The starting point for the two levels hypothesis. A hand has an inside and an outside, and both sides are available for observation. In a similar way, the activity in some parts of the brain can be observed from two different angles.

 
  Part of: Johannisson T (2020) Variation in length of alpha waves reveals how forebrain activity is organized. Research Ideas and Outcomes 6: e49942. https://doi.org/10.3897/rio.6.e49942