How Brain Damage Affects Knowledge: Behavioral Evidence of Category- and Modality-Specific Semantic Deficits in Tunisian Aphasic Patients with Left Sylvian Lesions

This paper aims at contributing evidence to the cerebral organisation of semantic memory using Arab Tunisian aphasic data. It seeks to confirm the existence of category and modality-specificity of semantic deficits, to investigate the dissociable nature within domains of knowledge, as well as to explore the existence of a preferential impairment between a specific type of knowledge and a specific domain of knowledge. A testing battery consisting of a picture-naming task, a word-to-picture matching task, and a semantic task was designed to test living things, non-living things, body parts and colours. The battery was applied on 28 participants equally divided between patients and controls. Patients were Tunisians with left post stroke aphasia. To our knowledge, this is the first study investigating category-and-modality-specific semantic deficits in an Arab-speaking population. The study documents a dissociation of semantic knowledge across semantic categories (p=.008) and tasks (p <.001) with finer grained distinctions within domains of knowledge (e.g. Tools vs. Furniture; p= 0.033). In the naming task, the knowledge of Tools was significantly more impaired than all other categories (p= 0.006). The reported discrepancies between naming and comprehension were highly significant (p=.000008). Findings confirmed the absence of a preferential impairment between a specific type of knowledge and a specific domain of knowledge. The study allows criticizing the neural structure principle while challenging the assumptions made by the sensory/functional hypothesis. KeywordsAphasia; Category-specific semantic deficits; Dissociations; Left sylvian stroke; Modality-specificity; Semantic memory; Tunisian patients