Dissociation (neuropsychology)

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In neuropsychology, dissociation involves identifying the neural substrate of a particular brain function through identification of case series, 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 localisation of function in the normal brain.

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) ^[1].

To make the difference between single and double dissociations easier to understand, Parkin ^[2] gives the following example:
If your TV set suddenly looses the colour you can conclude that picture transmission and colour information must be separate processes (single dissociation: they cannot be independent because you cannot loose the picture and still have the colour). 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).

[edit] References

1. ^ E. Bruce Goldstein: Sensation and Perception. Wadsworth, Pacific Grove (USA), 2002.
2. ^ A.J. Parkin: Explorations in Cognitive Neuropsychology. Blackwell, Oxford, 1996.