Understanding the neural processes underlying limb apraxia

Limb apraxia, a neurological disorder, occurs after left hemisphere brain damage and impairs a person's ability to use tools, even though their motor control and cognition remain normal. This condition affects around a quarter of stroke survivors and is also observed in other conditions like Alzheimer's disease. Researchers at Thomas Jefferson University have proposed a new theory challenging the existing belief that understanding how to use tools and understanding how objects physically interact are interconnected.

In a study published in Cortex, researchers led by senior author Aaron Wong tested 11 patients who had experienced a left-hemisphere stroke. Participants were shown photos of familiar tools and asked to demonstrate how they would use them. The researchers assessed participants' physical reasoning by having them predict how objects would interact based on factors such as mass and velocity.

They found interesting dissociations between tool use and physical reasoning abilities. While one patient with limb apraxia performed poorly on physical reasoning tests, another patient without apraxia demonstrated proper tool use but performed poorly on physical reasoning tests.

The researchers' findings suggest that these two skills may be independent of each other, contradicting the assumption that impairment in one area would necessarily lead to impairment in the other. Wong explained, "It turns out that's not true." In their ongoing research, the team developed tool-free tasks modeled after a Rube Goldberg machine.

Participants were asked to predict how a ball would move through systems involving ramps and levers, and also completed separate tool-use tasks with unfamiliar tools. The results confirmed that there are individuals who tend to be better at one skill than the other.

Laurel Buxbaum, a co-investigator on the study, suggested that if tool use does not rely on physical reasoning, it may instead be linked to mental simulation of body movements or be a specialized ability. Understanding these mechanisms could ultimately help researchers develop more targeted therapies for limb apraxia. The findings were published in Cortex and preprint on bioRxiv.