How much of our brain do we actually need?

I attended a gifted and talented conference a few months ago where we were told about a man who had walked into a hospital with enlarged ventricles meaning that his actual brain tissue was reduced massively. I was so fascinated by this because I could not get my head around the fact that the brain can be so significantly damaged and for someone to be able to function with almost no real problems. I read further into this and found that this man had an IQ of 75, which is of course below average but by no means unusual, and definitely does not indicate the level of damage to his brain tissue which was seen. To me this shows just how much our brains can adapt to certain circumstances. This is especially impressive considering that the man had suffered from hydrocephalus as a child, which can often kill. The man must have had immensely increased intracranial pressure which, in his severe case should have caused some sort of mental disability. 

Another interesting case of brain injury (that I read about in Impulse) was the case of Phineus Gage, who had a metal rod driven through his brain, but managed to survive reasonably unchanged, given the circumstances. This is due to the fact that the rod penetrated his left frontal lobe, the area of the brain associated with recognising consequences and knowing the difference between right and wrong (among other things of course), which would explain why Gage's injury only resulted in a change in his voluntary behaviour. This was, of course, famously recognised by his peers, and the reason he was fired from his job as a foreman. 

I find it incredibly interesting that the brain is such a complex and vital organ, but can take serious damage without fatal consequences. When even the smallest thing goes wrong, for example in 'silent strokes', it can have significant effects, such as the onset of vascular dementia, but others can have a huge steel rod pushed through and remain mentally competent. It is definitely fair to say that the human body is a well oiled machine and something I often wonder if I will ever truly get my head around.