Tool use and concept representation in the brain: a study on three neurodegenerative patients

Objectives: Apraxic patients with left-hemisphere lesions are impaired in semantic tasks tapping knowledge of objects manipulation, but the specific locus of actions representations is still debated: the same brain regions involved in action production, according to the sensory-motor theory, or the left posterior middle/superior temporal gyrus (MTG/STG), according to a more recent hypothesis. To address this issue, we investigated brain FDG-PET correlates of action knowledge in three neurodegenerative cases with left-hemisphere damage. Materials and methods: We included one patient with Cortico-basal Syndrome (CBS), limb apraxia and parietal-occipital hypometabolism, one patient with Alzheimer's Disease (AD), mild apraxia and aphasia and extensive posterior temporal hypometabolism including MTG/STG, and one patient with Semantic Dementia (SD) showing anterior temporal hypometabolism. Their knowledge of actions vs context of use was assessed with a visual object-to-object association task and a verbal categorical decision task. Results: The CBS case showed no impairment of action knowledge, the AD patient had a worse performance in the gesture condition, with a strong dissociation in the verbal task, and the SD case was equally impaired in gesture and context in the verbal task but had a normal performance with gestures in the visual task. Discussion and conclusions: Contrary to the sensory-motor prediction, in the CBS patient parietal damage causing apraxia dissociated from loss of action knowledge. In the SD case, general semantic impairment affected action representations, sparing access to affordances. Finally, the AD patients' performance supports a specific role for the left posterior MTG/STG in storing/processing action semantics. References: Binkowski, F., & Buxbaum, L. J. (2012). Two action system in the human brain. Brain and Language, 127, 222-229. Buxbaum, L. J., et al. (2014). Critical brain regions for tool-related and imitative actions: a componential analysis. Brain, 137, 1971-1985. Ishibashi, R., et al. (2011). Different roles of lateral anterior temporal lobe and inferior parietal lobule in coding function and manipulation tool knowledge: Evidence from an rTMS study. Neuropsychologia, 49, 1128-1135.