Neuropsychology appeared within psychiatry and was a part of it for a long time.
It has own specifics, as neuropsychology studies the brain-behaviour relationship. It means that the investigation is aimed to learn what is the relation between the information processing of the brain with the behaviour of the person.
Frankly speaking, the abnormal behaviour is the result of brain damage or information processing dysfunction, as states neuropsychology.
Regarding history, the role of neuropsychology was minimal. The only thing trusted to a scientist in a field was a differentiation between organic and functional psychoses. This might be the ground of the ‘long history’ you’ve asked about.
Only in the past decade neuropsychology has gained a stable place of a separate science. (That is why it has a short past, actually). Its importance now is beyond any doubt. Its first breakthrough was in the study of schizophrenia and brain damages.
It is interesting to know, that neuropsychology actively uses loads of technologies in studies. F.e. SPECT, PET and fMRI techniques are actively used for the findings validation.
Neuropsychology now has the new field, ‘cognitive neuropsychiatry,’ that actively elaborates its studies and is fascinating in first results. Check them out when there’s a chance.
Also, I’d recommend you to go through these several exciting materials to get a better understanding of the topic