The P600 is an event-related potential, or peak in electrical brain activity measured by electroencephalography. It is a language-relevant ERP and is thought to be elicited by hearing or reading grammatical errors and other syntactic anomalies. Therefore, it is a common topic of study in neurolinguistic experiments investigating sentence processing in the human brain. The P600 can be elicited in both visual and auditory experiments, and is characterized as a positive-going deflection with an onset around 500 milliseconds after the stimulus that elicits it; it often reaches its peak around 600 milliseconds after presentation of the stimulus, and lasts several hundred milliseconds. In other words, in the EEG waveform it is a large peak in the positive direction, which starts around 500 milliseconds after the subject sees or hears a stimulus. It is typically thought of as appearing mostly on centro-parietal electrodes, but frontal P600s have also been observed in several studies. In EEG, however, this distribution at the scalp does not mean the P600 is coming from that part of the brain; a 2007 study using magnetoencephalography speculates that the generators of the P600 are in the posterior temporal lobe, behind Wernicke's area. The P600 was first reported by Lee Osterhout and Phillip Holcomb in 1992. It is also sometimes called the syntactic positive shift, since it has a positive polarity and is usually elicited by syntactic phenomena.
Elicitation
The P600 was originally considered a "syntactic" ERP, as it is elicited by several types of syntactic phenomena, including ungrammatical stimuli, garden path sentences that require reanalysis, complex sentences with a large number of thematic roles, and the processing of filler-gap dependencies. ;Grammatical errors A P600 may be elicited by several kinds of grammatical errors in sentences, such as problems in agreement, such as "the child *throw the toy". In addition to this sort of subject-verb disagreement, P600s have also been elicited by disagreements in tense, gender, number, and case, as well as phrase structure violations. A 2009 study has suggested that these errors elicit stronger P600s than the other syntactic stimuli that have been implicated. ;Garden paths P600s are also known to occur when a sentence contains no outright grammatical error, but must be parsed in a different way than the reader originally expects. These sentences are known as "garden path" sentences, because the reader follows one interpretation of the sentence only to realize later that this interpretation was wrong and he must backtrack to understand the sentence. For example, found P600s elicited by the wordto in sentences such as
The broker persuaded to sell the stock was tall.
In sentences such as this, the preferred reading is to interpret "persuaded" as the main verb of the sentence, and upon seeing the word to the reader has to re-analyze the sentence to mean something more like "the broker that was persuaded to sell the stock, he was tall". ;Syntactic errors in music P600s are also elicited by errors in musical harmony, such as when a chord is played out of key with the rest of a musical phrase. This implies that P600s are not "language-specific," but "can be elicited in nonlinguistic sequences." ;Dependencies and complexity Some studies have found a P600 elicited by words where there is no grammatical error and no "garden path", but when the sentence is complex because there are a number of noun phrasesactive. This has most often been the case when the reader has to "re-activate" a word that appeared earlier in the sentence. For example, in a sentence like "Who did you imitate?", the word who appears in the beginning of the sentence but is actually the direct object of imitate, and must be interpreted in that way ; several studies have found that after the reader sees the word imitate he or she has a P600 response, possibly as a result of re-activating who. These sorts of P600s get stronger as the number of noun phrases active in the sentences increases, suggesting that the P600 generator is sensitive to the level of complexity in a sentence. ;Semantic attraction demonstrated a so-called "semantic P600" in sentences that are grammatically correct but semantically anomalous, and in which syntactic reanalysis is more appealing than semantic reanalysis. For example, a P600 may be elicited in the following sentence:
The hearty meal was devouring the kids.
This suggests that the reader would rather interpret the sentence as containing a morphosyntactic error rather than a semantic one. The interpretation of "semantic P600s" has attracted considerable attention and controversy in the literature.
Interpretations
There are several theories about what computational processes the P600 may be triggered by. Because it often happens in response to grammatical violations or garden path sentences, one theory is that the P600 reflects processes of revision and reanalysis. On the other hand, other models suggest that the P600 may not reflect these processes in particular, but just the amount of time and effort in general it takes to build up coherent structure in a sentence, or the general processes of creating or destroying syntactic structure. Another proposal is that the P600 does not necessarily reflect any linguistic processes per se, but is similar to the P300 in that it is triggered when a subject encounters "improbable" stimuli—since ungrammatical sentences are relatively rare in natural speech, a P600 may not be a linguistic response but simply an effect of the subject's "surprise" upon encountering an unexpected stimulus. Another account is that the P600 reflects error/surprisal propagation due to learning processes that take place during linguistic adaptation and this account has been implemented in a connectionist model that explains several P600/N400 results.
