Category-specific deficits in semantic dementia: links between perception and semantic knowledge

The nature of knowledge and its relationship with the perceptual processes are among the most central issues in the study of human cognition. Should knowledge be abstract, then semantic memory and perception should be relatively independent. On the contrary, if knowledge is sensory-dependent, then memory and perception should be very close. The first view is supported by the multiple system approach of memory, whereas the second view is supported by the single-store memory theories. One way to study these links is through the category-specific impairment and the sensory-functional theory (SFT). Category-specific impairment is generally observed for living items compared to artefacts. The SFT explains this deficit by defining living items as essentially based on perception. In the abstract view of knowledge, a living deficit should be related to a deficit in processing sensory knowledge. On the opposite, the sensory-dependent view states that this deficit results from perception impairment. This article focuses on the relations between knowledge and perception in semantic dementia (SD). SD is characterized by a progressive loss of semantic knowledge, making it particularly interesting to study. This article first focuses on the SFT, to explain the category-specific impairment. The issue of perceptual processing in SD is then reviewed from the lowest level (senses) to the highest level of perception (multimodal integration). The data demonstrated normal perception for these patients. However, visual integration appeared to be impaired for existing knowledge. This result is discussed in relation with a possible involvement of the anterior temporal lobes. These regions are known to be the most vulnerable in SD. Recently these regions have also been shown to be involved in the multimodal integration. Taken together, these data suggest that perception and knowledge could be linked and partially explained by the SFT. Finally, the data support the sensory-dependent approaches of memory.