The study of brain behaviour relationships under very specific circumstances which are both controlled and standardised. Generally this means using tests which have been validated and shown to have acceptable levels of sensitivity and specificity.
Neuropsychologists have extensive training in the anatomy, physiology, and pathology of the nervous system. Some neuropsychologists specialise in research while others specialise in evaluating and treating people who are thought to have something wrong with the way in which their nervous system is functioning.