A functional model of visuo-verbal disconnection and the neuroanatomical constraints of optic aphasia

Abstract

In this paper, we discuss the case of a patient, AB, who presented a pattern of performance corresponding to that usually known as optic aphasia. In particular, her visual object naming was severely impaired, while tactile naming and naming to definition were significantly better. In addition to the classical visual anomia, the patient also showed a deficit in tasks requiring categorization and access to associative knowledge. We interpret the results of our patient in line with the explanation proposed by Coslett and Saffran (Brain, 1989; 112: 1091–110), I.e. a disconnection between right hemisphere and left hemisphere semantic knowledge. Damage to the left occipital region requires the initial processing of visual information to be carried out in the right hemisphere only, and a lesion of the splenium of the corpus callosum interrupts the flow of information from the right to the left hemisphere. However, the pattern of symptoms observed in our patient can only be fully explained by combining this framework with a model which distinguishes visual from verbal semantics (Shallice, 1988. From Neuropsychology To Mental Structure, Cambridge University Press). While the right hemisphere has a complete visual semantic organization, it has only a basic and concrete associative semantic representation. AB's difficulties in categorizing and in accessing associative knowledge as the result of a visuo-verbal disconnection were also interpreted in this light. Furthermore, we suggest that the variable patterns of optic aphasia and the different behaviour of associative visual agnosic patients may be explained by interindividual differences in the levels of verbal and visual semantics in the right hemisphere.