control and conflict monitoring for action. We summarize the extant body of empirical work by suggesting that the FAT plays a domain general role in the planning, timing, and coordination of sequential motor movements through the resolution of competition among potential motor plans. However, we also propose some domain specialization across the hemispheres. On the left hemisphere, the circuit is proposed to be specialized for speech actions. On the right hemisphere, the circuit is proposed to be specialized for general action control of the organism, especially in the visuo-spatial domain. We close the review with a discussion of the clinical significance of the FAT, and suggestions for further research on the pathway.

A Role for the Frontal Aslant Tract in Speech Planning: A Neurosurgical Case Study

Frontal and temporal white matter pathways play key roles in language processing, but the specific computations supported by different tracts remain a matter of study. A role in speech planning has been proposed for a recently described pathway, the frontal aslant tract (FAT), which connects the posterior inferior frontal gyrus to the pre-SMA. Here, we use longitudinal functional and structural MRI and behavioral testing to evaluate the behavioral consequences of a lesion to the left FAT that was incurred during surgical resection of a frontal glioma in a 60-year-old woman, Patient AF. The pattern of performance in AF is compared, using the same measures, with that in a 37-year-old individual who underwent a left anterior temporal resection and hippocampectomy (Patient AG). AF and AG were both cognitively intact preoperatively but exhibited specific and doubly dissociable behavioral deficits postoperatively: AF had dysfluent speech but no word finding difficulty, whereas AG had word finding difficulty but otherwise fluent speech. Probabilistic tractography showed that the left FAT was lesioned postoperatively in AF (but not AG) whereas the inferior longitudinal fasciculus was lesioned in AG (but not AF). Those structural changes were supported by corresponding changes in functional connectivity to the posterior inferior frontal gyrus: decreased functional connectivity postoperatively between the posterior inferior frontal gyrus and pre-SMA in AF (but not AG) and decreased functional connectivity between the posterior inferior frontal gyrus and the middle temporal gyrus in AG (but not AF). We suggest from these findings that the left FAT serves as a key communicative link between sentence planning and lexical access processes.

Age differences in the motor control of speech: An fMRI study of healthy aging

Healthy aging is associated with a decline in cognitive, executive, and motor processes that are concomitant with changes in brain activation patterns, particularly at high complexity levels. While speech production relies on all these processes, and is known to decline with age, the mechanisms that underlie these changes remain poorly understood, despite the importance of communication on everyday life. In this cross-sectional group study, we investigated age differences in the neuromotor control of speech production by combining behavioral and functional magnetic resonance imaging (fMRI) data. Twenty-seven healthy adults underwent fMRI while performing a speech production task consisting in the articulation of nonwords of different sequential and motor complexity. Results demonstrate strong age differences in movement time (MT), with longer and more variable MT in older adults. The fMRI results revealed extensive age differences in the relationship between BOLD signal and MT, within and outside the sensorimotor system. Moreover, age differences were also found in relation to sequential complexity within the motor and attentional systems, reflecting both compensatory and de-differentiation mechanisms. At very high complexity level (high motor complexity and high sequence complexity), age differences were found in both MT data and BOLD response, which increased in several sensorimotor and executive control areas. Together, these results suggest that aging of motor and executive control mechanisms may contribute to age differences in speech production. These findings highlight the importance of studying functionally relevant behavior such as speech to understand the mechanisms of human brain aging. Hum Brain Mapp 38:2751-2771, 2017. © 2017 Wiley Periodicals, Inc.

Structural correlates of spoken language abilities: A surface-based region-of interest morphometry study

Brain structure can predict many aspects of human behavior, though the extent of this relationship in healthy adults, particularly for language-related skills, remains largely unknown. The objective of the present study was to explore this relation using magnetic resonance imaging (MRI) on a group of 21 healthy young adults who completed two language tasks: (1) semantic fluency and (2) sentence generation. For each region of interest, cortical thickness, surface area, and volume were calculated. The results show that verbal fluency scores correlated mainly with measures of brain morphology in the left inferior frontal cortex and bilateral insula. Sentence generation scores correlated with structure of the left inferior parietal and right inferior frontal regions. These results reveal that the anatomy of several structures in frontal and parietal lobes is associated with spoken language performance. The presence of both negative and positive correlations highlights the complex relation between brain and language.

Speech networks at rest and in action: interactions between functional brain networks controlling speech production

Speech production is one of the most complex human behaviors. Although brain activation during speaking has been well investigated, our understanding of interactions between the brain regions and neural networks remains scarce. We combined seed-based interregional correlation analysis with graph theoretical analysis of functional MRI data during the resting state and sentence production in healthy subjects to investigate the interface and topology of functional networks originating from the key brain regions controlling speech, i.e., the laryngeal/orofacial motor cortex, inferior frontal and superior temporal gyri, supplementary motor area, cingulate cortex, putamen, and thalamus. During both resting and speaking, the interactions between these networks were bilaterally distributed and centered on the sensorimotor brain regions. However, speech production preferentially recruited the inferior parietal lobule (IPL) and cerebellum into the large-scale network, suggesting the importance of these regions in facilitation of the transition from the resting state to speaking. Furthermore, the cerebellum (lobule VI) was the most prominent region showing functional influences on speech-network integration and segregation. Although networks were bilaterally distributed, interregional connectivity during speaking was stronger in the left vs. right hemisphere, which may have underlined a more homogeneous overlap between the examined networks in the left hemisphere. Among these, the laryngeal motor cortex (LMC) established a core network that fully overlapped with all other speech-related networks, determining the extent of network interactions. Our data demonstrate complex interactions of large-scale brain networks controlling speech production and point to the critical role of the LMC, IPL, and cerebellum in the formation of speech production network.

Sensory-motor integration during speech production localizes to both left and right plana temporale

Speech production relies on fine voluntary motor control of respiration, phonation, and articulation. The cortical initiation of complex sequences of coordinated movements is thought to result in parallel outputs, one directed toward motor neurons while the "efference copy" projects to auditory and somatosensory fields. It is proposed that the latter encodes the expected sensory consequences of speech and compares expected with actual postarticulatory sensory feedback. Previous functional neuroimaging evidence has indicated that the cortical target for the merging of feedforward motor and feedback sensory signals is left-lateralized and lies at the junction of the supratemporal plane with the parietal operculum, located mainly in the posterior half of the planum temporale (PT). The design of these studies required participants to imagine speaking or generating nonverbal vocalizations in response to external stimuli. The resulting assumption is that verbal and nonverbal vocal motor imagery activates neural systems that integrate the sensory-motor consequences of speech, even in the absence of primary motor cortical activity or sensory feedback. The present human functional magnetic resonance imaging study used univariate and multivariate analyses to investigate both overt and covert (internally generated) propositional and nonpropositional speech (noun definition and counting, respectively). Activity in response to overt, but not covert, speech was present in bilateral anterior PT, with no increased activity observed in posterior PT or parietal opercula for either speech type. On this evidence, the response of the left and right anterior PTs better fulfills the criteria for sensory target and state maps during overt speech production.

A rostro-caudal axis for language in the frontal lobe: the role of executive control in speech production

The present article promotes a formal executive model of frontal functions underlying speech production, bringing together hierarchical theories of adaptive behavior in the (pre-)frontal cortex (pFC) and psycho- and neurolinguistic approaches to spoken language within an information-theoretic framework. Its biological plausibility is revealed through two Activation Likelihood Estimation meta-analyses carried out on a total of 41 hemodynamic studies of overt word and continuous speech production respectively. Their principal findings, considered in light of neuropsychological evidence and earlier models of speech-related frontal functions, support the engagement of a caudal-to-rostral gradient of pFC activity operationalized by the nature and quantity of speech-related information conveyed by task-related external cues (i.e., cue codability) on the one hand, and the total informational content of generated utterances on the other. In particular, overt reading or repetition and picture naming recruit primarily caudal motor-premotor regions involved in the sensorimotor and phonological aspects of speech; word and sentence generation engage mid- ventro- and dorsolateral areas supporting its basic predicative and syntactic functions; finally, rostral- and fronto-polar cortices subsume domain-general strategic processes of discourse generation for creative speech. These different levels interact in a top-down fashion, ranging representationally and temporally from the most general and extended to the most specific and immediate. The end-result is an integrative theory of pFC as the main executive component of the language cortical network, which supports the existence of areas specialized for speech communication and articulation and regions subsuming internal reasoning and planning. Prospective avenues of research pertaining to this model's principal predictions are discussed.

The role of domain-general cognitive control in language comprehension

What role does domain-general cognitive control play in understanding linguistic input? Although much evidence has suggested that domain-general cognitive control and working memory resources are sometimes recruited during language comprehension, many aspects of this relationship remain elusive. For example, how frequently do cognitive control mechanisms get engaged when we understand language? And is this engagement necessary for successful comprehension? I here (a) review recent brain imaging evidence for the neural separability of the brain regions that support high-level linguistic processing vs. those that support domain-general cognitive control abilities; (b) define the space of possibilities for the relationship between these sets of brain regions; and (c) review the available evidence that constrains these possibilities to some extent. I argue that we should stop asking whether domain-general cognitive control mechanisms play a role in language comprehension, and instead focus on characterizing the division of labor between the cognitive control brain regions and the more functionally specialized language regions.

The neurobiology of speech perception decline in aging

Speech perception difficulties are common among elderlies; yet the underlying neural mechanisms are still poorly understood. New empirical evidence suggesting that brain senescence may be an important contributor to these difficulties has challenged the traditional view that peripheral hearing loss was the main factor in the etiology of these difficulties. Here, we investigated the relationship between structural and functional brain senescence and speech perception skills in aging. Following audiometric evaluations, participants underwent MRI while performing a speech perception task at different intelligibility levels. As expected, with age speech perception declined, even after controlling for hearing sensitivity using an audiological measure (pure tone averages), and a bioacoustical measure (DPOAEs recordings). Our results reveal that the core speech network, centered on the supratemporal cortex and ventral motor areas bilaterally, decreased in spatial extent in older adults. Importantly, our results also show that speech skills in aging are affected by changes in cortical thickness and in brain functioning. Age-independent intelligibility effects were found in several motor and premotor areas, including the left ventral premotor cortex and the right supplementary motor area (SMA). Age-dependent intelligibility effects were also found, mainly in sensorimotor cortical areas, and in the left dorsal anterior insula. In this region, changes in BOLD signal modulated the relationship between age and speech perception skills suggesting a role for this region in maintaining speech perception in older ages. These results provide important new insights into the neurobiology of speech perception in aging.

Competitive mechanisms in sentence processing: common and distinct production and reading comprehension networks linked to the prefrontal cortex

Despite much interest in language production and comprehension mechanisms, little is known about the relationship between the two. Previous research suggests that linguistic knowledge is shared across these tasks and that the left inferior frontal gyrus (LIFG) may be commonly recruited. However, it remains unclear the extent to which production and comprehension share competition mechanisms. Here we investigate this issue and specifically examine competition in determining the event roles in a sentence (agent or affected participant). We used both behavioral and fMRI methods and compared the reading and production of high- and low-competition sentences, specifically targeting LIFG. We found that activity in pars opercularis (PO), independently identified by a competition-driven localizer, was modulated by competition in both tasks. Psychophysiological interaction analyses seeded in PO revealed task-specific networks: In comprehension, PO only interacted with the posterior temporal lobe, whereas in production, it interacted with a large network including hippocampal, posterior temporal, medial frontal and subcortical structures. Production and comprehension therefore recruit partially distinct functional networks but share competitive processes within fronto-temporal regions. We argue that these common regions store long-term linguistic associations and compute their higher-order contingencies, but competition in production ignites a larger neural network implementing planning, as required by task demands.

Language repetition and short-term memory: an integrative framework

Short-term maintenance of verbal information is a core factor of language repetition, especially when reproducing multiple or unfamiliar stimuli. Many models of language processing locate the verbal short-term maintenance function in the left posterior superior temporo-parietal area and its connections with the inferior frontal gyrus. However, research in the field of short-term memory has implicated bilateral fronto-parietal networks, involved in attention and serial order processing, as being critical for the maintenance and reproduction of verbal sequences. We present here an integrative framework aimed at bridging research in the language processing and short-term memory fields. This framework considers verbal short-term maintenance as an emergent function resulting from synchronized and integrated activation in dorsal and ventral language processing networks as well as fronto-parietal attention and serial order processing networks. To-be-maintained item representations are temporarily activated in the dorsal and ventral language processing networks, novel phoneme and word serial order information is proposed to be maintained via a right fronto-parietal serial order processing network, and activation in these different networks is proposed to be coordinated and maintained via a left fronto-parietal attention processing network. This framework provides new perspectives for our understanding of information maintenance at the non-word-, word- and sentence-level as well as of verbal maintenance deficits in case of brain injury.

The neostriatum and response selection in overt sentence production: an fMRI study

A number of premotor and prefrontal brain areas have been recently shown to play a significant role in response selection in overt sentence production. These areas are anatomically connected to the basal ganglia, a set of subcortical structures that has been traditionally involved in response selection across behavioral domains. The putamen and the caudate, the two major inputs to the basal ganglia, have been shown to undertake motor- as well as non-motor-related selection operations in language processing. Here we investigate the role of these basal ganglia structures in sentence repetition and generation in healthy adults. Although sentence generation is known to activate prefrontal and premotor cortical areas that reciprocally connect with these two neostriatal structures, their specific contributions are not known. We present evidence suggesting that that the putamen undertakes articulation-related aspects across tasks, while the caudate selectively supports selection processes in sentence generation.

Increased Facilitatory Connectivity from the Pre-SMA to the Left Dorsal Premotor Cortex during Pseudoword Repetition

Previous studies have demonstrated that the repetition of pseudowords engages a network of premotor areas for articulatory planning and articulation. However, it remains unclear how these premotor areas interact and drive one another during speech production. We used fMRI with dynamic causal modeling to investigate effective connectivity between premotor areas during overt repetition of words and pseudowords presented in both the auditory and visual modalities. Regions involved in phonological aspects of language production were identified as those where regional increases in the BOLD signal were common to repetition in both modalities. We thus obtained three seed regions: the bilateral pre-SMA, left dorsal premotor cortex (PMd), and left ventral premotor cortex that were used to test 63 different models of effective connectivity in the premotor network for pseudoword relative to word repetition. The optimal model was identified with Bayesian model selection and reflected a network with driving input to pre-SMA and an increase in facilitatory drive from pre-SMA to PMd during repetition of pseudowords. The task-specific increase in effective connectivity from pre-SMA to left PMd suggests that the pre-SMA plays a supervisory role in the generation and subsequent sequencing of motor plans. Diffusion tensor imaging-based fiber tracking in another group of healthy volunteers showed that the functional connection between both regions is underpinned by a direct cortico-cortical anatomical connection.

Functional and structural aging of the speech sensorimotor neural system: functional magnetic resonance imaging evidence

The ability to perceive and produce speech undergoes important changes in late adulthood. The goal of the present study was to characterize functional and structural age-related differences in the cortical network that support speech perception and production, using magnetic resonance imaging, as well as the relationship between functional and structural age-related changes occurring in this network. We asked young and older adults to observe videos of a speaker producing single words (perception), and to observe and repeat the words produced (production). Results show a widespread bilateral network of brain activation for Perception and Production that was not correlated with age. In addition, several regions did show age-related change (auditory cortex, planum temporale, superior temporal sulcus, premotor cortices, SMA-proper). Examination of the relationship between brain signal and regional and global gray matter volume and cortical thickness revealed a complex set of relationships between structure and function, with some regions showing a relationship between structure and function and some not. The present results provide novel findings about the neurobiology of aging and verbal communication.

A review and synthesis of the first 20 years of PET and fMRI studies of heard speech, spoken language and reading

The anatomy of language has been investigated with PET or fMRI for more than 20 years. Here I attempt to provide an overview of the brain areas associated with heard speech, speech production and reading. The conclusions of many hundreds of studies were considered, grouped according to the type of processing, and reported in the order that they were published. Many findings have been replicated time and time again leading to some consistent and undisputable conclusions. These are summarised in an anatomical model that indicates the location of the language areas and the most consistent functions that have been assigned to them. The implications for cognitive models of language processing are also considered. In particular, a distinction can be made between processes that are localized to specific structures (e.g. sensory and motor processing) and processes where specialisation arises in the distributed pattern of activation over many different areas that each participate in multiple functions. For example, phonological processing of heard speech is supported by the functional integration of auditory processing and articulation; and orthographic processing is supported by the functional integration of visual processing, articulation and semantics. Future studies will undoubtedly be able to improve the spatial precision with which functional regions can be dissociated but the greatest challenge will be to understand how different brain regions interact with one another in their attempts to comprehend and produce language.

The contribution of the inferior parietal cortex to spoken language production

This functional MRI study investigated the involvement of the left inferior parietal cortex (IPC) in spoken language production (Speech). Its role has been apparent in some studies but not others, and is not convincingly supported by clinical studies as they rarely include cases with lesions confined to the parietal lobe. We compared Speech with non-communicative repetitive tongue movements (Tongue). The data were analyzed with both univariate contrasts between conditions and probabilistic independent component analysis (ICA). The former indicated decreased activity of left IPC during Speech relative to Tongue. However, the ICA revealed a Speech component in which there was correlated activity between left IPC, frontal and temporal cortices known to be involved in language. Therefore, although net synaptic activity throughout the left IPC may not increase above baseline conditions during Speech, one or more local systems within this region are involved, evidenced by the correlated activity with other language regions.