The role of semantic vs. motor processing information in knowledge of tool manipulation

On the one hand, apraxic patients are often reported as being impaired in tool manipulation knowledge, suggesting that integrity of motor production processes is necessary for action recognition. On the other hand, evidence from patients with Semantic Dementia indicates that semantic knowledge is necessary for correct use of objects. Our study explores the role of praxis and general semantics in action knowledge for object use. Performance of 10 apraxic patients with Posterior Cortical Atrophy (n= 7), Corticobasal Syndrome (n= 2) or Alzheimer’s Disease (n= 1) was compared with that of six patients with Semantic Dementia in two experimental tasks requiring the matching of objects (Picture-to-Picture matching, PtP; name-to-name matching, NtN) according to how they are manipulated or to their context of use. We also correlated the two tasks with tests addressing semantic memory and limb apraxia. Repeated measures ANOVA showed a significant group x condition interaction in the PtP matching task, due to similar performance in the two conditions for Semantic Dementia patients, and a significantly worse performance in manipulation than context of use for the apraxic patients; a similar trend also occurred for the NtN matching task. Significant direct correlations emerged for the PtP context condition with measures of semantic memory, and for both NtN conditions with verbal tasks; no correlation emerged with the apraxia measures. Altogether, these findings confirm that apraxic patients are critically impaired in knowledge of tool manipulation, and support a crucial role of semantic memory deficits rather than apraxia.