Do we have brain to spare?

Within the liter and a half of human brain, stereologic studies estimate that there are approximately 20 billion neocortical neurons, with an average of 7,000 synaptic connections each.1 The cerebral cortex has about 0.15 quadrillion synapses—or about a trillion synapses per cubic centimeter of cortex. The white matter of the brain contains approximately 150,000 to 180,000 km of myelinated nerve fibers at age 20, connecting all these neuronal elements. Despite the monumental number of components in the brain, Szentagothai estimated that each neuron is able to contact any other neuron with no more than six interneuronal connections—“six degrees of separation.” The human brain would seem to have enough redundancy and plasticity so that the loss of a modest number of neurons or connections might have little consequence. But because the brain—unlike the liver, with its array of nearly identical portal lobules—has exquisitely specialized substructures involved in sensory, motor, and integrative functions, impairment following structural loss may depend not only on how much brain is lost, but where. Modest focal neuronal losses in eloquent areas of the brain may produce dramatic deficits; more extensive diffuse losses of neurons in so-called “silent” areas may be less obvious or go unnoticed. Hebb was moved to write his classic Organization of Behavior …