Visual agnosia in the era of behavioral and neural investigations

Single case studies have long been used to provide insights into the mechanisms underlying normal cognition, including in the domains of memory, language and visuoperceptual function, and standardized testing has been a steadfast companion in such investigations. Experimental approaches designed to address specific hypotheses have also been conducted and analytic methods have been developed for the comparison of single subject data to a control group. However, a seismic shift has occurred in the last decade or two in which neuroimaging, primarily magnetic resonance imaging, has been added to the experimental toolbox. The question addressed in this article is whether, with these newer methodologies offering novel and previously unattainable evidence, single case studies have become obsolete. Here, in a single patient with integrative visual agnosia, tested repeatedly over three decades, behavioral, neuroimaging and joint behavioral-neuroimaging studies are described and their yield evaluated. Behavioral investigations have served to characterize the perceptual deficit well, and structural and functional neuroimaging data have furthered our understanding of the distributed circuit engaged in object recognition. However, imaging studies executed in concert with a behavioral task have offered more direct causal evidence, providing a more complete understanding of brain-behavior correspondences that goes beyond the sum of the parts. The conclusion reached is that the contribution of causal evidence from single cases remains a powerful methodology in advancing our knowledge of the neural basis of cognition.