Why would a neuroscientist immediately doubt that 90 percent of the average brain lies perpetually fallow? First of all, it is obvious that the brain, like all our other organs, has been shaped by natural selection. Brain tissue is metabolically expensive both to grow and to run, and it strains credulity to think that evolution would have permitted squandering of resources on a scale necessary to build and maintain such a massively underutilized organ. Moreover, doubts are fueled by ample evidence from clinical neurology. Losing far less than 90 percent of the brain to accident or disease has catastrophic consequences. What is more, observing the effects of head injury reveals that there does not seem to be any area of the brain that can be destroyed by strokes, head trauma, or other manner, without leaving the patient with some kind of functional deficit. Likewise, electrical stimulation of points in the brain during neurosurgery has failed so far to uncover any dormant areas where no percept, emotion or movement is elicited by applying these tiny currents (this can be done with conscious patients under local anesthetic because the brain itself has no pain receptors).
Scientific American March 2004