N400

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. In models of speech comprehension, N400 is often associated with the semantic integration of words in sentence context; its finding is interpreted as pointing to the activation of a process working on semantics in the general time frame. This article deals with the effect in itself as well as with the interpretations it has earned in psycholinguistic/neurolinguistic research.

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"
 * Some sentences will have unexpected or deviant ends, e.g., "I like my coffee with cream and dog" (a control condition)
 * 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 approximately 400ms after the unexpected stimulus.

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.

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 "walkmen". 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.

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 semantical way. The comparison between 1. Songs sung correctly, 2. Songs sung with a wrong note but the right words, 3. Songs sung with a wrong note and an N400-triggering word and 4. A song sung correctly, but with a N400-eliciting word shows that words, sung or not, can elicit a N400; but what causes the N400 is not the music but the word.

Musical interpretation probably uses different resources that lie in a similar but different time frame, for example the P600, corresponding to syntactical structured information in general.

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.