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.
History The N400 was first discovered by Marta Kutas and Steven A. 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, where the final word is unexpectedly fully capitalized) 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 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, in studies using the N400 as a dependent measure, the relative amplitude 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 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 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. 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.
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