It has kind of been understood for a couple of generations at LEAST that this is not the case? There are three decision making centres in the body and only one of those is in the head. The sense of self is largely centered there so it 'feels' like it should be the answer. Plus, intuitively, you can lose arms and legs and function but not the head. So it made sense to assume that before they realised Neurons exited throughout the entire body.