The N200, or N2, is an event-related potential (ERP) component. An ERP can be monitored using a non-invasive electroencephalography (EEG) cap that is fitted over the scalp on human subjects. An EEG cap allows researchers and clinicians to monitor the minute electrical activity that reaches the surface of the scalp from post-synaptic potentials in neurons, which fluctuate in relation to cognitive processing. EEG provides millisecond-level temporal resolution and is therefore known as one of the most direct measures of covert mental operations in the brain. The N200 in particular is a negative-going wave that peaks 200-350ms post-stimulus and is found primarily over anterior scalp sites. Past research focused on the N200 as a mismatch detector, but it has also been found to reflect executive cognitive control functions, and has recently been used in the study of language (Folstein & Van Petten, 2008; Schmitt, Münte, & Kutas, 2000).
History The N2 component starts with the discovery of EEG which dates back as early as 1929 with Hans Berger demonstrating the ability to record electrical activity of the brain by simply placing electrodes over the scalp and then amplifying the signal. Later, in 1936, researcher Pauline and Hallowell Davis manipulated events in the environment and recorded the first known ERP's. One of the first experiments to find evidence of an N200 was by Sutton, Braren, Zubin, and John (1965) when examining the effects of stimulus uncertainty on sensory potentials. In their study, participants were presented with two types of paired stimuli. In the certain condition, a cue stimulus was presented that was predictive of the modality of the target stimulus, which was either clicks or light flashes. In the uncertain condition, the cue stimulus was not predictive and could be followed by either a click or a light flash. The researchers occasionally found a negativity that peaked on average 190ms post-stimulus in the uncertain condition (N200), in addition to a positivity 300ms post-stimulus (P300). Following the experiment by Sutton et al. (1965), subsequent research further manipulated stimulus uncertainty in an attempt to elicit a more robust N200. The N200 has been found in a variety of different experimental conditions, and is now thought to consist of several subcomponents. The N200 in response to attended or unattended deviant auditory stimuli, similar to what was originally seen in Sutton et al. (1965), is referred to as the mismatch negativity. Additionally, there is the no-go N200, which is elicited on no-go trials in go/no-go tasks. More generally, the N2 component has been described in tasks that reflect stimulus identification, attentional shifts, inhibition of motor responses, overcoming stereotypical responses or conflict monitoring, maintenance of context information, response selection timing, and detection of novelty or mismatch.
Main paradigms The N200 is seen in a variety of experimental paradigms. A commonly used experimental design is the Eriksen flanker task. In this task, participants are shown an array of items (usually letters), with each letter corresponding to a left or right-handed response. For example, the letter 'A' could indicate a left-handed response, and the letter 'B' a right-handed response. It is the job of the participants to respond to the central item of the array, which is flanked by the same item on compatible trials (AAAAA) or a different item on incompatible trials (BBABB). The N200 is normally seen on incompatible trials. Another task that has been utilized to elicit a N200 is the go/no-go task. This task presents participants with two different stimuli that indicate which hand to respond with (e.g. 'A' indicates a left-handed response and 'B' a right-handed response). The stimuli also vary on another dimension that indicates whether a response is necessary (e.g. small letter requires a response, large letter means do not respond). For example, a small 'A' would indicate a left-handed go, and a large 'B' would be a right-handed no-go. The go/no-go mapping is then reversed to test for differences (e.g. letter size would indicate the hand and letter identity the go/no-go). The N200 is most often seen on no-go trials.
In the study of language Since the go/no-go paradigm with N200 can be used to indicate the timing of information noting, it is a good candidate to examine the order of language processing and production. Schmitt et al. (2000) utilized the occurrence of N200 in the go/no-go paradigm to determine the timing of semantic and phonological information processing. Participants were presented with a series of pictures. In one trial instance, the participant was asked to respond (by pressing a button) or not to make a respond based on the semantic feature of the picture - whether the picture depicted an animal or a non-animal object; in the paralleled instance, the participant made a response or no response based upon whether the name of the pictured item began with a vowel or consonant (phonology-dependent). EEG of the participants were analyzed, and the researchers found that the peak latency of the N200 occurred earlier when the response was contingent on semantic information than on phonological information. Thus, they were able to conclude that semantic information becomes available earlier than phonological information in language processing. Researchers have also been able to show that some forms of knowledge are available from written words as quickly as 160 ms by capitalizing on the go/nogo paradigm associated with N200 to .
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