A Superadditive-Impairment Theory of Optic Aphasia

Martha Farah Dept. of Psychology University of Pennsylvania Phila., PA 19104-6196 Accounts of neurological disorders often posit damage to a specific functional pathway of the brain. Farah (1990) has proposed an alternative class of explanations involving partial damage to multiple pathways. We explore this explanation for optic aphasia, a disorder in which severe perfonnance deficits are observed when patients are asked to name visually presented objects, but surprisingly, performance is relatively nonnal on naming objects from auditory cues and on gesturing the appropriate use of visually presented objects. We model this highly specific deficit through partial damage to two pathways-one that maps visual input to semantics, and the other that maps semantics to naming responses. The effect of this damage is superadditive, meaning that tasks which require one pathway or the other show little or no performance deficit, but the damage is manifested when a task requires both pathways (i.e., naming visually presented objects). Our model explains other phenomena associated with optic aphasia, and makes testable experimental predictions. Neuropsychology is the study of disrupted cognition resulting from damage to functional systems in the brain. Generally, accounts of neuropsychological disorders posit damage to a particular functional system or a disconnection between systems. Farah (1990) suggested an alternative class of explanations for neuropsychological disorders: partial damage to multiple systems, which is manifested through interactions among the loci of damage. We explore this explanation for the neuropsychological disorder of optic aphasia. Optic aphasia, arising from unilateral left posterior lesions, including occipital cortex and the splenium of the corpus callosum (Schnider, Benson, & Scharre, 1994), is marked by a deficit in naming visually presented objects, hereafter referred to as visual naming (Farah, 1990). However, patients can demonstrate recognition of visually presented objects nonverbally, for example, by gesturing the appropriate use of an object or sorting visual items into their proper superordinate categories (hereafter, visual gesturing). Patients can also name objects by nonvisual cues such as a verbal definition or typical sounds made by the objects (hereafter, auditory naming). The highly specific nature of the deficit rules out an explanation in terms of damage to a single pathway in a standard model of visual naming (Figure 1), suggesting that a more complex model is required, involving A Superadditive-Impainnent Theory of Optic Aphasia FIGURE 1. A standard box-and-arrow model of visual naming. The boxes denote levels of representation, and the arrows denote pathways mapping from one level of representation to another. Although optic aphasia cannot be explained by damage to the vision-to-semantics pathway or the semantics-ta-naming pathway, Farah (1990) proposed an explanation in terms of partial damage to both pathways (the X's). visual 67