Although brain damage can result in various mental deficiencies, we shouldn’t forget the remarkable capacity of the brain to re organize itself (brain plasticity). There have even been people who had hardly any brain, but functioned very well. For example, before improved standards of antenatal care were available, Dr. Lorber (1981) had studied hundreds of patients who displayed normal and above normal IQ's in spite of having severely reduced brain tissue – in fact, they had almost no brain! Check out the following reference:

Lorber J. Is your brain really necessary? Nurs Mirror. 1981 Apr 30;152(18):29-30