Cognitive models indicate that there are multiple ways of completing the same task. This implicit degeneracy cannot be revealed by functional imaging studies of normal subjects if more than one of the sufficient neural systems is activated. Nor can it be detected by neuropsychological studies of patients because their performance might not be impaired when only one degenerate system is damaged. We propose that degenerate sets of sufficient neural systems can only be identified by an iterative approach that integrates the lesion-deficit model and functional imaging studies of normal and neurologically damaged subjects.