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Visual P200 The visual P2 waveform is a component or feature of the ERP (event-related potential) measured at the human scalp. Like other potential changes measurable from the scalp, this effect is believed to reflect the post-synaptic activity of a specific neural process. The P2 component is also known as the P200, and is so named because it is a positive going electrical potential that peaks at about 200 milliseconds after the onset of some external stimulus (varying between about 150 and 275 ms). The distribution of this component in the brain, as measured by electrodes placed across the scalp, is located around the centro-frontal area and the parieto-occipital region. It is generally found to be maximal around the vertex (frontal region) of the scalp, however there have been some topographical differences noted in ERP studies of the P2 in different experimental conditions.

Research on the visual P2 is in an early stage compared to other more established ERP components and there is much that we still do not yet know about it. Part of the difficulty of clearly characterizing this component is that it appears to be modulated by a large and diverse number of cognitive tasks. Functionally, there seems to be partial agreement amongst researchers in the field of cognitive neuroscience that the P2 represents some sort of metric of stimulus evaluation. It is known that the P2 is typically elicited as part of the normal response to visual stimuli and has been studied in relation to visual search and attention, language context information, and memory and repetition effects. The amplitude of the peak of the waveform may be modulated by many different aspects of visual stimuli which allow it to be used for studies of visual cognition and disease. In general, the P2 may be a part of cognitive matching system which compares sensory inputs with memory (Freunberger, Klimiesch, Doppelmayr & Holler, 2007; Luck & Hillyard, 1994).

History
The first mentions of an ERP component similar to that of the modern P2 were characterized in studies of basic visual and auditory evoked-response potentials. One of the first of such studies involved the presentation of flashing lights. Using this method, researchers found that a series of potential changes were consistent observed across repeated trials. These would later be classified as components of the visual evoked response (VER), part of which includes the P2.

The P2 follows the N1 and P1 waveforms (negativity and positivity at 100ms respectively) and is followed by N2, P3, and N4 waveforms. Other components may overlap with the P2 to some extent, making it difficult to distinguish clearly between them, depending on the location of measurement. Originally, the P2 was characterized as a subcomponent of a complex involving the N1, P1and P2, which was known as the vertex potential which was classically studied as a unitary phenomenon. In particular, the relationship between the N1 and P2 was thought to be important. The difference between the N1 and P2, known as the vertex amplitude, and was found to be significantly larger for target than non-target stimuli and for rapid attention switching task, (Furutsuka, 1989). Further studies have subsequently examined the P2 separately from the N1 and have found that the amplitude of the P2 itself is larger for target stimuli that are less frequent. This is similar to the P3, though the P2 is usually seen for more simple features than the P3, (Crowley & Colrain, 2004; Luck & Hillyard, 1994).

In terms of modality, the visual P2 is similar to the auditory P2 and both have been studied in similar contexts. There are most likely multiple distinct P2s in different modalities, including both frontal and posterior visual P2s, which may or may not have similar origins or functional similarities. It is not yet understood whether the visual, auditory or other P2s reflect the same functional and neural activities.