There has been a long-running debate about whether the brain behaves "holistically" (all parts are responsible for all functions) or whether each little piece of the brain has its own particular job. In a sense, both views seem to be correct. Each part of the cerebral cortex has a distinct function, but most of these parts are multiply connected to each other and to the rest of the brain. As a result, if some areas in the cerebral cortex are destroyed, their functions are lost; but other areas are more "flexible" - if they are damaged, the rest of the cortex can compensate.
Blind cats are better at locating sounds than sighted cats: the auditory part of the brain partially compensates for the defective vision. Similarly, blind people who learn to read braille have better tactile processing systems in their brains; so have jewellers and others who perform fine manual work. A braille message activates a part of a blind person's brain that in sighted people responds to visual stimuli. (The information processing capacity of the visual channels is vast and alternative inputs cannot compensate fully, but the fact that there is any compensation at all is striking.) A congenitally deaf person reading sign language uses a part of the brain that is activated by speech sounds in hearing people. Mammalian brains retain such plasticity throughout life, though it might decrease with ageing.
However, if the primary visual and auditory regions in the human cerebral cortex are lost (the regions where information from the retina or inner ear is first recorded and sorted), the sufferer is blind or deaf and these losses cannot be compensated. Plasticity is confined to the secondary regions, where the perceived information is interpreted. Similarly: if a stroke destroys part of the motor cortex then the patient might recover well. But if other parts of the brain, for example the basal ganglia or cerebellum, have been damaged, then there is hardly any recovery.
Like most terminally differentiated cells, mature neurones cannot divide. In most vertebrate brains, neurones are not replaced when they die, so their number in the brain decreases with advancing age. After their late teens, humans lose brain neurones at the rate of about a million a day. This sounds alarming, but the human brain contains something like a million million neurones altogether. Nevertheless, losses can accumulate significantly over a long life-time. If the brain had less plasticity, if new circuits were less capable of compensating for damage, then senile dementia might develop much earlier.
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