Study reveals brain mechanisms behind speech impairment in Parkinson’s | News Center

The subthalamic nucleus is known for its role in inhibiting motor activity, but there are clues to its involvement in other functions. For example, deep brain stimulation, which uses implanted electrodes to stimulate the subthalamic nucleus, has been shown to be a powerful way to relieve motor symptoms in Parkinson’s patients, but a common side effect is worsening speech impairment.

Same test, different scores

In the new study, 27 participants with Parkinson’s disease and 43 healthy controls, all over age 60, performed standard tests of motor and cognitive functioning. Participants with Parkinson’s took the tests while taking and not taking medication.

As expected, the drug improved the patients’ motor functioning, and those who had the most severe symptoms improved the most.

The cognitive functioning test offered a surprise. The test, known as the Digit and Symbol Modalities Test, is administered in two forms: oral and written. Patients are given nine symbols, each associated with a number (a plus sign for the number 7, for example). They are then asked to translate a series of symbols into numbers, either by speaking or writing their answers, depending on the version of the test.

As a group, patients’ performance on both versions of the cognitive test was little affected by medication. But upon taking a closer look, the researchers noticed that the subset of patients who performed particularly poorly on the spoken version of the test without medication improved their oral performance with medication. Their written test scores did not change significantly.

“It was quite interesting to find this dissociation between the written and oral versions of the same test,” Cai said.

The dissociation suggested that the drug did not improve general cognitive functions such as attention and working memory, but did selectively improve speech.

“Our research revealed a previously unrecognized impact of dopaminergic drugs on the speech function of Parkinson’s patients,” Menon said.

Discovering connections

Next, the researchers analyzed fMRI brain scans of the participants and looked at how the subthalamic nucleus interacted with brain networks dedicated to various functions, including hearing, vision, language, and executive control.