Neurological Research Changes Our Understanding of Stuttering

Stuttering during childhood is a relatively common occurrence. Roughly one in 20 kids (5 percent) go through a period of stuttering, but only about one in a hundred (1 percent) of adults are stutterers. Disfluent speech or having a stammer isn't referred to as "stuttering" in the DSM-5. The clinical terminology used to describe speech disfluency is Childhood-Onset Fluency Disorder. That said, "stuttering" is still a widely-used term.

Throughout the 20th century, stuttering was commonly viewed as a "psychological problem." However, accumulating evidence suggests that stuttering's root cause may be structural and functional anomalies in the cortico-basal ganglia-thalamocortical motor loop (i.e., cortico-BG loop).

Severe stuttering in adulthood is also linked to decreased functional connectivity between the cerebellum and a specific area in the prefrontal cortex just behind our eye sockets called the orbitofrontal cortex or OFC.

What Is the Cortico-Basal Ganglia-Thalamocortical Motor Loop?

The cortico-basal ganglia-thalamocortical motor loop is a system of neural circuits that connect cortical and subcortical (i.e., below the cerebral cortex) brain regions.

This three-part motor loop facilitates functional connectivity between gray matter "bark" that encases the cerebrum's cortical brain areas (1), the basal ganglia (2), which is a cluster of subcortical nuclei that are primarily responsible for motor control, and the largest subcortical structure called the thalamus (3), which acts as a type of relay station between the brainstem and the cerebral cortex.

This week, Frank Guenther of Boston University presented his recent paper, A Neurocomputational View of Developmental Stuttering (Guenther, 2021), at the 181st Meeting of the Acoustical Society of America (Nov. 29 to Dec. 3) in Seattle, Washington.

Guenther's research builds on prior theoretical work, most notably that of Dr. Per Alm of Uppsala University in Sweden, in positing that the cortico-basal ganglia-thalamocortical motor loop is heavily involved in the initiation of speech motor programs and that anomalies in the structure and function of these motor loops disrupt speech initiation in ways that cause people to stammer.

Of note: another fMRI study (Thibault et al., 2021) published on Nov. 12 in Science found that tool use and language syntax share the same neural circuits rooted in the basal ganglia. Astonishingly, Simon Thibault and colleagues also found that tool-use training appears to hack into the basal ganglia's circuitry in ways that improve people's language skills. (See "Can Mastering Tool Use Make You a Better Wordsmith?")

Anomalies in the Basal Ganglia's Circuitry Disrupt Speech Initiation

Oftentimes, people who stutter can fluently speak the same words that trip them up at the beginning of a sentence if the exact same words are spoken later in the same sentence. Hence, there's reason to believe that stuttering is primarily an impairment rooted in the initiation of speech and isn't the result of impaired motor skills.

Because most stutterers can fluidly say stammer-inducing words if they come later in a sentence, Guenther speculates that stuttering stems from having trouble with speech initiation, not from atypical sensory-motor encoding of the actual motor programs that facilitate speech fluency.

To help visualize how anomalies in the cortico-basal ganglia-thalamocortical loop can disrupt speech initiation, Guenther uses a jukebox metaphor such that if you put some money into an old jukebox with vinyl 45s, one circuit in the machine would choose the record, and another circuit would play it. Using this jukebox analogy, Guenther explains that the neural circuitry required to play the song (i.e., smooth speech fluency) is working fine inside a stutterer's brain. However, the neural mechanisms needed to initiate the selection of a jukebox song (e.g., spoken words for a stutterer) are impaired.

Inside a neurotypical human brain, one circuit in the basal ganglia initiates fluid speech, and another facilitates the perfectly-timed coordination of muscles needed to articulate fluently spoken words. Guenther's hypothesis that stuttering stems from problems with speech initiation posits that only the first circuit used during this process is impaired inside a stutterer's brain.

Stuttering Is a Speech Initiation Issue, Not a Motor Impairment Issue

More specifically, through the lens of the cortico-BG loop theory, the basal ganglia's initiation circuitry causes disfluent speech. Based on this cortico-BG loop model, stuttering is not the direct result of impaired motor skills rooted in the other cortical-subcortical circuitry. In a paper (Chang & Guenther, 2020) about this theoretical framework from last year, Guenther and coauthor Soo-Eun Chang sum up:

"We propose that the primary impairment underlying stuttering behavior is a malfunction in the cortico-basal ganglia-thalamocortical loop that is responsible for initiating speech motor programs. The basal ganglia and their connections to cortical regions involved in speech form critical networks that support fluent speech production."

"This [model] gives us a fighting chance of finding the specific problems underlying stuttering and addressing them with highly targeted drugs or technological treatments that have minimal unwanted side effects," Guenther noted in a recent news release about his ASA181 presentation.

The Cerebellum May Help Compensate for Speech Initiation Problems

Lastly, although the cortico-basal ganglia-thalamocortical motor loop model of developmental stuttering helps explain the neural bases of disfluent speech, other brain regions such as the cerebellum impact and offset stuttering behaviors.

For example, in another recent study (Sitek et al., 2016), Guenther and colleagues at Harvard Medical School and MIT found that decreased cerebellar-orbitofrontal connectivity correlated with stuttering severity. Conversely, robust neural connections between the cerebellum and the OFC were negatively correlated with stuttering severity. As first author Kevin Sitek and coauthors explain:

"Our findings support the hypothesis that both cerebellum and OFC are involved in successful compensation for stuttering symptoms and suggest that the best compensation occurs when the two compensatory networks—subcortical (cerebellar) and cortical (orbitofrontal)—are synchronized."

More research is needed to examine how neurological deficits rooted in the cortico-BG network cause chronic stuttering by disrupting speech initiation and how other brain mechanisms compensate for these neural vulnerabilities.

Future studies could pave the way for neuroscience-guided treatments that alleviate speech disfluency in adults who stutter and early interventions based on a neurological assessment that could help offset developmental stuttering during childhood by remedying the disruptions of flow that occur during speech initiation.

Neurocomputational modeling and stuttering (IMAGE) by Frank Guenther via EurekAlert