[silleknarf]

This exists! You can use the mediation measurements on a Neurosky Mindwave [1] to measure when you're thinking of nothing.

I wrote an app with a friend about 10 years ago where we used the headset and the python mindwave library [2]. The app displays a longer and longer spiral on screen when you aren't thinking about anything.

[1] The headset seems to still exist in some form: https://store.neurosky.com/pages/mindwave [2] https://github.com/faturita/python-mindwave