Н400 толқыны: мидың семантикалық өңдеуінің электрофизиологиялық белгісі

The N400 is a component of time locked EEG signals known as event related potentials (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 sites. The N400 is part of the normal brain response to words and other meaningful (or potentially meaningful) stimuli, including visual and auditory words, sign language signs, pictures, faces, environmental sounds, and smells.

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, 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.

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 affects the timing of its peak. 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.

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.

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. 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. 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 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. 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.