Systematicity and Specialization in Semantics: A Computational Account of Optic Aphasia

Optic aphasic patients are selectively impaired at naming visually presented objects but demonstrate relative intact comprehension of those objects (e.g., by gesturing or categorization) and are able to name them when presented in other modalities (e.g., via tactile input). This and other modality-specific naming deficits have been taken as evidence that semantics is organized into distinct modality-specific subsystems. We adopt an alternative view in which semantics is a set of learned, internal representations within a parallel distributed processing system that maps between multiple input and output modalities. We account for the critical aspects of optic aphasia in terms of the effects of damage to such a system, despite its lack of modality-specific specialization. We show that the robustness of a task in such a system depends critically on its systematicity, and that modality-specific naming deficits can arise because naming is an unsystematic task.