What Has Direct Cortical and Subcortical Electrostimulation Taught Us about Neurolinguistics?

Investigating the neural and physiological basis of language is one of the most important challenges in neurosciences. Direct electrical stimulation (DES), usually performed in awake patients during surgery for cerebral lesions, is a reliable tool for detecting both cortical and subcortical (white matter and deep grey nuclei) regions crucial for cognitive functions, especially language. DES transiently interacts locally with a small cortical or axonal site, but also nonlocally, as the focal perturbation will disrupt the entire subnetwork sustaining a given function. Thus, in contrast to functional neuroimaging, DES represents a unique opportunity to identify with great accuracy and reproducibility, in vivo in humans, the structures that are actually indispensable to the function, by inducing a transient virtual lesion based on the inhibition of a subcircuit lasting a few seconds. Currently, this is the sole technique that is able to directly investigate the functional role of white matter tracts in humans. Thus, combining transient disturbances elicited by DES with the anatomical data provided by pre- and postoperative MRI enables to achieve reliable anatomo-functional correlations, supporting a network organization of the brain, and leading to the reappraisal of models of language representation. Finally, combining serial peri-operative functional neuroimaging and online intraoperative DES allows the study of mechanisms underlying neuroplasticity. This chapter critically reviews the basic principles of DES, its advantages and limitations, and what DES can reveal about the neural foundations of language, that is, the large-scale distribution of language areas in the brain, their connectivity, and their ability to reorganize.

Language and Aging

Language processing in older adulthood is a model of balance between preservation and decline. Despite widespread changes to physiological mechanisms supporting perception and cognition, older adults’ language abilities are frequently well preserved. At the same time, the neural systems engaged to achieve this high level of success change, and individual differences in neural organization appear to differentiate between more and less successful performers. This chapter reviews anatomical and cognitive changes that occur in aging and popular frameworks for age-related changes in brain function, followed by an examination of how these principles play out in the context of language comprehension and production.

Neuroplasticity

With respect to language, it has long been observed that children who experience early unilateral brain injury do not show the same irreparable damage as do adults with homologous late-onset strokes. Neural plasticity has been proposed as the explanation for such differential linguistic profiles; that is, the plasticity of the young, developing brain allows the possibility for extensive adaptation and organization following a neural insult. Recent research, however, suggests that there are limits to this ability to adapt and organize. Results from a another communicative system, affect, suggest that children with unilateral pre- or perinatal stroke show similar (albeit subtler) effects to adults with homologous late-onset injuries. This chapter presents findings on language development in children who sustained a pre- or perinatal unilateral stroke, and complements these studies with a discussion of affective expression in these same children. These prospective studies of children with perinatal stroke provide a unique window into the development of the neural substrates for language and affect. Specifically, they afford a context to investigate the degree to which particular brain regions may be privileged for specific behavioral functions, as well as how the developing brain adapts to organize alternative pathways in the wake of an early insult.

Magnetoencephalography and the Cortical Dynamics of Language Processing

When seeking to uncover the brain correlates of language processing, timing and location are of the essence. Magnetoencephalography (MEG) offers them both, with the highest sensitivity to cortical activity. MEG has shown its worth in revealing cortical dynamics of reading, speech perception, and speech production in adults and children, in unimpaired language processing as well as developmental and acquired language disorders. The MEG signals, once recorded, provide an extensive selection of measures for examination of neural processing. Like all other neuroimaging tools, MEG has its own strengths and limitations of which the user should be aware in order to make the best possible use of this powerful method and to generate meaningful and reliable scientific data. This chapter reviews MEG methodology and how MEG has been used to study the cortical dynamics of language.

Studying Language with Functional Magnetic Resonance Imaging (fMRI)

Since the discoveries of language-sensitive brain areas in the late nineteenth century, the localization of the language network in the brain has been the subject of neurolinguistics research. Especially during the times of the two world wars and until the 1980s, head and brain injuries in soldiers as well as in civil patients served as the main data source. The advent of neuroimaging techniques roughly 100 years later was a milestone, providing online data from the living brain. This chapter presents functional magnetic resonance imaging (fMRI) as the most frequently used technique, the physical basics, appropriate experimental study designs, and perspectives for novel developments for neurolinguistics research in the active and passive brain.

Neural Mechanisms of Music and Language

Music and language are uniquely human forms of communication. What neural structures facilitate these abilities? This chapter conducts a review of music and language processing that follows these acoustic signals as they ascend the auditory pathway from the brainstem to auditory cortex and on to more specialized cortical regions. Acoustic, neural, and cognitive mechanisms are identified where processing demands from both domains might overlap, with an eye to examples of experience-dependent cortical plasticity, which are taken as strong evidence for common neural substrates. Following an introduction describing how understanding musical processing informs linguistic or auditory processing more generally, findings regarding the major components (and parallels) of music and language research are reviewed: pitch perception, syntax and harmonic structural processing, semantics, timbre and speaker identification, attending in auditory scenes, and rhythm. Overall, the strongest evidence that currently exists for neural overlap (and cross-domain, experience-dependent plasticity) is in the brainstem, followed by auditory cortex, with evidence and the potential for overlap becoming less apparent as the mechanisms involved in music and speech perception become more specialized and distinct at higher levels of processing.

Neuromotor Organization of Speech Production

The production of speech is a multistep process requiring close coordination between neurolinguistic, neurocognitive, and neuromotor processes to communicate fluently and seemingly effortlessly. This complex process, which combines speech-specific and domain-general neural mechanisms, involves a closed repertoire of motor programs to control over 100 muscles distributed over the face, neck, and abdomen. The process requires neuromotor mechanisms to implement phonological planning, response selection, sequencing, and timing, contextual adjustments of the motor programs, as well as action execution and response monitoring. Recent advances in neuroimaging and neuromodulation techniques have led to the emergence of neurobiologically realistic models of speech production, leading to more comprehensive understanding of the mechanisms involved in producing speech. This chapter reviews the most up-to-date knowledge on the neural organization of the brain systems involved in producing speech.

The How and What of Object Knowledge in the Human Brain

Humans recognize, grasp, and manipulate objects on a daily basis. Critical to those behaviors is the ability to integrate information about visual structure, object function, and object-associated manipulation. This chapter reviews several lines of evidence that have documented a dissociation between representations of object function and representations of object manipulation. Drawing on a prior suggestion by Rothi, Heilman, and colleagues, the authors argue that this distinction runs parallel to a distinction made in the context of language processing, between abstract semantic representations of words and modality-specific representations of word forms. The studies that are reviewed in the chapter use a range of methods, including transcranial magnetic stimulation (TMS), functional magnetic resonance imaging (fMRI), and cognitive neuropsychological investigations of patients with brain injuries. The chapter also reviews studies on the functional and structural connectivity between temporal and parietal areas that are implicated in processing function and manipulation knowledge, respectively. It concludes by outlining key issues that lie ahead, emphasizing the role that connectivity-based measures will likely play in developing an explicit model of how the brain deploys the right actions to the right objects. The authors suggest that there are further lessons to be learned from models of lexical access in developing a computationally explicit model of object-directed action.

Lateralization of Language

It is intriguing that the two brain halves of the human brain look so similar, but are in fact quite different at the anatomical level, and even more so at the functional level. In particular, the highly frequent co-occurrence of right-handedness and left hemisphere dominance of language has led to an abundance of laterality research. This chapter discusses the most important recent finding on laterality (i.e., left or right hemisphere) and degree of hemispheric specialization for speech production, auditory speech processing, and reading. Following a descriptive overview of these three core sub-processes of language, the chapter summarizes possible influences on the lateralization of each, including anatomical, evolutionary, genetic, developmental, and experiential factors, as well as handedness and impairment. It will become clear that language is a heterogeneous cognitive function driven by a variety of underpinning origins. Next, the often-underestimated role of the right hemisphere for language is discussed with respect to prosody and metaphor comprehension, as well as individual differences in the lateralization of healthy and language-impaired brains. Finally, recent insights into the relationship between lateralized language and non-language functions are discussed, highlighting the unique contribution of lateralization research to the growing knowledge of general human brain mechanisms.

Motor Speech Disorders

This chapter reviews speech motor impairments resulting from neurologic disorders, that is, dysarthria and apraxia of speech. The architecture of the brain’s speech motor network is used as a framework to describe the symptom patterns of the most relevant syndromes and their underlying pathomechanisms, with a focus on some of the more controversial issues. The chapter’s final section discusses whether speech motor impairments should be understood as domain-general dysfunctions of respiratory, laryngeal, and vocal tract movements, or as disorders of a motor system specialized for vocal communication.