There are two main neurobiological theories of the possible causes of stuttering. The first one suggests that feedback change improves speech fluency, which is violated by reduced auditory cortex activity, abnormal motor cortex, and cerebellum activity. The second one says that external temporal cues improve fluency, which is impaired by excess dopamine metabolism, and stress activity correlates with severity and normalizes with successful therapy.
With the help of fMRI technology, it was possible to see a complete picture of brain activity during the speech process. Stuttering people were asked to speak aloud sentences with normal, delayed, and frequency-shifted feedback. The effect was the same for the groups PWS were mostly fluent in the scanner, probably because of the noise. Using diffusion-weighted MRI, it was possible to analyze the brain’s white matter of stuttering people. By measuring the diffusion of water in each voxel, it was possible to conclude the predominant direction of the fibers. Disorganized white matter, impaired tract demyelination, fewer fibers, and smaller axons result in lower fractional anisotropy (FA).
People who stutter show activity in different brain parts than fluent speakers — there is no activity in the motor cortex in PWS. White matter FA is reduced in adults and children with stuttering underlying the ventral sensory cortex. The auditory and motor cortex activity was weakly correlated in PWS during speech reproduction with normal feedback. A possible mechanism underlying the effects of altered auditory feedback on speech fluency is amplifying the auditory signal to synchronize activity in the sensorimotor network better.
Thus, brain imaging studies suggest abnormal brain activity during speech production in the sensory and motor speech cortex in PWS. Consistent results support structural white matter abnormalities underlying sensory and motor areas of the brain in adults and children who stutter (even in those who have recovered). Signal amplification in the auditory cortex by altering auditory feedback improves speech fluency and synchrony of speech-related activity in the sensory and motor cortex in RWS.