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Editorial: Special collection on single case studies.

Mainstream research in psychology or neuroscience is currently biased towards large sample group studies, with single case research being neglected. However, single cases have always been milestones in the progress of science. Throughout history there are many examples of single cases that have brought insights to our understanding of clinical and psychological phenomena. In medicine, for example, the first case report of a patient with melancholic depression and insomnia, described by Hippocrates, dates back to 400 BCE. A breakthrough in neurology was made in the nineteenth century by the famous description of the case of “Tan-tan” by Paul Broca. Our understanding of brain functions was also facilitated by analyses of the consequences of brain injuries. A renowned example is the case of Phineas Gage, a mild-mannered railroad worker who was injured with an iron rod shooting up from his left cheek through the top of his skull. The large destruction of his brain’s left frontal lobe, leading to unexpected severe personality changes without a loss of language capability, greatly influenced nineteenthcentury debates on the localization of cerebral functions. Brain-injured patients also inspired Alexander Luria, providing a basis for his formulation of a theory of systems and the dynamic localization of cognitive functions. The famous case of Henry M., who lost his referential memory after a bilateral hippocampal ablation, opened a new window of research on memory functions. A unique constellation of phenomena in a patient, or a specific individual or a group, provides a distinctive perspective for looking into brain functions or dysfunctions. Medical science may thus benefit from a newly described syndrome or disease, an unexpected association between different diseases, or a case with a surprising clinical development that suggests therapeutic or adverse drug effects. In psychology, the emphasis on testable theories within mainstream research should be complemented by single case studies, which can be central with respect to the confirmation, disconfirmation, or rejection of a theory. Furthermore, “outlier” cases representing variations from the expected patterns may indicate the extreme borders of a particular psychological phenomenon, thus enriching the existing understanding or even generating new hypotheses. A good example of such an “extreme” case is the journalist S. V. Shereshevsky with his seemingly unlimited memory as described by Alexander Luria. It should also be stressed that the unique benefit of conducting single case studies lies in the constellations of particular psychological phenomena which include even unexpected details, and in the potential holistic understandings researchers can gain from such cases. Taken as a whole, exploratory or hypothesis-generating single case research should be treated as equal in importance to confirmatory hypothesis-testing studies. A balance between the two types of research, with their different methodological concepts, is necessary as they complement each other and enrich our understanding of how the mind functions in its complexity. The current issue of PsyCh Journal, along with a forthcoming sequel in preparation, aims to reorient attention from the presently observed bias for group studies towards a complementary focus on single case research. Contributions to this special collection come from researchers working in Austria, Brazil, China, the Czech Republic, Germany, Italy, Latvia, Russia, and the USA. They comprise discussions of the theoretical frames of single case studies, clinical observations and experimental paradigms for rare psychiatric and neurological conditions, psychological cases with unusual subjective phenomena, and descriptions of specific groups of patients and healthy individuals. Despite this diversity in perspective, they all contribute to uncovering mechanisms of the cognitive machinery.