N400 (neuroscience)
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The N400 is a component of time-locked EEG signals known as
event-related potential An event-related potential (ERP) is the measured brain response that is the direct result of a specific sense, sensory, cognition, cognitive, or motor system, motor event. More formally, it is any stereotyped electrophysiology, electrophysiologi ...
s (ERP). It is a negative-going deflection that peaks around 400 milliseconds post-stimulus onset, although it can extend from 250-500 ms, and is typically maximal over centro-parietal
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
sites. The N400 is part of the normal
brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...
response to words and other meaningful (or potentially meaningful) stimuli, including visual and auditory words,
sign language Sign languages (also known as signed languages) are languages that use the visual-manual modality to convey meaning, instead of spoken words. Sign languages are expressed through manual articulation in combination with #Non-manual elements, no ...
signs, pictures,
face The face is the front of the head that features the eyes, nose and mouth, and through which animals express many of their emotions. The face is crucial for human identity, and damage such as scarring or developmental deformities may affect th ...
s, environmental sounds, and smells.


History

The N400 was first discovered by Marta Kutas and Steven Hillyard in 1980. They conducted the first experiment looking at the response to unexpected words in read
sentences The ''Sentences'' (. ) is a compendium of Christian theology written by Peter Lombard around 1150. It was the most important religious textbook of the Middle Ages. Background The sentence genre emerged from works like Prosper of Aquitaine's ...
, expecting to elicit a P300 component. The P300 had previously been shown to be elicited by unexpected stimuli. Kutas and Hillyard therefore used sentences with anomalous endings (i.e.''I take coffee with cream and dog''), expecting to see a P300 to the unexpected sentence-final words. However, instead of eliciting a large positivity, these anomalous endings elicited a large negativity, relative to the sentences with expected endings (i.e. ''He returned the book to the library'') In the same paper, they confirmed that the negativity was not just caused by any unexpected event at the end of a sentence, since a semantically expected but physically unexpected word (i.e. ''She put on her high-heeled SHOES'') elicited a P300 instead of negativity in the N400 window. This finding showed that the N400 is related to semantic processing, and is not just a response to unexpected words.


Component characteristics

The N400 is characterized by a distinct pattern of electrical activity that can be observed at the scalp. As its name indicates, this waveform peaks around 400 ms post-stimulus onset, with negativity that can be observed in the time window ranging from 250-500 ms. This latency (delay between stimulus and response) is very stable across tasks—little else besides
age Age or AGE may refer to: Time and its effects * Age, the amount of time someone has been alive or something has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...
affects the timing of its peak. The N400 is a negative component, relative to reference electrodes placed on the mastoid processes (the bony ridge behind the ear), and relative to a 100 ms pre-stimulus baseline. Its amplitude can range from -5 to 5 microvolts. However, it is important to note that in studies using the N400 as a dependent measure, the relative
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
of the waveform compared to another experimental condition (the "N400 effect") is more important than its absolute amplitude. The N400 itself is not always negative—it is just a more negative-going deflection than that seen to other conditions. Its distribution is maximal over centro-parietal electrode sites, and is slightly larger over the left side of the head for visual words, although the distribution can change slightly depending on the eliciting stimulus.


Main paradigms

A typical experiment designed to study the N400 will usually involve the visual presentation of words, either in sentence or list contexts. In a typical visual N400 experiment, for example, subjects will be seated in front of a
computer monitor A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a electronic visual display, visual display, support electronics, power supply, Housing (engineering), housing, electri ...
while words are presented one-by-one at a central screen location. Stimuli must be presented centrally because eye movements will generate large amounts of electrical noise that will mask the relatively small N400 component. Subjects will often be given a behavioral task (e.g., making a word/nonword decision, answering a comprehension question, responding to a
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
probe), either after each stimulus or at longer intervals, to ensure that subjects are paying attention. Note, however, that overt responses by the subject are not required to elicit the N400—passively viewing stimuli will still evoke this response. An example of an experimental task used to study the N400 is a priming paradigm. Subjects are shown a list of words in which a prime word is either associatively related to a target word (e.g. bee and honey), semantically related (e.g. sugar and honey) or a direct repetition (e.g. honey and honey). The N400 amplitude seen to the target word (honey) will be reduced upon repetition due to semantic priming. The amount of reduction in amplitude can be used to measure the degree of relatedness between the words. Another widely used experimental task used to study the N400 is sentence
reading Reading is the process of taking in the sense or meaning of symbols, often specifically those of a written language, by means of Visual perception, sight or Somatosensory system, touch. For educators and researchers, reading is a multifacete ...
. In this kind of study, sentences are presented to subjects centrally, one word at a time, until the sentence is completed. Alternatively, subjects could listen to a sentence as natural auditory speech. Again, subjects may be asked to respond to comprehension questions periodically throughout the experiment, although this is not necessary. Experimenters can choose to manipulate various linguistic characteristics of the sentences, including contextual constraint or the cloze probability of the sentence-final word (see below for a definition of cloze probability) to observe how these changes affect the waveform's amplitude. As previously mentioned, the N400 response is seen to all meaningful, or potentially meaningful, stimuli. As such, a wide range of paradigms can be used to study it. Experiments involving the presentation of spoken words, acronyms, pictures embedded at the end of sentences, music, words related to current context or orientation and videos of real-word events, have all been used to study the N400, just to name a few.


Functional sensitivity

Extensive research has been carried out to better understand what kinds of experimental manipulations do and do not affect the N400. General findings are discussed below.


Factors that affect N400 amplitude

The frequency of a word's usage is known to affect the amplitude of the N400. With all else being constant, highly frequent words elicit reduced N400s relative to infrequent words. As previously mentioned, N400 amplitude is also reduced by repetition, such that a word's second presentation exhibits a more positive response when repeated in context. These findings suggest that when a word is highly frequent or has recently appeared in context, it eases the semantic processing thought to be indexed by the N400, reducing its amplitude. N400 amplitude is also sensitive to a word's orthographic neighborhood size, or how many other words differ from it by only one letter (e.g. ''boot'' and ''boat''). Words with large neighborhoods (that have many other physically similar items) elicit larger N400 amplitudes than do words with small neighborhoods. This finding also holds true for pseudowords, or pronounceable letter strings that are not real words (e.g. flom), which are not themselves meaningful but look like words. This has been taken as evidence that the N400 reflects general activation in the comprehension network, such that an item that looks like many words (regardless of whether it itself is a word) partially activates the representations of similar-looking words, producing a more negative N400. The N400 is sensitive to priming: in other words, its amplitude is reduced when a target word is preceded by a word that is semantically, morphologically, or orthographically related to it. In a sentence context, an important determinant of N400 amplitude elicited by a word is its cloze probability. Cloze probability is defined as the probability of the target word completing that particular sentence frame. Kutas and Hillyard (1984) found that a word's N400 amplitude has a nearly inverse linear relationship with its cloze probability. That is, as a word becomes less expected in context, its N400 amplitude is increased relative to more expected words. Words that are incongruent with a context (and thus have a cloze probability of 0) elicit large N400 amplitudes as well (although the amplitude of the N400 for incongruent words is also modulated by the cloze probability of the congruent word that would have been expected in its place Relatedly, the N400 amplitude elicited by open-class words (i.e. nouns, verbs, adjectives, and adverbs) is reduced for words appearing later in a sentence compared to earlier words. Taken together, these findings suggest that when the prior context builds up meaning, it makes the processing of upcoming words that fit with that context easier, reducing the N400 amplitude they elicit.


Factors that do not affect N400 amplitude

While the N400 is larger to unexpected items at the end of a sentence, its amplitude is generally unaffected by
negation In logic, negation, also called the logical not or logical complement, is an operation (mathematics), operation that takes a Proposition (mathematics), proposition P to another proposition "not P", written \neg P, \mathord P, P^\prime or \over ...
that causes the last word to be untrue and thus anomalous. For example, in the sentence ''A sparrow is a building'', the N400 response to ''building'' is more negative than the N400 response to bird in the sentence ''A sparrow is a bird''. In this case, building has a lower cloze probability, and so it is less expected than ''bird''. However, if
negation In logic, negation, also called the logical not or logical complement, is an operation (mathematics), operation that takes a Proposition (mathematics), proposition P to another proposition "not P", written \neg P, \mathord P, P^\prime or \over ...
is added to both sentences in the form of the word ''not'' (i.e. ''A sparrow is not a building'' and ''A sparrow is not a bird''), the N400 amplitude to ''building'' will still be more negative than that seen to ''bird''. This suggests that the N400 responds to the relationship between words in context, but is not necessarily sensitive to the sentence's
truth value In logic and mathematics, a truth value, sometimes called a logical value, is a value indicating the relation of a proposition to truth, which in classical logic has only two possible values ('' true'' or '' false''). Truth values are used in ...
. More recent research, however, has demonstrated that the N400 can sometimes be modulated by quantifiers or adjectives that serve negation-like purposes, or by pragmatically licensed negation. Additionally, grammatical violations do not elicit a large N400 response. Rather, these types of violations show a large positivity from about 500-1000 ms after stimulus onset, known as the P600.


Factors that affect N400 latency

A striking feature of the N400 is the general invariance of its peak latency. Although many different experimental manipulations affect the amplitude of the N400, few factors (aging and disease states and language proficiency being rare examples) alter the time it takes for the N400 component to reach a peak amplitude.


Sources

Although localization of the neural generators of an ERP signal is difficult due to the spreading of current from the source to the sensors, multiple techniques can be used to provide converging evidence about possible neural sources. Using methods such as recordings directly off the surface of the brain or from electrodes implanted in the brain, evidence from brain damaged patients, and magnetoencephalographic (MEG) recordings (which measure magnetic activity at the scalp associated with the electrical signal measured by ERPs), the left
temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in pr ...
has been highlighted as an important source for the N400, with additional contributions from the right
temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in pr ...
. More generally, however, activity in a wide network of brain areas is elicited in the N400 time window, suggesting a highly distributed neural source.


Theories

There is still much debate as to exactly what kind of neural and comprehension processes the N400 indexes. Some researchers believe that the underlying processes reflected in the N400 occur after a stimulus has been recognized. For example, Brown and Hagoort (1993) believe that the N400 occurs late in the processing stream, and reflects the integration of a word's meaning into the preceding context (see Kutas & Federmeier, 2011, for a discussion). However, this account has not explained why items that themselves have no meaning (e.g. pseudowords without defined associations) also elicit the N400. Other researchers believe that the N400 occurs much earlier, before words are recognized, and represents
neurolinguistics Neurolinguistics is the study of Nervous system, 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 fie ...
, orthographic or
phonological Phonology (formerly also phonemics or phonematics: "phonemics ''n.'' 'obsolescent''1. Any procedure for identifying the phonemes of a language from a corpus of data. 2. (formerly also phonematics) A former synonym for phonology, often prefer ...
analysis. More recent accounts posit that the N400 represents a broader range of processes indexing access to
semantic memory Semantic memory refers to general world knowledge that humans have accumulated throughout their lives. This general knowledge (Semantics, word meanings, concepts, facts, and ideas) is intertwined in experience and dependent on culture. New concep ...
. According to this account, it represents the binding of information obtained from stimulus input with representations from short- and long-term
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
(such as recent context, and accessing a word's meaning in long term memory) that work together to create meaning from the information available in the current context (Federmeier & Laszlo, 2009; see Kutas & Federmeier, 2011). Another account is that the N400 reflects prediction error or surprisal. Word-based surprisal was a strong predictor of N400 amplitude in an ERP corpus. In addition, connectionist models make use of prediction error for learning and linguistic adaptation, and these models can explain several N400/P600 results in terms of prediction error propagation for learning. It may also be that the N400 reflects a combination of these or other factors. Nieuwland et al. (2019) argue that the N400 is actually made up of two sub-components, with predictability affecting the early part of the N400 (200-500 ms after stimulus onset) and plausibility affecting it later (350-650 ms after stimulus onset). This suggests that the N400 reflects both access to semantic memory (which is sensitive to prediction), and semantic integration (sensitive to plausibility). As research in the field of electrophysiology continues to progress, these theories will likely be refined to include a complete account of just what the N400 represents.


See also

*
Bereitschaftspotential In neurology, the Bereitschaftspotential or BP (German language, German for "readiness potential"), also called the pre-motor potential or readiness potential (RP), is a measure of activity in the motor cortex and supplementary motor area of the b ...
* C1 and P1 *
Contingent negative variation The contingent negative variation (CNV) is a negative slow surface potential, as measured by electroencephalography (EEG), that occurs during the period between a warning stimulus or signal and an imperative ("go") stimulus. The CNV was one of the ...
*
Difference due to memory Difference due to memory (Dm) indexes differences in neural activity during the study phase of an experiment for items that subsequently are remembered compared to items that are later forgotten. It is mainly discussed as an event-related potential ...
*
Early left anterior negativity The early left anterior negativity (commonly referred to as ELAN) is an event-related potential in electroencephalography (EEG), or component of brain activity that occurs in response to a certain kind of stimulus. It is characterized by a negativ ...
*
Error-related negativity Error-related negativity (ERN), sometimes referred to as the Ne, is a component of an event-related potential (ERP). ERPs are electrical activity in the brain as measured through electroencephalography (EEG) and time-locked to an external event (e. ...
*
Late positive component The late positive component or late positive complex (LPC) is a positive-going event-related brain potential (ERP) component that has been important in studies of explicit recognition memory.Munte, T. F., Urbach, T. P., Duzel, E., & Kutas, M., (200 ...
*
Lateralized readiness potential In neuroscience, the lateralized readiness potential (LRP) is an event-related brain potential, or increase in electrical activity at the surface of the brain, that is thought to reflect the preparation of motor activity on a certain side of the b ...
*
Mismatch negativity The mismatch negativity (MMN) or mismatch field (MMF) is a component of the event-related potential (ERP) to an odd stimulus in a sequence of stimuli. It arises from electrical activity in the brain and is studied within the field of cognitive neu ...
*
N2pc N2pc refers to an ERP component linked to selective attention.Luck, S. J. (2005). "The operation of attention—millisecond by millisecond—over the first half second." In H. Ogmen & B. G. Breitmeyer (Eds.), ''The first half second: The microgen ...
* N100 * N170 * N200 * P3a * P3b *
P200 In neuroscience, the visual P200 or P2 is a waveform component or feature of the event-related potential (ERP) measured at the human scalp. Like other potential changes measurable from the scalp, this effect is believed to reflect the post-synap ...
*
P300 (neuroscience) The P300 (P3) wave is an event-related potential (ERP) component elicited in the process of decision making. It is considered to be an endogenous potential, as its occurrence links not to the physical attributes of a stimulus, but to a person' ...
* P600 *
Somatosensory evoked potential Somatosensory evoked potential (SEP or SSEP) is the electrical activity of the brain that results from the stimulation of touch. SEP tests measure that activity and are a useful, noninvasive means of assessing somatosensory system functioning. By c ...
*
Visual N1 The visual N1 is a visual evoked potential, a type of event-related electrical potential (ERP), that is produced in the brain and recorded on the scalp. The N1 is so named to reflect the polarity and typical timing of the component. The "N" ind ...


References


External links


Video of the N400 and P600 visualized as animated scalp topographies
{{EEG Electroencephalography Evoked potentials Neurolinguistics