What does it take to grasp a tool? A fine-grained multiple-case analysis of hand-tool and tool-object interaction errors in apraxia.

Apraxia of tool use is a neuropsychological syndrome characterized by difficulties in selecting or using familiar tools that cannot be better explained by primary motor, sensory, attentional, or language deficits. The neuropsychological literature indicates that tool use is a complex, multifaceted ability relying on heterogeneous cognitive processes and neural substrates. Patients may commit errors at different stages of the grasp-to-use sequence, including hand-tool interaction errors (e.g., holding tools upside-down), tool-object association errors (i.e., incorrect matching of the functional parts of tools and objects), and tool-object interaction errors (i.e., inappropriate mechanical actions), resulting in reduced performance efficiency. However, the cognitive origins of these distinct error types remain unclear, making their interpretation challenging for both researchers and clinicians. We report the case of D.B., a 69-year-old right-handed woman who sustained multiple strokes. D.B. demonstrated flawless tool selection and subnormal tool-object associations, but clearly abnormal hand-tool and tool-object interactions, in the context of impaired technical reasoning and preserved semantic tool knowledge and planning skills. This profile is discussed in light of previously reported cases of apraxia and optic ataxia. This analysis suggests that tool selection and tool-object association errors may stem from deficits in semantic tool knowledge, whereas hand-tool and tool-object interaction errors are more plausibly explained by deficits in technical reasoning. Motor imagery may also represent a contributing factor and warrants further investigation.