When dissecting complex mental tasks into their subcomponents, a researcher can establish a "single dissociation" between functions.
[citation needed] Dr. Oliver Sacks has described many famous cases of dissociation in his books.
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).Paul Broca and Carl Wernicke were two physicians of the 1800s whose patients were evidence of the double dissociation between generating language (speech) and understanding language.
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).
[8] The conditions Capgras delusion and prosopagnosia have also been argued to represent a double dissociation.