I received this email the other day:
These are valid criticisms of a cliché that's glibly made about 'brain power'. Certainly all the brain cells are normally in use, at varying degrees of arousal. However, I was referring to mental abilities, not just to brain cells. In our culture with a typical upbringing and psychological development, we only normally use a small proportion of our potential mental abilities - those such as memory, speed-reading, intuition, psychic, creativity, rationality, awareness, and so on.
These are skills that can be learned rather than neurons magically coming to life. And in terms of the whole person, the mind is more than the brain and the spirit is more than the mind. However, there are indeed nervous system changes that accompany development of mental skills, such as increased development of the (normally under-used) corpus callosum (L/R communication), corresponding neural interconnections, changes in balance of brain-wavelength arousal, changes in ability to arouse particular brain centers consciously, changes in stress level (sympathetic vs parasympathetic arousal), and in neurotransmitter chemistry. Nutrition affects these factors too. There is a great deal of room for positive development!