Study Explores Parkinson’s Effect on How Brain Represents Tool Embodiment

A new way to understand and study how the brain represents tool embodiment — the physical actions for using tools — was proposed in a new research article.

The study recommends methods for understanding Parkinson’s disease signs and symptoms relating to the brain’s ability to represent motor output, which can aid in the diagnosis, treatment, and monitoring of neurological conditions.

The research article, “Defective Tool Embodiment in Body Representation of Individuals Affected by Parkinson’s Disease: A Preliminary Study,” was published in Frontiers in Psychology.

People have the capacity to use tools for acting in the environment. For example, they can use a rod because something is out of reach; the tool helps them get an object that would otherwise be unreachable. To do so, the human brain takes the object into account as if it were part of the body; when using a rod, the brain represents the hand as if it were the tip of the rod — the brain “embodies” the tool.

This ever-changing representation of the body, derived from all sensory input around the body that the brain uses to plan and execute actions, has been termed the “body schema.” Although there is increasing research on understanding the body schema and tool embodiment in healthy individuals, the body schema is thought to be affected in several pathological conditions. These include people with neurological diseases affecting motor control or spinal cord injury, as well as amputees who use a prosthesis. Yet there is little known about the body schema and tool embodiment in Parkinson’s disease, a neurological disease characterized by sensory and motor symptoms affecting motor action.

To understand how the body schema is affected by neurological disease, researchers from the Istituto Auxologico Italiano and the University of Turin, Italy, studied tool embodiment in Parkinson’s disease patients.

The study included 14 people affected by Parkinson’s disease and 18 healthy controls. The researchers had the participants use a rod to point toward a far target and studied the accuracy and how long it took for the patients to point toward the target before and after they had spent time training with the rod.

They also investigated changes in the brain’s estimation of arm length through a test called the Tactile Estimation Task. In this task, participants estimated the distance between two tactile stimuli presented simultaneously on the arm. If the rod is correctly embodied, the arm should be represented from its base to the tip of the rod, and, consequently, the distance between the two might be perceived larger than the actual gap.

All patients were tested when their symptoms were efficiently managed by Parkinson’s medicine, or “on” state of medication.

After tool-use training, control participants’ behaviors completely changed: they were slower to perform movements after training to try to hit the target. More specifically, after training, healthy individuals reported a higher value of deceleration — when individuals are nearest to the target after having achieved peak of velocity of movement. However, such differences did not emerge in the Parkinson’s participants. Researchers did not observe any differences in estimation of arm length before and after the tool-use training in either Parkinson’s or control participants.

These results suggest possible difficulties in the tool embodiment process for Parkinson’s patients. The lack of changes in these participants before and after training possibly reflects the absence of effective tool embodiment into the body schema. This study proposes a new way to understand Parkinson’s signs and symptoms in terms of how they affect a patient’s body schema.

“This study suggests a novel way to conceive Parkinson’s sensory motor signs and symptoms: the disease might affect the tool embodiment in cognitive body representation, as a possible secondary effect of altered plasticity of body schema, since the sensory and motor symptoms, or altered multisensory integration process due to the degeneration of dopaminergic neurons in the basal ganglia,” researchers stated.

“Tool embodiment in body representation can extend the potentiality of individual’s action; however, if deficient, it might have remarkable consequences and implications on motor behavior, specifically in those clinical conditions like Parkinson’s, in which the body and action are primarily affected by symptoms,” they said.