In neuropsychology, dissociation involves identifying the neural substrate of a particular brain function through identification of case studies, neuroimaging, or neuropsychological testing.
One method is "double dissociation". In a series of patients with traumatic brain injury, one might find two patients, A and B. Patient A has difficulty performing cognitive tests for, say auditory memory but has no problem with visual memory. Patient B has the opposite problem. By using neuroimaging (or neuropathology post mortem) to identify the overlap and dissociation between lesioned areas of the brain, one can infer something about the localization of function in the normal brain.
Broca and Wernicke were two physicians of the 1800s. In observation of their patients an apparent double dissociation was observed. Broca's patients could no longer speak but could understand language (non-fluent aphasia) while Wernicke's patients could no longer understand language but could produce jumbled speech (fluent aphasia). Postmortems revealed lesions in separate areas of the brain in each case (now referred to as Broca's and Wernicke's areas respectively). Although the neurophysiology of language is now known to be more complicated than described by Broca or Wernicke, this classic double dissociation acted to begin modern neuropsychological investigation of language.
Dr. Oliver Sacks has described many famous cases in his books. There are patients who cannot name an object when they can only see it, but can when they use other senses like touching or smelling. Patient D.F. was unable to place a card in a slot, but could do so when told to place it "as if mailing a letter". From this the conclusion was drawn that judging orientation is one ability (which D.F. had lost) and visual control of an action another (which D.F. could still do) .
To make the difference between single and double dissociations easier to understand, Parkin  gives the following example:
- If your TV set suddenly loses the color you can conclude that picture transmission and color information must be separate processes (single dissociation: they cannot be independent because you cannot lose the picture and still have the color). If on the other hand you have two TV sets, one without sound and one without a picture you can conclude that these must be two independent functions (double dissociation).