For those who now speak smoothly, it may seem quite straightforward. You think of something, and you can say it. You do not need to consider the multiple signaling pathways to and from your brain that make speaking possible!
In reality, the production of speech is a complex process. It not only involves your mouth and larynx but several parts of your brain as well.
One such lesser-known part of the brain that supports the production of spoken language is the basal ganglia.
How Did Researchers Learn About The Relationship Of Basal Ganglia With Speech?
When three patients with injuries to the basal ganglia were observed, researchers noted some anomalies in their speech. Two of the subjects were disfluent. One subject spoke fluently, but all three produced agrammatic sentences with semantic errors!
Voila! If injuries to the basal ganglia prevent the production of grammatically correct sentences and fluent speech, then its proper function must be crucial for the production of speech as well.
So, what’s New with Basal Ganglia and Speech?
Developmental stuttering affects around 8% of children around the globe. Recent stats show that approximately 1% of the total population stutters.
Existing theoretical evidence states that the dysfunction of the basal ganglia is associated with developmental stuttering. This theory originates from observing similarities between Parkinson's disease and dystonia (disorders involving the basal ganglia) and the changes in fluency upon deep brain stimulation (TMI). The use of dopaminergic medication elicits similar effects on fluency.
Early PET scans show that abnormal basal ganglia function may be responsible for disfluent speech in individuals. Unfortunately, recent MRI scans and functional MRI (fMRI) studies have failed to identify the dysfunctions in the basal ganglia that may cause or characterize developmental stuttering.
Traditional MRI scans can determine the volume of gray matter, cortical thickness, and the diffusion properties of white matter fiber tracts. Conventional MRI or fMRI cannot provide enough imaging evidence to substantiate or contradict this theory!
So, researchers used a multi-parameter mapping (MPM) protocol for creating semi-quantitative whole-brain maps. These maps can reveal the differences in the tissue microstructures that quantify the myelin and iron content in targeted parts of the brain.
The aim of this study was to compare the scans of the brains of individuals who stutter with individuals of similar age and gender who are fluent. The imaging study produced semi-quantitative whole-brain maps of the subjects for a thorough comparison.
Low concentrations of iron in the basal ganglia have been associated with poor cognitive ability in adolescents. On the other hand, a higher concentration of iron in the basal ganglia is the hallmark of neurodegenerative disorders like Parkinson's disease.
The most recent MPM MRI study shows there are considerable differences in the microstructures that may contribute to disfluent speech. In simpler words, those who stutter have a higher concentration of iron in the gray matter.
Since iron and dopamine have complex interactions in the brain, researchers say that higher iron concentrations in the basal ganglia implicate a higher dopamine presence that plays a significant role in the precipitation of stuttering.
Elevated Iron Concentration in the Basal Ganglia
The study included 73 participants. 41 of them were individuals who stutter, and 32 were individuals who are typically fluent. The whole-brain volume of those who stuttered didn't differ from those who were fluent.
The researchers state, "The study provides proof of microstructural differences in the cortico-basal ganglia-thalamocortical loops through the putamen among those who have developmental stuttering. However, further investigation is necessary to link the differences observed with the genetic profiles associated with a childhood-onset fluency disorder."
The new finding that links the increased iron concentration in the basal ganglia may shed more light on the increased dopamine release, lysosomal dysfunction, or other neurobiological factors that contribute to developmental stuttering.
In 2019, a team of researchers led by Dr. Drayna used mouse models to demonstrate that mutations in the GNTAB genes cause a dip in the volume of astrocytes in the corpus callosum of the brain. These astrocytes are star-shaped cells that support the transmission of messages inside the brain, and the corpus callosum is a bridge-like structure that connects the two hemispheres.
The findings from the latest research like these opens new avenues for more studies on medication and therapy for developmental stuttering in all ages.
Very soon, it may also become possible to predict whether an infant will grow up with a stutter from the analysis of the fetal genes or MRI of the newborns.
Stuttering and Gray Matter Volume
Despite studying a large sample size, researchers have found no inconsistencies in the gray matter volume in the same age group.
No evidence links stuttering to cognitive functions. Those who stutter may be just as capable and intelligent as those with fluent speech. The higher concentration of iron in specific parts of one's brain may simply prevent them from speaking as smoothly as 99% of the world's population!
