Source: Research gate
EEG is a very low frequency.. and literature will give us the region where Alpha, Beta, Mu, signals are generated in Brain... and to reduce the complexity and to avoid interference as much as possible.. we go for few electrodes even tho we get 128 channels。
I would say that strongly depends on your research interests or, more specifically, the signals you are interested in. For instance, if you are interested in event-related potentials such as the ERN or FRN, which are maximal at frontocentral and central electrodes, or the P3b, which is maximal at midline parietal electrodes, you can focus on these electrodes in your statistical analysis (and also in data acquisition in general). In contrast, you need other (or more) than midline electrodes when you are interested in certain other signals, such as lateralized readiness potentials.
For instance, in one of my papers (see below), we investigated the FRN and P3 and focused on electrode sites FCz and Cz for the FRN and Pz for the P3b. These electrodes totally sufficed for my research questions (well, along with a reference, ocular electrodes, etc.). However, I acquired data from 61 electrodes, since I was also doing a source analysis, but without that I wouldn't have needed them.
PS: I would like to qualify my comment above. The full range of electrodes was not just needed for the source analysis, but also to have a look at the topographic maps and whether the signals of interest were indeed pronounced at the expected locations (from what is known in the literature) - a robustness check.
Nevertheless, many researchers then focus on these electrode sites of maximal activity (in particular for ERPs). I think this resembles procedures in other domains, e.g., in a region of interest (ROI) analysis in fMRI, where you also focus on activity within specific brain regions. Of course, such an approach needs to be based on previous evidence to be valid, but for many signals in ERP research we already know quite a bit where and when they can be expected.
However, I agree that other techniques that use more of the available temporal and spatial information are particularly valuable.
Jon A Frederick
Generally, the closer your get to Cz, the less muscle artifact.
However, your question is a good one to keep asking when you see this pattern of selective reporting, because it suggests that the results might be cherry picked and p values aren't adjusted for all the unreported significance tests.
The midline electrodes are, hope so, balanced. So, they synthesize the behavior of all the others. If you need to write an abstract about your work, is easier to focus only over the midline, than try to write a large explanation of the statistical differences over more than 30 channels, that such differences maybe are not correlated with the mental task realization.
Perhaps there is one good explanation for a focus on midline electrodes: a frequent enrollment of homologous cortices, making for a bilateral signal whose combined field is maximal medially. But other than that, it is generally a mistake to focus only on some electrodes, midline or otherwise. This is true both under the aim of interpreting one signal (its spatial organization is important: even as one can get good measurement of some properties at peak electrodes, the investigation should not stop there), and under the aim of interpreting several simultaneous signals. If your question was tongue-in-cheek, then I share your stupefaction that so many investigators would go through the trouble of preparing, collecting and analyzing 64, 128 or more electrodes, only to redact a report that shows very few of them. A lot of EEG misconceptions are hidden in this limited release of spatial information, such as inappropriately identified rhythmic activity, e.g. an alpha rhythm interpreted as mu just because it has residual power at electrode CZ.