ABSTRACT
The traditional neuropsychological understanding of the breakdown of disorders of vision and more broadly visuo-spatial cognition has been one of a bewildering variety of disorders. There are “visual” components to disorders as diverse as agnosia, alexia, apraxia, and topographical disorientation—as well as more esoteric disorders such as optic aphasia and simultanagnosia. This traditional pattern of classification has, by and large, involved discussing this range of disorders independently, in terms of disorders of visual and spatial ability. This classification system has also involved identifying the lesion site typically associated with each disorder. The rationale for identifying these disorders as being independent, or unrelated to each other, is partly undermined by the fact that, in many cases, the lesion sites for different disorders largely overlap. Thus, there has been debate on the necessity of a bilateral lesion in prosopagnosia, of a lesion involving the splenium of the corpus callosum in optic aphasia, and of the possibility of “dorsal” and “ventral” simultanagnosia (De Renzi, 1982; Farah, 1990; Grusser & Landis, 1991). This approach has served the diagnostic needs of clinical work very well, and it has offered a body of sound and replicable observational findings that form the basis of any science. However, the very complexity, and diversity, of the clinical disorders have made it difficult to conceptualise their relationship to the overall manner in which the visual system operates. Certainly, there are many disorders of vision, but are there any underlying trends or patterns that might help us to understand the general landscape of the visual system? Thus, in order to move from observation to theory, it might be useful to suggest a parsimonious unifying principle which simplifies this world of complex clinical descriptions, and may also provide a framework for a cognitive neuroscience of high-level vision.
