Researchers study actual physical brain of a deceased person who displayed a unique/particular behaviour whilst alive - such as rare disorder, cognitive deficit etc.
Broca - posterior left frontal lobe lesion
Knowledge of other disorders have been found too e.g Iverson found higher levels of dopamine in deceased schizophrenics brains compared to non-schizophrenics
These also allow detailed study of neurochemical and neuroanatomical aspects of the brain