P200

In neuroscience, the visual P200 or P2 is a waveform component or feature of the event-related potential (ERP) 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, also known as the P200, is so named because it is a positive going electrical potential that peaks at about 200 milliseconds (varying between about 150 and 275 ms) after the onset of some external stimulus. This component is often distributed around the centro-frontal and the parieto-occipital areas of the scalp. 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. In neuroscience, the visual P200 or P2 is a waveform component or feature of the event-related potential (ERP) 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, also known as the P200, is so named because it is a positive going electrical potential that peaks at about 200 milliseconds (varying between about 150 and 275 ms) after the onset of some external stimulus. This component is often distributed around the centro-frontal and the parieto-occipital areas of the scalp. 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 at an early stage compared to other more established ERP components and there is much that we still do not 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 aspect of higher-order perceptual processing, modulated by attention. 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 that compares sensory inputs with stored memory. The first mentions of an ERP component similar to that of the modern P2 were characterized in studies of basic visual and auditory evoked 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 consistently 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 visual N1 (or auditory N100) and P1 waveforms (negativity and positivity at 150 and 100ms respectively) and is followed by the N200, P3, and N400 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 sub-component of a complex involving the N1, P1 and P2, which was known as the vertex potential and 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, was found to be significantly larger for target than non-target stimuli and for rapid attention switching task. 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. In the auditory domain, there is evidence of enhanced P2 amplitudes even when a target stimuli is not embedded in a series of identical stimuli. In these instances, enhanced P2 amplitudes have been associated with auditory learning and repeated stimulus exposure. Enhanced P2 amplitudes have been reported in musicians with extensive listening experience as well as laboratory based auditory training experiments. A significant finding is that P2 amplitude changes are sometimes seen independent of N1 amplitude changes, again suggesting some degree of independence of N1, and P2 latencies and amplitudes appear to be affected by old age.