Competition between tool and hand motion impairs movement planning in limb apraxia

Tool use is a complex motor planning problem. Prior research suggests that planning to use tools involves resolving competition between different action representations. We reasoned that competition may be exacerbated with tools for which the motions of the tool and the hand are incongruent, such as pinching the fingers to open a clothespin. If this hypothesis is correct, then we should observe particularly marked deficits in planning the use of incongruent as compared to congruent tools in individuals with limb apraxia, a common disorder after left-hemisphere stroke (LCVA) that is associated with abnormal action competition. In a first experiment, we asked 34 individuals with chronic LCVA and 16 matched neurotypical controls to use novel tools that we developed in which the correspondence between the motions of the hand and tool-tip were either congruent or incongruent. Individuals with LCVA also completed background assessments to quantify apraxia severity. We observed increased planning time for incongruent as compared to congruent tools as a function of the severity of apraxia, particularly when the tools were first introduced. Exploratory lesion-symptom mapping analyses revealed that lesions to posterior portions of the tool-use network were associated with impaired planning for incongruent tools. A second experiment on the same individuals with LCVA revealed that the ability to demonstrate the use of conventional tools was impaired for tools rated as more incongruent by a normative sample. Together, these findings suggest tool-hand incongruence evokes action competition and influences the tool-use difficulties experienced by people with apraxia.