Levodopa treatment can prevent movement impairment in patients with Parkinson’s disease by increasing overall sensory attenuation, or the ability to fine-tune information received from the senses before a motor action is performed, a study suggests.
Based on these findings, researchers suggest that dopamine, which increases as a result of levodopa treatment, may be important for regulating brain activity to effectively integrate predictions of action with sensory information — a process required for the control of voluntary movements.
“This may provide a common framework for understanding the role of dopamine in perceptual, cognitive, and motor function,” they wrote.
The study, “Sensory attenuation in Parkinson’s disease is related to disease severity and dopamine dose,” was published at the journal Scientific Reports.
Parkinson’s disease is often characterized by slower movements, which are associated with the impaired ability of patients to plan, initiate, and execute voluntary movements. However, the underlying mechanisms that promote these deficits are still not very well understood.
Researchers evaluated the sensorial and motor response of 18 patients with idiopathic Parkinson’s disease and 175 age and gender-matched healthy volunteers used as controls. All Parkinson’s patients were receiving treatment with levodopa, one of the main therapies used to increase the levels of dopamine.
To quantify participants’ sensorimotor response, researchers used the force matching task, in which a torque motor applies one of four force levels through a lever to the left index finger. Participants are then asked to match the force they just sensed either by pressing the lever with their right index finger (direct condition), or by sliding a linear potentiometer that controls the torque motor (slider condition).
In response to this test, people often apply a stronger force when exposed to the direct condition, while they tend to use a more accurately matched force in the slider condition. The overcompensation of forces that occur in the direct condition has been associated with the integrity of the fronto-striatal network — an area of the brain strongly affected by dopamine deficits in Parkinson’s disease.
Task results revealed that Parkinson’s patients had less sensitivity than controls. Still, the overall force response to matching the applied motor force was similar between patients and controls.
Further analysis showed that overall sensory attenuation was negatively related to Parkinson’s motor severity, but positively linked to individual patient dopamine levels, as measured by levodopa dose equivalent.
In general, patients who were taking higher levodopa doses were also the ones showing greater overcompensation on the direct condition of the task.
“These results support the hypothesis that dopamine alleviates disorders of movement in Parkinson’s disease by restoring the precision and hence the typical reliance on sensorimotor predictions,” the researchers wrote.