Here is a fascinating account of a man whose skull was mostly filled with fluid rather than brain tissue.  He had a lower than average IQ but was able to lead a normal and productive life.  The fact that he had little brain tissue challenges current notions of consciousness:

“Any theory of consciousness has to be able to explain why a person like that, who’s missing 90% of his neurons, still exhibits normal behavior,” says Cleeremans. A theory of consciousness that depends on “specific neuroanatomical features” (the physical make-up of the brain) would have trouble explaining such cases.

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