A Cognitive Overview of Limb Apraxia.

Since the first studies on limb apraxia carried out by Hugo Liepmann more than a century ago, research interests focused on the way humans process manual gestures by assessing gesture production after patients suffered neurologic deficits. Recent reviews centered their attention on deficits in gesture imitation or processing object-related gestures, namely pantomimes and transitive gestures, thereby neglecting communicative/intransitive gestures. This review will attempt to reconcile limb apraxia in its entirety. To this end, the existing cognitive models of praxis processing that have been designed to account for the complexity of this disorder will be taken into account, with an attempt to integrate in these models the latest findings in the studies of limb apraxia, in particular on meaningful gestures. Finally, this overview questions the very nature of limb apraxia when other cognitive deficits are observed.

Handling or being the concept: An fMRI study on metonymy representations in coverbal gestures.

In "Two heads are better than one," "head" stands for people and focuses the message on the intelligence of people. This is an example of figurative language through metonymy, where substituting a whole entity by one of its parts focuses attention on a specific aspect of the entity. Whereas metaphors, another figurative language device, are substitutions based on similarity, metonymy involves substitutions based on associations. Both are figures of speech but are also expressed in coverbal gestures during multimodal communication. The closest neuropsychological studies of metonymy in gestures have been nonlinguistic tool-use, illustrated by the classic apraxic problem of body-part-as-object (BPO, equivalent to an internal metonymy representation of the tool) vs. pantomimed action (external metonymy representation of the absent object/tool). Combining these research domains with concepts in cognitive linguistic research on gestures, we conducted an fMRI study to investigate metonymy resolution in coverbal gestures. Given the greater difficulty in developmental and apraxia studies, perhaps explained by the more complex semantic inferencing involved for external metonymy than for internal metonymy representations, we hypothesized that external metonymy resolution requires greater processing demands and that the neural resources supporting metonymy resolution would modulate regions involved in semantic processing. We found that there are indeed greater activations for external than for internal metonymy resolution in the temporoparietal junction (TPJ). This area is posterior to the lateral temporal regions recruited by metaphor processing. Effective connectivity analysis confirmed our hypothesis that metonymy resolution modulates areas implicated in semantic processing. We interpret our results in an interdisciplinary view of what metonymy in action can reveal about abstract cognition.

Neural correlates of transitive and intransitive action imitation: an investigation using voxel-based morphometry.

The ability to reproduce visually presented actions has been studied through neuropsychological observations of patients with ideomotor apraxia. These studies include attempts to understand the neural basis of action reproduction based on lesion-symptom mapping in different patient groups. While there is a convergence of evidence that areas in the parietal and frontal lobes within the left hemisphere are involved in the imitation of a variety of actions, questions remain about whether the results generalize beyond the imitation of tool use and whether the presence of a strong grasp component of the action is critical. Here we used voxel-based lesion-symptom mapping to assess the neural substrates of imitating meaningful (familiar, MF) and meaningless (unfamiliar, ML) tool-related (transitive) and non-tool related (intransitive) actions. The analysis showed that the left parietal cortex was involved in the imitation of transitive gestures, regardless of whether they were meaningful or not. In addition there was poor reproduction of meaningless actions (both transitive and intransitive) following damage of the right frontal cortex. These findings suggest a role of right frontal regions in processing of unfamiliar actions.