Table 2.

Models of optic aphasia2.

Models	Theories	Limitations
Canonical Model	A visual processing system feeds its output into a semantic system which in turn feeds its output into a naming system. One cannot name a visually pre­sented object until one first knows what the object is.	One cannot place the lesion in vision, semantics, or the pathway connecting them, because patients can non-verbally demonstrate their recognition of visually presented objects. Neither can one place the lesion in naming or the pathway between seman­tics and naming, because patients are unimpaired in their ability to name objects presented in the tactile or auditory modalities.
Direct Visual Naming Pathway	There is a direct, uninterrupted pathway between vi­sion and naming. Optic aphasia results when the di­rect visual naming pathway becomes disconnected.	There are no documented cases of individuals who can name visual objects without any knowledge of what the objects are.
Modality-Specific Semantic Systems	Each modality has a corresponding semantic system. Optic aphasia arises when there is a disconnection between verbal semantics and visual semantics.	It does not explain the ability of optic aphasics. To sort visually dissimilar items into the same super­ordinate category.
Impaired Access to Semantics from Vision	There is an impairment in accessing a unified seman­tic system from vision. Whereas nonverbal responses may be initiated by activation of isolated semantic features from isolated visual features, naming re­quires access to a complete semantic representation.	Studies indicating poor performance on difficult nonverbal tasks may simply point to the fact that some patients indeed have a greater semantic deficit than others, apart from their inability to name visu­ally presented objects.
Hemisphere-Specific Semantic System	There is an independent semantic system for each hemisphere. Optic aphasia occurs when there is a disconnection between visual input and left hemi­sphere semantics	The major assumption behind this hypothesis - qualitatively distinct semantic for each hemisphere system - was questioned