EEG recordings revealed that reindeer's brainwaves during rumination resemble the brain waves present during non-REM sleep, and these brainwave patterns suggest that the reindeer are more "rested" after ruminating.

Source: https://phys.org/news/2023-12-reindeer-cud-strategy-summer.html

For example, if the goal is to increase activity in a particular brainwave frequency, you might receive positive feedback like a pleasant sound or a moving image on a screen when your brain produces the desired waves.

Source: https://londonlovesbusiness.com/ten-benefits-of-neurofeedback-and-brain-training/