Modality-specific naming deficit: cognitive and neural mechanisms implicated in naming to definition.

We report the case of an anomic patient who had a significantly greater impairment in naming to definition than in picture naming. His difficulty did not depend on the number of semantic attributes (two vs. four) or type of information (visual vs. non-visual) carried in the definition. When asked to visualize the perceptual characteristics of the stimulus by means of drawing, his performance on the naming to definition task improved significantly. We first discuss this performance pattern in terms of the debate between the modality access deficit hypothesis and the multiple semantics position. The patient's more severe impairment in naming to definition suggests that his dissociation may be due to a more selective involvement of verbal semantics. In line with recent PET findings indicating that top-down semantic-to-visual sensory neural feedback improves the retrieval of semantic information in naming to definition tasks, we hypothesize that the patient's difficulty in retrieving the name was due to his inability to automatically activate this pathway. Picture drawing facilitated his naming because it activated the visual system directly (through the structure description level), implementing the retrieval of semantic information.