... the other patient demonstrates the opposite pattern with inability to perform cognitive function B and at least a relatively intact ability to perform function A (Ashcraft, 1994). The underlying logic here is that these cognitive functions are psychologically and anatomically separate and distinct, because if they were not, the brain damage should then disrupt both functions equally (Ashcraft, 1994). We shall now continue to explain the concept of a double dissociation with a concrete demonstration of the phenomenon. Thus we shall contrast two types of aphasias (disruption of language): Broca's and Wernicke's aphasias, as they together offer an excellent example of a double dissociation. We will also examine why information gained through studying a single dissociation is inappropriate on its own - that is without a double dissociation study- in providing valid evidence about the modularity of mind. We shall then continue by pointing out some critical aspects within the methodology ...

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