Altered Brain Activity May Explain Why Speech Is Perceived as Yelling
A particular region of the brain responsible for sensing and processing speech and speech-like sounds is overly active in people with Parkinson’s disease, according to a new study.
These findings may help provide a biological explanation for why Parkinson’s patients tend to speak more quietly than a given situation warrants. They might also explain why, as emerging research suggests, patients experience their own speech as louder than it is and feel as if they’re yelling, even though they’re actually talking at a normal volume.
The study, “Early auditory responses to speech sounds in Parkinson’s disease: preliminary data,” was published in Scientific Reports.
Many people with Parkinson’s disease have problems with speech. Motor symptoms associated with the disease can contribute to speech difficulties by impairing the muscles used for talking, but Parkinson’s speech-related complications commonly extend beyond the purely mechanical.
When a person hears speech, or any other sound, specialized cells in the ears convert the vibrations of sound waves into electrical signals that are sent to the brain, which sorts through the noise and makes sense of it. Hearing speech-like sound activates a circuit of electrical activity toward the base of the brain (brainstem) called the frequency-following response (FFR), which helps it decode aspects of speech, such as pitch.
Parkinson’s is characterized by a loss of dopamine signaling in the brain. Dopamine is a molecule that brain cells use to communicate with each other. Among its many other functions in the brain, dopamine signaling is known to regulate FFR, helping to “tune” the response and prevent unneeded activity.
Since dopamine is known to affect FFR, which is important for how speech and sound is perceived, it’s possible that Parkinson’s patients have an irregular FFR that contributes to altered perceptions of speech-like sounds. In this study, a quartet of scientists in the U.K. and Canada conducted a series of experiments to test this idea.
Using electrodes placed on the scalp, the researchers recorded FFR in response to speech-like sounds, or sounds outside the normal frequencies of human speech, among 15 people with Parkinson’s and 15 people with no known health conditions. All the participants were in their mid-60s, on average, and just over half of them were men.
Results broadly indicated that Parkinson’s patients had more FFR activity in response to speech-like sounds, whereas there was no difference between the groups for sounds outside normal speech frequencies.
“These findings provide a neural basis for the sensory processing deficits of vocal pitch and loudness at the brainstem level” in Parkinson’s disease, the researchers wrote.
“It appears that the increased FFR response in [Parkinson’s] participants reflects a reduction in the inhibitory (or tuning) function of the auditory brainstem pathway which enhances the frequency response within the vocal pitch range,” they added.
This was a small study, the researchers emphasized, and most of the Parkinson’s participants had mild to moderate disease.
Further study is needed on how Parkinson’s affects the way the brain processes sounds, and how other factors, such as playing a musical instrument or speaking multiple languages, might affect these brain processes, the researchers wrote.