Your brain is an expensive organ

Chapter 4 endnote 14, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

In brain-imaging experiments, when we show photographs to test subjects or ask them to perform tasks, only a small portion of the signal we measure is due to the photos and tasks; most of the signal represents intrinsic activity. [...] This intrinsic activity is metabolically expensive;

Your brain is your most expensive organ—it consumes about 20% of your body’s budget—and the biggest consumer is intrinsic neural activity. There is a limit on how many connections your brain can support metabolically and still run. It must transfer information efficiently by passing activity to the fewest number of neurons possible.