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Understanding Speech Impairment in Cerebellar Ataxia: Why It Sounds Different but Can Still Be Understood

Written by Allison Hilger, PhD, CCC-SLP
Edited by Celeste Suart, PhD

Did you know cerebellar ataxia affects how speech sounds more than how it’s understood? Learn why speech naturalness matters.

Have you ever heard someone speak in a way that sounds a bit unnatural, even if you can understand them perfectly? This is often the case for people with cerebellar ataxia, a neurological condition that affects movement coordination, including speech.

When the cerebellum (the part of the brain that helps control movement) is damaged, it can make speech sound different. This condition is called ataxic dysarthria. But how does it affect speech exactly? Is it harder to understand what is being said, or does it just sound unusual?

This study, conducted by researchers at the University of Colorado Boulder, set out to answer that question by comparing people with ataxia to healthy individuals. The researchers focused on two aspects of speech:

Intelligibility: How well can listeners understand the words spoken?
Naturalness: Does the speech sound rhythmically and melodically normal, or does it sound unusual and robotic?

Their findings revealed that people with ataxia do not necessarily struggle with being understood, but their speech sounds less natural. This distinction is important because even if a person is understandable, their speech might still impact their confidence and communication.

How was the Study Done?

The researchers recorded speech samples from 27 people with ataxia and 28 control participants of similar age and gender. Control participants are research participants with no neurological diagnosis to compare against the participants with ataxia. Speech-language pathology students then listened to these recordings and rated intelligibility and naturalness. Intelligibility was measured by having the students transcribe what was said and calculating how many words they transcribed correctly. Naturalness was measured on a scale of 1-7, from very unnatural to very natural. The study also examined other factors like age, gender, severity of ataxia, and time since diagnosis to see if these played a role in how speech was affected.

Key Findings: What Did They Discover?

What did they find? Overall, people with ataxia had relatively unimpacted intelligibility, but there were differences in speech naturalness. In other words, people with ataxia were almost as understandable as the control speakers. On average, the people with ataxia were 94% intelligible, meaning that listeners could accurately understand 94% of what was being said, compared to 99% for the control speakers. This means that listeners could generally understand what people with ataxia were saying, even if their speech wasn’t perfect.

However, speech naturalness was affected by the speakers with ataxia. People with ataxia had an average naturalness score of 3.37 out of seven, while the control speakers had a score of 5.60 out of seven. Furthermore, the more severe the ataxia, the more unnatural it was rated.

However, the amount of time since the ataxia diagnosis also played a role: longer-term patients with ataxia had slightly worse naturalness ratings. Factors like age, gender, and self-reported quality of life did not strongly affect intelligibility or naturalness. These findings suggest that speech in people with ataxia sounds less natural. This is likely due to impaired prosody for controlling pitch, loudness, and timing. However, speech is still generally understandable.

Why does this matter?

These findings provide valuable insights into the role of the cerebellum in speech production. In particular, they show how the cerebellum contributes to the coordination, rhythm, and flow of speech. The study highlights that while the cerebellum may not be essential for producing intelligible speech, it is crucial in ensuring that speech sounds natural and fluid. This explains why individuals with cerebellar ataxia often experience speech that is understandable but sounds uneven or unnatural. Having less naturally sounding speech can affect a person’s ability to communicate effectively in social situations. Understanding these specific impairments can help speech-language pathologists develop targeted therapies that focus not just on clarity, but also on improving the prosody, pitch, and rhythm of speech for individuals with cerebellar damage or disease.

How do these findings fit with what we already know about this disease?

This discovery aligns with what we already know about cerebellar ataxia and its effects on movement coordination, including speech. Previous research has established that the cerebellum plays a key role in fine-tuning motor actions. This includes the precise timing and rhythm required for clear and natural speech.

However, past studies have primarily focused on speech intelligibility, or whether or not someone with ataxia can be understood. This study builds on that knowledge by showing that while intelligibility remains relatively preserved, the naturalness of speech is significantly impaired. This finding reinforces the idea that the cerebellum is essential for regulating the prosody, pitch, and smoothness of speech rather than just articulation.

Additionally, these results complement what we know about motor control in ataxia. People with ataxia struggle with coordinating smooth movements. This leads to symptoms like unsteady walking, difficulty with fine motor tasks, and now, a clearer understanding of irregular speech patterns. This study bridges the gap between motor function and speech. It emphasizes that the cerebellum’s role extends beyond balance and movement. The cerebellum also helps to shape the way speech sounds in a social and communicative context.

Key Words

Intelligibility: The percentage of words that are correctly understood.

Naturalness: How speech patterns fit with expected patterns of rate, rhythm, intonation, and loudness

Ataxic dysarthria: Speech impairment caused by cerebellar dysfunction

Conflict of Interest Statement

The author and editor have no conflicts of interest to declare. The writer of this summary was part of the original research team being summarized. This research was funded by the NIH NIDCD F31, DC017877-01A1, and the Council of Academic Programs in Communication Sciences and Disorders.

Citation of Article Reviewed

Hilger, A., Cloud, C., & Fahey, T. (2023). Speech impairment in cerebellar ataxia affects naturalness more than intelligibility. The Cerebellum, 22(4), 601-612.