On drawing inferences about the structure of normal cognitive systems from the analysis of patterns of impaired performance: the case for single-patient studies.

An analysis of the logic of valid inferences about the structure of normal cognitive processes from the study of impaired cognitive performance in brain-damaged patients is presented. The logic of inferences from group studies and single-case studies is compared. It is shown that given certain assumptions, only the single-case method allows valid inferences about the structure of cognitive systems from the analysis of impaired performance. It is also argued that although the single-case approach is not entirely problem-free, the difficulties encountered are relatively minor.