Spared structural knowledge in a case of semantic dementia: implications for models of object recognition, semantic memory and structural description.

Most models of visual object recognition assume that items belonging to known categories are represented in long-term memory in terms of both structural descriptions and semantic representations. The former specify the visual appearance of category members and the latter allow recognising them as meaningful objects. Nevertheless, the format of these two kinds of representations and their relationships are still a matter of debate. Recently, the independence of structural and semantic representations has been questioned on the basis of the finding of an impaired performance of subjects suffering from semantic dementia on the object decision task, which was originally devised to tap the structural description system. In the present case study of a patient with semantic dementia, we provide data supporting the independence of these two systems. Our results allowed us to better qualify the content and format of structural descriptions in terms of purely geometric non-verbalizable information, specifying the appearance of exemplars at a rather coarse level of categorisation.