I've just started looking into EEG data using a Muse s and mind monitor. I've noted this very regular 4 Hertz (approximately) pattern and wonder if this is real or an artifact. Is it just a very regular delta wave? My understanding is that this is the raw real time levels from different sensors. The time between dashed lines is one second.
TP9 and AF7 are definitely not brain waves. AF8 is more realistic, but still too clean; I suspect both it and TP10 were heavily filtered. Possibly TP9 and AF7 had negligible activity and were dominated by equipment noise, where a single frequency prevails (e.g. 60Hz), so its bandpass filtering would show such regularity.
I'd dig around in the app for a way to display the data in raw form.