The N400 is an event-related potential (ERP) component typically elicited by unexpected linguistic stimuli. It is characterized as a negative deflection (topologically distributed over central-parietal sites on the scalp), peaking approximately 400ms (300-500ms) after the presentation of the stimulus.[1]
The N400 plays a significant role in language processing. It was "discovered" in 1980 by Marta Kutas and Steven Hillyard, in a study that is considered to have first introduced the concept of using ERPs to study language processing, and one of the first studies in what is now the field of neurolinguistics.[2] The N400 response is often elicited by semantically inappropriate words in an otherwise acceptable sentential context,[3] and has also been shown to occur in response to words at the end of a sentence when there was a problem earlier in the sentence.[1] In general, the more difficult it is to "integrate" a word into the preceding context, the bigger an N400 that word will elicit.[1] In models of speech comprehension, N400 is often associated with the semantic integration of words in a sentence context[4] and general "wrap-up" and decision-making processes at the end of a sentence.[1] The N400 remains a common topic of study in neurolinguistics and psycholinguistics.
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[edit]A typical experiment
A typical experiment that demonstrates the N400 might be set up as follows:
- The subject is presented a sentence one word at a time, and advances through each word by pressing a key.
- Some sentences will be semantically unmarked, e.g., "I enjoy walking in the evenings" (a control condition)
- Some sentences will have unexpected or deviant ends, e.g., "I like my coffee with cream and dog"
- By recording the subject's neural activity using an Electroencephalogram and comparing the effects between the expectable and the unexpected words, the N400 can be measured. It will be seen as a wave-effect on the EEG curve peaking approximately 400ms after the unexpected stimulus.
[edit]Scope of the N400 effect and the underlying process
The N400 is one of the most researched neurolinguistic topics with many papers on the subject. It is generally a very stable finding. In which conditions it is found and what exactly its finding shows is a hotly discussed topic.
[edit]Processing costs
It is generally observed that the amplitude of the N400 effect grows with stimuli that are harder to integrate semantically. That means that in a context where, for example, "lions" would be assumed ("The zebras ran away, chased by the ..."), "rabbits" will induce a greater N400 effect than "tigers", but smaller than "bicycles" or "Walkmans". This is interpreted as showing greater cognitive processing costs for the integration of some words—it is harder to conjure up a meaningful context with rabbits chasing zebras, and zebras in general, in particular chased zebras, leads one to think about lions. Researchers are not in full agreement about the syntactic associations of the N400; some report findings, more don't.
[edit]Music and N400
Music cognition researchers find N400-like effects in response to the relation between pieces of meanings and words that can be interpreted as being in some way descriptive of this music. That would mean that music is often at least partially interpreted in a semantical way. Words, sung or not, can elicit an N400, but comparison of results in the following cases shows that the N400 may be caused not by the music but by the word: (1) Songs sung correctly with the right words and the right notes; (2) Songs sung with non-N400-eliciting words but with a wrong note; (3) Songs sung with an N400-triggering word as well as a wrong note; and (4) Songs sung correctly but with an N400-eliciting word. Further research could also consider pure music with unexpected notes.
[edit]N400 and pictures
A clearly observable phenomenon is that nonlinguistic stimuli can elicit N400s, too: Pictures with expected or unexpected meanings in sentences (like the picture of a dog instead of the word in the sentence "I like my coffee with cream and dog") can be used to show that the effect is not exclusive to linguistics, but has to be in some way related to general semantic processes.
More broadly, cognitive scientists believe the N400 is not limited to linguistic stimuli, but instead an effect showing the processing of semantical formed and structured information in general.
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