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P3a
The P3a, or novelty P3, is a component of time-locked (EEG) signals known as event-related potentials (ERP). The P3a is a positive-going scalp-recorded brain potential that has a maximum amplitude over frontal/central electrode sites with a peak latency falling in the range of 250-280 ms. The P3a has been associated with brain activity related to the engagement of attention (especially orienting and involuntary shifts to changes in the environment) and the processing of novelty.〔Polich, J. (2003). Overview of P3a and P3b. In J. Polich (Ed.), Detection of Change:Event-Related Potential and fMRI Findings (pp. 83-98). Kluwer Academic Press: Boston.〕
== History ==
In 1975 Squires and colleagues conducted a study attempting to resolve some of the questions surrounding what neural process the P300 reflects. At the time, several researchers suggested that there needed to be active attention towards the target stimuli in order to elicit a P300, in part because stimuli that were ignored resulted in a P300 with a smaller amplitude or no P300 at all. On the other hand, some research had shown that subjects exhibit a P300 to unpredictable stimuli in an ongoing repetitive series of stimuli, even when the stimuli were classified as irrelevant and subjects were asked to ignore them while completing another task (i.e. reading a book). It was intriguing that you could elicit a P300 in conditions with active attention and those of non-attention. Upon further investigation it turned out that when comparing the two types of P300 potentials, they differed in latency and scalp topography. This led Squires ''et al.'' to suggest that there were two distinct psycho-physiological entities that had been referred to collectively as the P300
More specifically, Squires ''et al.'' recorded EEG during an auditory odd-ball paradigm with various conditions. The two types of stimuli were 90 dB and 70 db tone bursts that occurred 1.1 sec apart. Loud tones occurred with a probability of .9, .5, or .1, while the soft tones occurred with complementary probability. In addition, subjects completed blocks of stimuli under instruction to count the number of loud tones, count the number of soft tones, or ignore the tones and quietly read. Therefore, each set of instructions was performed at each of the probability combinations. Squires et al. found that when subjects were told to ignore the tones, the less frequent or rare tone (probability of .1) elicited a positive-going potential which occurred between 220 and 280 ms. They termed this potential the P3a in order to distinguish it from its relative, the P3b, which was a positive-going potential that occurred at 310-380 ms when the infrequent tones were attended to. Scalp distribution helped them differentiate the two potentials as well. The newly coined “P3a” had a peak amplitude occurring at frontal midline sites while the P3b peak amplitude occurred over parietal midline sites.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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