Apraxia, one of many disorders that can emerge after a stroke, involves difficulty executing complex skilled movements, such as those needed to use common everyday objects. This difficulty, which is caused not by paralysis or a weakness of corresponding muscles, but rather by an injury to the brain, is associated with significant disability and dependence on caregivers. While apraxia is a common stroke-related disorder, its causes are complex, and researchers still have a number of hurdles to overcome before they can develop effective treatment.
“Apraxia has been perplexing researchers and clinicians for over 100 years,” says Laurel Buxbaum, PsyD, director of the Cognition and Action laboratory at the Moss Rehabilitation Research Institute. “It’s a very heterogenous group of disorders of skilled action and tool use. Because it manifests in different ways in different patients, there has been a lot of confusion regarding how to best diagnose and classify patients with this disorder.”
According to Dr. Buxbaum, there has long been a need for a detailed neuroanatomic model or map of the brain that clearly shows how lesions in the ‘action system’ of the brain result in particular patient problems or deficits. Such a map might help explain and predict expected consequences of a stroke, including apraxia. She and her colleagues are already beginning to develop such maps.
“We already know,” says Dr. Buxbaum, “that one part of the brain’s left hemisphere, called the temporal lobe, is important for skilled tool use knowledge, and that another part, the parietal lobe, is critical for spatial orientation of the body. Strokes in these areas can lead to different types of apraxia. Our research is geared toward predicting, for example, what problems a patient might have in response to a lesion in the left temporal versus the parietal lobe. We can discover the answer to this kind of question by testing large samples of patients.”
At MRRI, researchers in Dr. Buxbaum’s lab have collected data from more than 140 stroke patients whose lesions have been carefully drawn and analyzed, and they are using a variety of sophisticated techniques to relate the site of a lesion to a patient’s performance during skilled action tasks.
One technique, known as voxel- based lesion symptom mapping (VLSM), is a powerful research tool that allows researchers to infer which part of the brain is responsible for specific behaviors by documenting what patients with a particular lesion are no longer able to do.
Together with colleagues at the University of Pennsylvania, researchers at MRRI used VLSM in the largest study to date of tool-related action and imitation in patients with left-hemisphere stroke. The results of this study were published in a 2014 article in the journal Brain.
“In this study we searched for brain regions common to tool-related imitation and tool-related production,” says Buxbaum. “We wanted to discover, for example, which regions of the brain were important for producing a hammering action when looking at a hammer, which regions were important for imitating another person hammering, and whether there was an overlap in these regions. We found indeed that there was an overlap, and it was in the left temporal lobe–a part of the brain not previously considered important for action. This has helped open up a new way of thinking about this part of the brain.”
Many projects in Buxbaum’s lab are conducted under an NIH grant titled Understanding the Conceptual-Motor Interface, which focuses on examining how our actions in the world are translated into action memories in the brain.
“It turns out that when you use a tool you store a trace of how you use it in your brain, and that trace is strongly linked to the identity of the tool,” says Buxbaum. “When we look at a tool, we automatically activate the ‘use information’ associated with it, even when we aren’t actually using it. So when a healthy individual is asked to find a specific tool in an array of different tools, that person’s brain calculates the use of the target tool, as well as the use information for all of the other tools that can be seen. The brain perceives tools that share similar actions as being similar, even if the tools look very different.”
Recently researchers in Buxbaum’s lab launched a project that will likely have more immediate applications—one involving a technology known as transcranial direct current stimulation (tDCS). They are using tDCS, a type of electromagnetic stimulation of the brain that is very mild and not painful, to facilitate activation of use memories for different tools. So far, they have found that this technique successfully facilitates activation in healthy individuals, and they have begun to explore whether it can be used as a therapeutic tool for stroke patients with apraxia.