Neurolinguistics
Neurolinguistics is the study of neural mechanisms in the human brain that control the comprehension, production, and acquisition of language. As an interdisciplinary field, neurolinguistics draws methods and theories from fields such as neuroscience, linguistics, cognitive science, communication disorders and neuropsychology. Researchers are drawn to the field from a variety of backgrounds, bringing along a variety of experimental techniques as well as widely varying theoretical perspectives. Much work in neurolinguistics is informed by models in psycholinguistics and theoretical linguistics, and is focused on investigating how the brain can implement the processes that theoretical and psycholinguistics propose are necessary in producing and comprehending language. Neurolinguists study the physiological mechanisms by which the brain processes information related to language, and evaluate linguistic and psycholinguistic theories, using aphasiology, brain imaging, electrophysiology, and computer modeling.
History
Neurolinguistics is historically rooted in the development in the 19th century of aphasiology, the study of linguistic deficits occurring as the result of brain damage. Aphasiology attempts to correlate structure to function by analyzing the effect of brain injuries on language processing. One of the first people to draw a connection between a particular brain area and language processing was Paul Broca, a French surgeon who conducted autopsies on numerous individuals who had speaking deficiencies, and found that most of them had brain damage on the left frontal lobe, in an area now known as Broca's area. Phrenologists had made the claim in the early 19th century that different brain regions carried out different functions and that language was mostly controlled by the frontal regions of the brain, but Broca's research was possibly the first to offer empirical evidence for such a relationship, and has been described as "epoch-making" and "pivotal" to the fields of neurolinguistics and cognitive science. Later, Carl Wernicke, after whom Wernicke's area is named, proposed that different areas of the brain were specialized for different linguistic tasks, with Broca's area handling the motor production of speech, and Wernicke's area handling auditory speech comprehension. The work of Broca and Wernicke established the field of aphasiology and the idea that language can be studied through examining physical characteristics of the brain. Early work in aphasiology also benefited from the early twentieth-century work of Korbinian Brodmann, who "mapped" the surface of the brain, dividing it up into numbered areas based on each area's cytoarchitecture and function; these areas, known as Brodmann areas, are still widely used in neuroscience today.The coining of the term neurolinguistics in the late 1940s and 1950s is attributed to Edith Crowell Trager, Henri Hecaen and Alexandr Luria. Luria's 1976 book "Basic Problems of Neurolinguistics" is likely the first book with "neurolinguistics" in the title. Harry Whitaker popularized neurolinguistics in the United States in the 1970s, founding the journal "Brain and Language" in 1974.
Although aphasiology is the historical core of neurolinguistics, in recent years the field has broadened considerably, thanks in part to the emergence of new brain imaging technologies and time-sensitive electrophysiological techniques, which can highlight patterns of brain activation as people engage in various language tasks. Electrophysiological techniques, in particular, emerged as a viable method for the study of language in 1980 with the discovery of the N400, a brain response shown to be sensitive to semantic issues in language comprehension. The N400 was the first language-relevant event-related potential to be identified, and since its discovery EEG and MEG have become increasingly widely used for conducting language research.
Discipline
Interaction with other fields
Neurolinguistics is closely related to the field of psycholinguistics, which seeks to elucidate the cognitive mechanisms of language by employing the traditional techniques of experimental psychology. Today, psycholinguistic and neurolinguistic theories often inform one another, and there is much collaboration between the two fields.Much work in neurolinguistics involves testing and evaluating theories put forth by psycholinguists and theoretical linguists. In general, theoretical linguists propose models to explain the structure of language and how language information is organized, psycholinguists propose models and algorithms to explain how language information is processed in the mind, and neurolinguists analyze brain activity to infer how biological structures carry out those psycholinguistic processing algorithms. For example, experiments in sentence processing have used the ELAN, N400, and P600 brain responses to examine how physiological brain responses reflect the different predictions of sentence processing models put forth by psycholinguists, such as Janet Fodor and Lyn Frazier's "serial" model, and Theo Vosse and Gerard Kempen's "unification model". Neurolinguists can also make new predictions about the structure and organization of language based on insights about the physiology of the brain, by "generalizing from the knowledge of neurological structures to language structure".
Neurolinguistics research is carried out in all the major areas of linguistics; the main linguistic subfields, and how neurolinguistics addresses them, are given in the table below.
| Subfield | Description | Research questions in neurolinguistics |
| Phonetics | the study of speech sounds | how the brain extracts speech sounds from an acoustic signal, how the brain separates speech sounds from background noise |
| Phonology | the study of how sounds are organized in a language | how the phonological system of a particular language is represented in the brain |
| Morphology and lexicology | the study of how words are structured and stored in the mental lexicon | how the brain stores and accesses words that a person knows |
| Syntax | the study of how multiple-word utterances are constructed | how the brain combines words into constituents and sentences; how structural and semantic information is used in understanding sentences |
| Semantics | the study of how meaning is encoded in language | how the brain combines words into constituents and sentences; how structural and semantic information is used in understanding sentences |
Topics considered
Neurolinguistics research investigates several topics, including where language information is processed, how language processing unfolds over time, how brain structures are related to language acquisition and learning, and how neurophysiology can contribute to speech and language pathology.Localizations of language processes
Much work in neurolinguistics has, like Broca's and Wernicke's early studies, investigated the locations of specific language "modules" within the brain. Research questions include what course language information follows through the brain as it is processed, whether or not particular areas specialize in processing particular sorts of information, how different brain regions interact with one another in language processing, and how the locations of brain activation differ when a subject is producing or perceiving a language other than his or her first language.Time course of language processes
Another area of neurolinguistics literature involves the use of electrophysiological techniques to analyze the rapid processing of language in time. The temporal ordering of specific patterns of brain activity may reflect discrete computational processes that the brain undergoes during language processing; for example, one neurolinguistic theory of sentence parsing proposes that three brain responses are products of three different steps in syntactic and semantic processing.Language acquisition
Another topic is the relationship between brain structures and language acquisition. Research in first language acquisition has already established that infants from all linguistic environments go through similar and predictable stages, and some neurolinguistics research attempts to find correlations between stages of language development and stages of brain development, while other research investigates the physical changes that the brain undergoes during second language acquisition, when adults learn a new language.Neuroplasticity is observed when both Second Language acquisition and Language Learning experience are induced, the result of this language exposure concludes that an increase of gray and white matter could be found in children, young adults and the elderly.
Language pathology
Neurolinguistic techniques are also used to study disorders and breakdowns in language, such as aphasia and dyslexia, and how they relate to physical characteristics of the brain.Technology used
Since one of the focuses of this field is the testing of linguistic and psycholinguistic models, the technology used for experiments is highly relevant to the study of neurolinguistics. Modern brain imaging techniques have contributed greatly to a growing understanding of the anatomical organization of linguistic functions. Brain imaging methods used in neurolinguistics may be classified into hemodynamic methods, electrophysiological methods, and methods that stimulate the cortex directly.Hemodynamic
Hemodynamic techniques take advantage of the fact that when an area of the brain works at a task, blood is sent to supply that area with oxygen. Such techniques include PET and fMRI. These techniques provide high spatial resolution, allowing researchers to pinpoint the location of activity within the brain; temporal resolution, on the other hand, is poor, since the BOLD response happens much more slowly than language processing. In addition to demonstrating which parts of the brain may subserve specific language tasks or computations, hemodynamic methods have also been used to demonstrate how the structure of the brain's language architecture and the distribution of language-related activation may change over time, as a function of linguistic exposure.In addition to PET and fMRI, which show which areas of the brain are activated by certain tasks, researchers also use diffusion tensor imaging, which shows the neural pathways that connect different brain areas, thus providing insight into how different areas interact. Functional near-infrared spectroscopy is another hemodynamic method used in language tasks.