Double dissociation refers to a logical progression of scientific assumptions in localizing functional areas in the brain. For example, if symptom A appears with lesions in brain str ucture X, but not with those in Y, and symptom B appears with lesions of Y, but not of X, then those specific areas of the brain each have a specific function.
In brain damage, when function A is present and function B is absent in one person, and function A is absent and function B is present in another. Presence of a double dissociation means that the two functions involve different mechanisms and operate independently of one another.
Moreover, Double dissociation is the finding that some individuals usually brain damaged perform normally on task A and poorly on task B, whereas others show the opposite pattern.
Lexicon of Psychology - Terms, Treatments, Biographies,