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Whilst data from individual normal subjects, or individual brain-damaged patients, can undoubtedly constrain cognitive models, certain patterns of deficit across populations of patients can provide much stronger constraints. If a patient A performs very much better on task 1 than on task 2, then we say that we have a strong single dissociation. If two patients, A and B, have opposite single dissociations, then together they form a double dissociation. This pattern of performance can be conveniently plotted as in Figure 3.1. The various types of dissociation and their implications have been discussed in

Figure 3.1. A strong crossover double dissociation for Tasks 1 and 2, Patients A and B.