A Superadditive-Impairment Theory of Optic Aphasia

Abstract

Accounts of neurological disorders often posit damage to a specific functional pathway of the brain. Farah (1990) has proposed an alterna(cid:173) tive class of explanations involving partial damage to multiple path(cid:173) ways. 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 rela(cid:173) tively 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 perfor(cid:173) mance 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.