Investigating a dual-pathway framework for praxis

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PROJECT SUMMARY Performing skilled actions relies on two fundamental abilities: imitating movements to quickly learn new behaviors, and using tools to more efficiently manipulate our environments. Imitation and tool use are critical for many activities of daily living; loss of these abilities in the clinical disorder of limb apraxia, frequently observed in many neurological disorders including in ~50% of individuals with left-hemisphere stroke, is the strongest predictor of increased caregiver dependence and poor post-stroke recovery. Despite the recognized importance of these abilities, the processing mechanisms underlying imitation and tool use remain unclear, in large part due to the challenges associated with studying them. Imitation and tool use both involve translating conceptual cognitive goals (i.e., knowing how to serve a tennis ball) into intricate sets of motor commands (i.e., accurately hitting a tennis ball with a racket). Hence, the study of imitation and tool use requires merging many concepts from cognitive and motor neuroscience. This proposal aims to bridge this cognitive-motor divide by developing a new theory of the common mechanisms supporting imitation and tool use that explains how conceptual goals are actually turned into motor commands. The proposed theory builds on our prior research by hypothesizing two interacting processing routes: (1) A route that specifies actions in terms of the trajectory of the end-effector (e.g., writing letters), affording an efficient means of defining desired tool or hand motion in an abstract (body-independent) manner, and (2) A route that specifies actions in terms of body configurations, providing a more detailed but still computationally tractable description of the positioning of the entire limb (and tool) throughout an action. While these two routes typically work together, they may be engaged to different extents due to task demands (e.g., focusing on racket trajectories or shoulder-elbow-wrist positions during a tennis serve) or when the different brain regions supporting these distinct processing routes become disrupted. Thus, evidence of these two routes will be established through novel task-switching paradigms in healthy individuals, measuring behavioral impairments due to transient disruptions of relevant brain regions using transcranial magnetic stimulation, or by quantifying chronic deficits in patients with left-hemisphere strokes. We will apply novel rigorous kinematic analyses to assess behavior, correlate patient impairments with clinical measures of apraxia, and use recently-developed multivariate lesion-symptom mapping approaches to confirm the neuroanatomical bases of these routes. Together, such efforts will identify and characterize how trajectory and body-configuration planning support the production and perception of imitation and tool use actions. Results will not only advance our understanding of apraxia, highlighting alternative compensatory pathways that might be targeted for rehabilitation treatment, but serves as a model for studying cognitive-motor interactions more broadly.