[stinos]

I can't find a good article atm, but afaik there's an established correlation between fMRI activity (indeed about 0.5/1 slices per second depending on settings) and beta waves.

Btw beta waves can also be recorded using a single electrode: normally these get used to record spikes from a single neuron, but they also pick up the lower frequency bands of all surrounding neurons. The nice thing is this provides a more local view (iirc everything that is closer than a couple of mm around the electrode), whereas an EEG gets you 'everything' so it's more an indication about general brain state without giving information about which part is active.