Brain differences in people who stutter. A systematic review of neuroimaging literature on developmental stuttering

“Overall… there are widespread functional and structural brain differences between [adults and children] who stutter and their fluent peers…” – Etchell et al 2018

 

A large number of studies have utilised neuroimaging to investigate developmental stuttering; a total of 111 studies were included in this review. Overall, the literature showed that “there are widespread functional and structural brain differences between [adults and children] who stutter and their fluent peers”.

 

What specifically did the results show?

 

Results were inconsistent across studies, but several patterns emerged from the literature:

• Stuttering in adults was associated with less excitement in motor areas of the brain, prior to speaking. Broadly, this means that in adults who stutter, there was less electrical activity in the parts of the brain that control movement, compared to those who did not have a stutter. This was shown from studies that utilised transcranial magnetic stimulation (TMS) and magneto/electroencephalography (M/EEG) - see below for explanations of these techniques.

• A similar pattern was seen in a number of studies that used a technique called functional magnetic resonance imaging (fMRI). These studies showed that in individuals who stutter, there were differences in brain activity during “the planning stages of speech production”, which could lead to disfluent speech. Some fMRI studies also found that prior to speech and periods of stuttering, motor areas were not appropriately activated. These findings come from studies which have examined adults who stutter. The authors suggested that further research was needed to determine whether similar patterns are seen in children.

• Stuttering was associated with atypical activation in other specific regions of the brain, meaning these parts of the brain functioned differently, and were anatomically different in individuals who stutter, compared to those who don’t stutter. Specifically, these differences were seen in the inferior frontal gyrus and right auditory regions of the brain, suggesting these regions may play an important role in stuttering.

 

Is this strong evidence?

A systematic review forms the highest level of evidence, as authors systematically search and appraise all relevant literature on the selected topic.

 

Glossary of neuroimaging techniques:

Positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and near infrared spectroscopy (NIRS): These techniques indirectly measure brain activity during speech tasks. They measure brain activity through the flow or magnetisation of blood. fMRI has the highest spatial resolution, i.e., it shows the clearest and most accurate image. These techniques are able to pinpoint precise regions in the brain that are activated during specific tasks. Contrastingly, they have poorer temporal resolution, meaning that the images may be delayed as it takes time for blood to flow to activated regions.

 

Transcranial Magnetic Stimulation (TMS): TMS is commonly used to probe “cortical excitability of motor areas” at specific time points. Excitability refers to how neurons respond to the electrical activity in the brain, which allows neurons to communicate. TMS can also be used to create temporary “virtual lesions” to test for their impact.

 

Magnetoencephalography (MEG) and electroencephalography (EEG): MEG and EEG use high temporal resolution to directly measure electrical currents occurring in brain. This means that they are able to efficiently track brain changes in real time. EEG takes measurement from the scalp, while MEG taps into a different magnetic field that is slightly above the scalp.

 

See Brain Basis of Stuttering for another summary of brain differences in people who stutter.

 

Etchell, A. C., Civier, O., Ballard, K. J., Sowman, P. F. (2018). A systematic literature review of neuroimaging research on developmental stuttering between 1995 and 2016. Journal of Fluency Disorders, 55, 6-45. doi: 10.1016/j.jfludis.2017.03.007