Study reveals brain mechanisms behind speech impairment in Parkinson’s

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 proven to be a powerful way to relieve motor symptoms for Parkinson’s patients — but a common side effect is worsened speech impairment.

Same test, different scores

In the new study, 27 participants with Parkinson’s disease and 43 healthy controls, all older than 60, took standard tests of motor and cognitive functioning. The participants with Parkinson’s took the tests while on and off their medication.

As expected, the medication improved motor functioning in the patients, with those having the most severe symptoms improving the most.

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

As a group, the patients’ performance on both versions of the cognitive test was little affected by medication. But 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 spoken performance on the medication. Their written test scores did not change significantly.

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

The dissociation suggested that the medication was not enhancing general cognitive functions such as attention and working memory, but it was selectively improving speech.

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

Uncovering connections

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