N400 (neuroscience) - Main Paradigms

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, and videos of real-word events, have all been used to study the N400, just to name a few.

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