The Brain Gauge is a Functional Measure
A functional measure simply tells you how well something works. In the physical world, there are lots of functional measures. How fast can you run a mile? How much weight can you lift? How fast can you swim across the pool? Can you run up 2 flights of stairs without getting out of breath? These measures tell you something about performance and how well your body functions when it tries to do something - when it is stressed or pushing the envelope.
Why are functional measures important? After all, I