The N400 is an ERP component that reflects aspects of language processing. It is a negative-going waveform that occurs about 400 ms after visual or auditory presentation of a word that does not semantically cohere with the sentence it is a part of. It does not occur when the terminal word of a sentence makes sense in context with the rest of the sentence. Thus, it reflects that an individual understands semantic content. When missing, language deficits are inferred.

The N400 is currently being employed to determine whether and to what degree coma victims' language abilities are injured. Rehabilitaion of cognitive language abilities is accomplished by auditory presentation of thousands of semantically coherent sentences throughout the coma.

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