The Structural Language Network

An adequate description of the neural basis of language processing must consider the entire network both with respect to its structural white matter connections and the functional connectivities between the different brain regions as the information has to be sent between different language-related regions distributed across the temporal and frontal cortex. This chapter discusses the white matter fiber bundles that connect the language-relevant regions. The chapter is broken into three sections. In the first, we look at the white matter fiber tracts connecting the language-relevant regions in the frontal and temporal cortices; in the second, the ventral and dorsal pathways in the right hemisphere that connect temporal and frontal regions; and finally in the third, the two syntax-relevant and (at least) one semantic-relevant neuroanatomically-defined networks that sentence processing is based on. From this discussion, it becomes clear that online language processing requires information transfer via the long-range white matter fiber pathways that connect the language-relevant brain regions within each hemisphere and between hemispheres.

The Neural Basis of Language

The findings discussed in this book lead to a first integrative view on the neurobiology of language, which proposes that BA 44 and the arcuate fasciculus are those brain structures that have evolved to subserve the human capacity to process syntax, which is at the core of the human language faculty. The chapter concludes with a brief statement of why syntax is important for the human being.

The Brain’s Critical Period for Language Acquisition

Whether a critical period for language learning exists (or not) is at the heart of ongoing debates over why second language learning appears to be easy early in life but much more difficult as we age. Neurocognitive studies on second language learning suggest that a unitary neural system is involved when processing more than one language, and the earlier a second language is learned, the more similar the neural language networks for the two languages will be. There appears to be a close relation between the developmental trajectory of white matter maturation and behavioral language skills. By looking at individuals coming from very different language backgrounds, such as native signers and hearing individuals, we find data that point towards a universal neural language system that is largely independent of input modality, but can be modulated slightly by the lifelong use of a given language.

The Functional Language Network

How information content is encoded and decoded in the sending and receiving brain areas is still an open issue. A possible though speculative view is that encoding and decoding requires similarity at the neuronal level in the encoding and decoding regions. This chapter discusses the functional neural network of language. It first describes the language network at the neurotransmitter level and then discusses the available data at the level of functional connectivity and oscillatory activity. Section 1 looks at the neural basis of information transfer, namely at the neurotransmitters which are crucially involved in the transmission of information from one neuron to the next. Section 2 uses functional connectivity analyses to provide information about how different brain regions work together. They allow us to make statements about which regions work together, and moreover, about the direction of the information flow between these. Section 3 models the language circuit as a a dynamic temporo-frontal network with initial input-driven information processed bottom-up from the auditory cortex to the frontal cortex along the ventral pathway, with semantic information reaching the anterior inferior frontal gyrus, and syntactic information reaching the posterior inferior frontal gyrus.

Evolution of Language

The past and current views of language evolution all center around a crucial question: What led to the human faculty of language and can it be explained by continuity of phylogenesis from non-human to human primates? The view that is presented here holds that the difference between human and non-human primates lies in the structure of their brains, particularly in the way the relevant brain areas are connected by white matter fiber tracts. During the evolution of language two crucial abilities had to evolve: these are first, sensory-motor learning, and second, the ability to process hierarchical structures. Across-species comparisons between the human and non-human primate brain reveal cytoarchitectonic and connectivity differences. Although still under discussion, the available paleoanthropological findings suggest a reorganization of the brain during phylogeny, and a possible rewiring which, due to the prolonged ontogeny in humans, is shaped by environmental input.

Ontogeny of the Neural Language Network

This chapter reviews the neural underpinning of normal language acquisition and asks not only at which age certain milestones in language acquisition are achieved, but moreover to what extent is this achievement dependent on the maturation of particular brain structures. In our recent model, the neural basis of the developing language system is described to reflect two major phases. The available data provide consistent evidence that very early on an infant is able to extract language-relevant information from the acoustic input. This first phase covers the first three years of life when language processing is largely input-driven and supported by the temporal cortex and the ventral part of the network. A second phase extends beyond age 3, when top-down processes come into play, and the left inferior frontal cortex and the dorsal part of the language network are recruited to a larger extent. Development towards full language performance beyond age 3 is dependent on maturational changes in the gray and white matter. An increased language ability is correlated with an increase in structural and functional connectivity between language-related brain regions in the left hemisphere, the inferior frontal gyrus and the posterior superior temporal gyrus/superior temporal sulcus.

Excursions

Historically, first production models were built on the basis of language deficits in patients with brain lesions, and later on the basis of speech errors in healthy people. More recently, attempts have been made to apply neuroscientific methods such as functional magnet-resonance imaging and electrocorticography during brain surgery using pictures or perceived words at controlled input to the production system. The available data suggest that language production, apart from brain structures supporting the motor act of speaking, involves Broca’s area in addition to temporal regions. There are a number of important aspects to be considered for communication that are beyond the core language system. These are contextual knowledge, known as pragmatics, as well as communicative hand gestures, which may interact with language during communication. At the neuroscientific level a number of brain regions beyond those involved in language such as the dorsomedial prefrontal cortex and the temporo-parietal junction have been identified to support aspects of social communication. Concerning the interplay between meaningful gestures with language it is interesting to note that BA 44 as the main syntactic processing region remains unaffected by communicative gestures.

Introduction

The introduction sets the scene for the book. Language is considered as a uniquely human trait, as a specific cognitive system and as a brain system with its basic neuroanatomy.

Language Functions in the Brain: From Auditory Input to Sentence Comprehension

Chapter 1 presents the neurocognitive model of language comprehension with its functional processing components and processing steps from auditory input to comprehension. The different functional components in the model are described with respect to their brain location and their time course. Topics covered include the identification of language-relevant sounds in speech input; the role of the temporal gyri together with the medial temporal lobe and the hippocampus in access to the lexicon and information encoded in the lexical entry; the combinatorics of language elements; neuroscientific studies relevant for the initial phrase structure building as assumed by syntax-first models; syntactic computation and the processing of syntactically complex sentences; the network of brain regions that support semantic processes; thematic role assignment (i.e., “who is doing what to whom”); and linguistic prosody and the processing of pitch information. Finally, the chapter concludes with a coherent view and respective model of the brain basis of language comprehension, with respect to the neuroanatomy of the brain regions supporting syntactic, semantic, and syntactic processes as well as the temporal relation and interaction of these different functions as comprehension proceeds.