Over the course of her career, Laurel Buxbaum, PsyD, has worked with collaborators and trainees to make substantial advances in our understanding of how the human brain allows us to successfully use tools and how these processes can become disrupted following stroke. Dr. Buxbaum is Associate Director of Moss Rehabilitation Research Institute (MRRI) and Director of the Cognition and Action Laboratory at MRRI. Research in Dr. Buxbaum’s lab has focused on how (and in which brain regions) tool-use information is learned, organized, activated, and selected.

Deficits in the ability to pantomime, imitate, and recognize tool actions occur in a neurological disorder known as limb apraxia. Apraxia may be present in people with a variety of neurological disorders, and it is most commonly studied in individuals who have experienced a stroke in the brain’s left hemisphere.

In 1982¹, Ungerlieder and Mishkin proposed that different kinds of visual information may be processed in two different pathways in the brain. This idea was further explored and refined over the years by Goodale and Milner². The dorsal stream was observed to be relevant for visual information required for executing actions, such as reaching, grasping, and eye movements to visual targets. In contrast, the ventral stream was found to be important for visual information necessary for perception such as object recognition and semantic knowledge.

This initial model accounted for some of the symptoms and patterns observed in patients with apraxia post-stroke, but Dr. Buxbaum and others in the field found that this model wasn’t sufficient to describe the complexity of the behaviors observed in these patients.

Developing New Models to Account for Tool Use Deficits

In 2001³, Dr. Buxbaum proposed a new functional neuroanatomic model that includes two action systems that are specialized for distinct aspects of tool actions: a bilateral dorso-dorsal network relevant for moving tools (the “Move Network”) and a left hemisphere ventro-dorsal network critical for tool use (the “Use Network”). The Move Network is specialized for actions to currently-viewed objects.  The information is short-lasting, and the network is responsible for processing information related to tool structure (size, shape, and orientation). In contrast, the Use Network is responsible for the representation of remembered trajectories and body postures necessary for learning the skilled use of tools and other manipulable objects.

Subsequent work in the Cognition and Action Laboratory has successfully tested and validated this proposed Two Action Systems model. Dr. Buxbaum and colleagues have demonstrated that tool use depends on the ventro-dorsal stream, and they have further identified that the posterior temporal lobe is an important repository for tool use knowledge and tool action semantics. In contrast, the inferior parietal lobe may be important for knowing how tool use actions are supposed to feel and for buffering the representations that are candidates for tool use actions prior to selection.

In individuals with apraxia, Dr. Buxbaum and colleagues have demonstrated that there is slower and weaker activation of took knowledge. This results in deficits in prediction and a dependence on visual feedback to complete tasks, such as imitating tool use. The impairments in activation of tool knowledge also lead to deficits in selecting task-relevant tool actions and resolving competition between tools with similar actions.

Dr. Buxbaum has also worked with collaborators to expand our understanding of the neural nodes and connections of the tool use network. Based on their data from neuroimaging studies, their Two Action Systems model was expanded to include two additional areas relevant for action selection⁴. This Two Action Systems Plus model further clarifies the brain regions that are likely involved in accumulating potential actions that could be executed and providing goal-relevant signals that may inform action selection.

Subtypes of Apraxia and Relevance for Rehabilitation

Different subtypes of apraxia have been identified based on lesion location and the impairments that result, and Dr. Buxbaum refers to these according to the underlying mechanisms that may be impacted. While individuals may demonstrate impairments that are considered hallmarks of more than one apraxia subtype, identifying underlying causes of different presentations of apraxia can aid in the identification of targeted rehabilitation approaches.

Preliminary work⁵ from the Cognition and Action Laboratory suggests that certain individuals may benefit from a treatment program that focuses on strengthening semantic associations between tool actions and other types of tool knowledge, while others may benefit from treatment programs that involve gesture training with repeated practice of actions. Dr. Buxbaum hopes to continue this line of research to improve treatment options for people with limb apraxia after left hemisphere stroke.