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I have a question about the averaging involved in the Bereitschaftspotential. On http://en.wikipedia.org/wiki/Bereitschaftspotential you can read that, because it is so small, the Bereitschaftspotential only becomes apparent after averaging.

My question: do they mean averaging over different electrodes, but in a single trial?

Or do they mean averaging over electrodes AND over trials?

Thanks in advance

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After reading more of the wiki page, I have found the answer in the Applications section: it can be detected from a single trial. –  Guest_______ Jun 13 at 7:00

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up vote 4 down vote accepted

Generally, EEG potentials are primarily averaged over trials, not electrodes. Averaging over electrodes is often done in addition to trial averaging, but this by itself does not make the potentials visible on a single-trial basis since the "noise" overshadowing the potentials is largely shared by all adjacent electrodes.

The Bereitschaftspotential is also averaged over trials, especially since it is usually specified to be observed over specific electrodes.

As you note in a comment, single-trial identification of EEG phenomena, such as the Bereitschaftspotential (or the P300), is being attempted using machine learning techniques. However, these work far from reliably, and the original work on the Bereitschaftspotential was done far before these powerful machine learning techniques became available. So in the original BP work, averaging was done over trials (and often, over subjects, too). This is apparent in for example the Methods section of Libet's famous paper from 1983 (p. 624):

For present purposes, analysis of RPs is made for those recorded at the vertex, where they were all maximal. ... 40 trials were performed and averaged by a computer of average transients

The vertex = electrode Cz.

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Thanks a lot for the answer, this is exactly the kind of information I was looking for. –  Guest_______ Jun 13 at 17:16

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