Neurolinguistics

numbered]] Neurolinguistics is the study of the neural mechanisms in the human brain that control the comprehension, production, and acquisition of language. As an interdisciplinary field, neurolinguistics draws methodology and theory from fields such as neuroscience, linguistics, cognitive science, neurobiology, communication disorders, neuropsychology, and computer science. 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

and Wernicke's area]] Neurolinguistics is historically rooted in the development in the 19th century of aphasiology, the study of linguistic deficits (aphasias) occurring as the result of brain damage. 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 (or lesions) 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 "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. 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 (cell structure) and function; these areas, known as Brodmann areas, are still widely used in neuroscience today.

The coining of the term "neurolinguistics" has been attributed to Harry Whitaker, who founded the Journal of Neurolinguistics in 1985.

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 (such as PET and fMRI) and time-sensitive electrophysiological techniques (EEG and MEG), which can highlight patterns of brain activation as people engage in various language tasks; The N400 was the first language-relevant brain response to be identified, and since its discovery EEG and MEG have become increasingly widely used for conducting language research.

Neurolinguistics as a 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 (such as neurons) 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."

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.

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 linguistics 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 differs 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. Research in first language acquisition has already established that infants from all linguistic environments go through similar and predictable stages (such as babbling), 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 (known as neuroplasticity) that the brain undergoes during second language acquisition, when adults learn a new language.

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. 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 (in what is known as the Blood Oxygen Level-Dependent, or BOLD, response). Such techniques include PET and fMRI. These techniques provide high spatial resolution, allowing researchers to pinpoint the location of activity within the brain; 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. thus providing insight into how different areas interact.

Electrophysiological

Electrophysiological techniques take advantage of the fact that when a group of neurons in the brain fire together, they create an electric dipole or current. The technique of EEG measures this electrical current using sensors on the scalp, while MEG measures the magnetic fields that are generated by these currents. On the other hand, the location of brain activity can be difficult to identify in EEG; consequently, this technique is used primarily to how language processes are carried out, rather than where. Research using EEG and MEG generally focuses on event-related potentials (ERPs), Some important and common ERP components include the N400 (a negativity occurring at a latency of about 400 milliseconds), and the lateralized readiness potential.

Experimental design

Experimental techniques

Neurolinguists employ a variety of experimental techniques in order to use brain imaging to draw conclusions about how language is represented and processed in the brain. These techniques include the mismatch design, violation-based studies, various forms of priming, and direct stimulation of the brain.

Mismatch paradigm

The mismatch negativity (MMN) is a rigorously documented ERP component frequently used in neurolinguistic experiments. It is an electrophysiological response that occurs in the brain when a subject hears a "deviant" stimulus in a set of perceptually identical "standards" (as in the sequence s s s s s s s d d s s s s s s d s s s s s d). Since the MMN is elicited only in response to a rare "oddball" stimulus in a set of other stimuli that are perceived to be the same, it has been used to test how speakers perceive sounds and organize stimuli categorically. For example, a landmark study by Colin Phillips and colleagues used the mismatch negativity as evidence that subjects, when presented with a series of speech sounds with acoustic parameters, perceived all the sounds as either /t/ or /d/ in spite of the acoustic variability, suggesting that the human brain has representations of abstract phonemes—in other words, the subjects were "hearing" not the specific acoustic features, but only the abstract phonemes.

Violation-based

Many studies in neurolinguistics take advantage of anomalies or violations of syntactic or semantic rules in experimental stimuli, and analyzing the brain responses elicited when a subject encounters these violations. For example, sentences beginning with phrases such as *the garden was on the worked, which violates an English phrase structure rule, often elicit a brain response called the early left anterior negativity (ELAN). Violation techniques have been in use since at least 1980, Using similar methods, in 1992, Lee Osterhout first reported the P600 response to syntactic anomalies. Violation designs have also been used for hemodynamic studies (fMRI and PET): Embick and colleagues, for example, used grammatical and spelling violations to investigate the location of syntactic processing in the brain using fMRI.

Priming

In psycholinguistics and neurolinguistics, priming refers to the phenomenon whereby a subject can recognize a word more quickly if he or she has recently been presented with a word that is similar in meaning and how structurally complex sentences are processed.

Stimulation

Transcranial magnetic stimulation (TMS), a new noninvasive technique for studying brain activity, uses powerful magnetic fields that are applied to the brain from outside the head. It is a method of exciting or interrupting brain activity in a specific and controlled location, and thus is able to imitate aphasic symptoms while giving the researcher more control over exactly which parts of the brain will be examined. The logic behind TMS and direct cortical stimulation is similar to the logic behind aphasiology: if a particular language function is impaired when a specific region of the brain is knocked out, then that region must be somehow implicated in that language function. Few neurolinguistic studies to date have used TMS;

Subject tasks

In many neurolinguistics experiments, subjects do not simply sit and listen to or watch stimuli, but also are instructed to perform some sort of task in response to the stimuli. Subjects perform these tasks while recordings (electrophysiological or hemodynamic) are being taken, usually in order to ensure that they are paying attention to the stimuli. At least one study has suggested that the task the subject does has an effect on the brain responses and the results of the experiment.

Lexical decision

The lexical decision task involves subjects seeing or hearing an isolated word and answering whether or not it is a real word. It is frequently used in priming studies, since subjects are known to make a lexical decision more quickly if a word has been primed by a related word (as in "doctor" priming "nurse").

Grammaticality judgment, acceptability judgment

Many studies, especially violation-based studies, have subjects make a decision about the "acceptability" (usually grammatical acceptability or semantic acceptability) of stimuli. Such a task is often used to "ensure that subjects [are] reading the sentences attentively and that they [distinguish] acceptable from unacceptable sentences in the way [the experimenter] expect[s] them to do."

Active distraction and double-task

Some experiments give subjects a "distractor" task to ensure that subjects are not consciously paying attention to the experimental stimuli; this may be done to test whether a certain computation in the brain is carried out automatically, regardless of whether the subject devotes attentional resources to it. For example, one study had subjects listen to non-linguistic tones (long beeps and buzzes) in one ear and speech in the other ear, and instructed subjects to press a button when they perceived a change in the tone; this supposedly caused subjects not to pay explicit attention to grammatical violations in the speech stimuli. The subjects showed a mismatch response (MMN) anyway, suggesting that the processing of the grammatical errors was happening automatically, regardless of attention

Further reading

Some relevant journals include the Journal of Neurolinguistics and Brain and Language. Both are subscription access journals, though some abstracts may be generally available.

Notes

References

External links

Society for Neuroscience (SfN)

Talking Brains, blog by neurolinguists Greg Hickock and David Poeppel

Category:Linguistics