Understanding the neural processes underlying limb apraxia

Limb apraxia is a neurological disorder that can occur after damage to the brain’s left hemisphere and is characterized by difficulty in using tools despite normal motor control and cognition. It affects roughly a quarter of stroke survivors and is observed in other conditions, including Alzheimer’s disease.

A popular theory proposes that understanding how to use tools, like a hammer, and understanding how objects physically interact are one and the same. New research from Thomas Jefferson University challenges this and could reshape how scientists understand limb apraxia and eventually inform treatment.

In a study published in Cortex, researchers led by senior author Aaron Wong, PhD, an institute scientist at Jefferson Moss Rehabilitation Research Institute, tested 11 patients who had a left-hemisphere stroke. Participants viewed photos of familiar tools and demonstrated how they would use them. Participants’ physical reasoning was assessed by predicting how objects would interact based on factors like mass and velocity.

The researchers found interesting dissociations. A patient without apraxia demonstrated proper tool use but performed poorly on physical reasoning tests, such as identifying which of two balls was heavier after they collided. Another patient with apraxia showed the opposite pattern. Even though these opposite behavioral patterns were only found in two patients, it showed that the behaviors are independent of one another.

“There’s an assumption that if you’re impaired in one area, you should be impaired in the other,” says Dr. Wong. “And it turns out that’s not true.”

In ongoing research, the researchers developed tool-free tasks modeled after a Rube Goldberg machine. Participants predicted how a ball would move through systems involving ramps and levers. They also completed separate tool-use tasks with unfamiliar tools.

“Again, we could see that there are people who tend to be better at one than the other,” says Dr. Wong.

“If being able to use tools does not rely on physical reasoning, there are other possibilities,” says Laurel Buxbaum, PsyD, co-investigator and institute scientist at Jefferson Moss Rehabilitation Research Institute. “It may rely, at least in part, on mental simulation of body movements. Or tool use may be its own special ability.”

Understanding these puzzling mechanisms could ultimately help researchers develop more targeted therapies for limb apraxia.