Neurolinguistics

Neurolinguistics is the study of

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.[9]

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

Discipline

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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.^[13][15]

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 (populations and networks of neurons) carry out those psycholinguistic processing algorithms.

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,^[19] whether or not particular areas specialize in processing particular sorts of information,^[20] how different brain regions interact with one another in language processing,^[21] and how the locations of brain activation differ when a subject is producing or perceiving a language other than his or her first language.^[22][23][24]

Time course of language processes

Another area of neurolinguistics literature involves the use of

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.^[23][27]

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.

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).

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 electric current using sensors on the scalp, while MEG measures the magnetic fields that are generated by these currents.^[34] In addition to these non-invasive methods, electrocorticography has also been used to study language processing. These techniques are able to measure brain activity from one millisecond to the next, providing excellent temporal resolution, which is important in studying processes that take place as quickly as language comprehension and production.^[34] On the other hand, the location of brain activity can be difficult to identify in EEG;^[31][35] 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),^[31] which are distinct brain responses (generally realized as negative or positive peaks on a graph of neural activity) elicited in response to a particular stimulus. Studies using ERP may focus on each ERP's latency (how long after the stimulus the ERP begins or peaks), amplitude (how high or low the peak is), or topography (where on the scalp the ERP response is picked up by sensors).^[36] Some important and common ERP components include the N400 (a negativity occurring at a latency of about 400 milliseconds),^[31] the mismatch negativity,^[37] the early left anterior negativity (a negativity occurring at an early latency and a front-left topography),^[38] the P600,^[14][39] and the lateralized readiness potential.^[40]

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 subtraction paradigm, mismatch design, violation-based studies, various forms of priming, and direct stimulation of the brain.

Subtraction

Many language studies, particularly in

The mismatch negativity (MMN) is a rigorously documented ERP component frequently used in neurolinguistic experiments.

Many studies in neurolinguistics take advantage of anomalies or violations of

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^[53] or morphological makeup (i.e., composed of similar parts).^[54] If a subject is presented with a "prime" word such as doctor and then a "target" word such as nurse, if the subject has a faster-than-usual response time to nurse then the experimenter may assume that word nurse in the brain had already been accessed when the word doctor was accessed.^[55] Priming is used to investigate a wide variety of questions about how words are stored and retrieved in the brain^[54][56] and how structurally complex sentences are processed.^[57]

Stimulation

Transcranial magnetic stimulation (TMS), a new noninvasive^[58] technique for studying brain activity, uses powerful magnetic fields that are applied to the brain from outside the head.^[59] 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.^[59] As such, it is a less invasive alternative to direct cortical stimulation, which can be used for similar types of research but requires that the subject's scalp be removed, and is thus only used on individuals who are already undergoing a major brain operation (such as individuals undergoing surgery for epilepsy).^[60] 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;^[2] direct cortical stimulation and cortical recording (recording brain activity using electrodes placed directly on the brain) have been used with macaque monkeys to make predictions about the behavior of human brains.^[61]

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.^[62] 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.^[63] 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.^[64]

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").^[53][54][55]

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.^{[64][65][66][67][68]} 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."^[66]

Experimental evidence has shown that the instructions given to subjects in an acceptability judgment task can influence the subjects' brain responses to stimuli. One experiment showed that when subjects were instructed to judge the "acceptability" of sentences they did not show an N400 brain response (a response commonly associated with semantic processing), but that they did show that response when instructed to ignore grammatical acceptability and only judge whether or not the sentences "made sense".^[64]

Probe verification

Some studies use a "probe verification" task rather than an overt acceptability judgment; in this paradigm, each experimental sentence is followed by a "probe word", and subjects must answer whether or not the probe word had appeared in the sentence.^[55][66] This task, like the acceptability judgment task, ensures that subjects are reading or listening attentively, but may avoid some of the additional processing demands of acceptability judgments, and may be used no matter what type of violation is being presented in the study.^[55]

Truth-value judgment

Subjects may be instructed not to judge whether or not the sentence is grammatically acceptable or logical, but whether the proposition expressed by the sentence is true or false. This task is commonly used in psycholinguistic studies of child language.^[69][70]

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^[37]—or at least that subjects were unable to consciously separate their attention from the speech stimuli.

Another related form of experiment is the double-task experiment, in which a subject must perform an extra task (such as sequential finger-tapping or articulating nonsense syllables) while responding to linguistic stimuli; this kind of experiment has been used to investigate the use of working memory in language processing.^[71]

Notes