Graded Modality-Specific Specialization in Semantics: A Computational Account of Optic Aphasia

A distributed connectionist model of semantics is presented in which semantic representations develop under the pressure of learning to mediate between multiple input and output modalities in performing various tasks. The system has a topographic bias on learning that favors short connections, leading to a graded degree of modality-specific functional specialization within semantics. The approach thus constitutes a middle ground between unitaryversus multiplesemantics accounts. As a result of the topographic bias, as well as the relative degrees of systematicity among tasks, damage to connections from vision to regions of semantics near phonology impairs visual object naming far more than visual gesturing or tactile naming, as observed in optic aphasia. Moreover, as in optic aphasia, the system is better at generating the name of an action associated with an object than at generating the name of the object itself, because action naming receives interactive support from the activation of action representations. The system also exhibits modality-specific impairments for grammatical categories (nouns vs. verbs) following lesions to or from regions of semantics which are partially specialized for objects (nouns) or actions (verbs).