Increasingly studies are finding physical differences in the organisation of brains of people who stammer, but what about children? Kate Watkins is lead author of one of two recent studies comparing the brains of young people who stammer with others.

Link to larger image. From the Watkins study. The left image shows the pattern of activity during speech production in the fluent control group (orange) and the group of young people who stammer (blue). Right image shows the regions of white matter (in orange) where the people who stammer have a disruption in the fibre tracts directly underlying the cortical areas that are not activated.

This winter two research groups published brain-imaging studies of young adults, adolescents and children who stammer. These studies reveal abnormalities in brain function and structure. Eventually, such studies aim to provide a better understanding of the causes of stammering and explain why some children recover from and others persist in stammering. Considering the results of these two studies together, we are beginning to answer some of these questions.

In our paper published in Brain, we describe differences in the physical organisation of the connections between brain areas in a group of adolescents and young people who stammer. These differences explained why the same group of people showed reduced brain activity in some regions during speech production.

Chang and colleagues published very similar findings in the journal NeuroImage. In children with persistent stammering but not those who had recovered from stammering, white matter connections were disrupted in the same region as in the people studied by us. Chang also found changes in the volume of cortical grey matter in both children with persistent stammering and children who had recovered from stammering, and pointed to differences from the brains of adults who stammer.

Disrupted connections

These studies used magnetic resonance imaging (MRI) scanners. MRI scanners provide detailed pictures of brain structure based on the movement of hydrogen atoms in water molecules. Both our group and that of Chang used a new kind of MRI scan known as diffusion-tensor imaging to look at the organisation of the white matter connections in the brain. White matter connects areas of the central nervous system together. It is white because it is mostly made up of a fatty tissue known as myelin. This fatty sheath provides insulation for the fibres that connect different brain areas and increases the speed of transmission of signals between communicating regions. Because the tissue is fatty, water molecules are restricted in their movement in the white matter. The restriction is limited mainly to movement across fibre bundles and less so along their length. By measuring these minute movements of water, we can obtain pictures showing how fibres are oriented.

Both studies found that the normally well-aligned fibre bundles connecting the brain areas involved in speech production were disrupted in young people who stammer. This disruption likely reduces the efficiency of communication between the brain areas involved in listening to and producing speech.

Measuring activity

Activity in the brain occurs close to the cell bodies of neurons, which are located in the grey matter. At the surface of the brain, the grey matter forms a layer about half a centimetre thick called the cortex. Deeper in the brain, grey matter is found in nuclei, such as the basal ganglia. In our study, we also used functional MRI to look at brain activity when people produced speech. This kind of scan detects changes in the amount of oxygen carried in the blood to the brain. When a brain area is active, it needs more oxygen and more blood flows to this area. These scans show how these patterns of brain activity change as people produce speech compared to silence.

Our study found that during speech production, people who stammer showed increased activity in brain areas not typically used by the fluent control group. Some of these regions were in the right hemisphere, and other studies suggest that activity here reflects compensation for stammering. We also found increased activity in people who stammer in an area called the midbrain at the level of the substantia nigra. The grey matter nuclei in this deep structure form part of the basal ganglia - a set of structures involved in the control and initiation of movement. The extra activity in this region in people who stammer is consistent with suggestions in previous studies that stammering is due to abnormal function of the basal ganglia or abnormal amounts of dopamine.

Interestingly, in our study the people who stammer showed reduced activity in one part of the 'normal' speech production system. This region - the ventral premotor cortex - lies directly above the area of disrupted white matter revealed by the diffusion scan. It seems that the region's activity was reduced due to a disruption of the normal connectivity and efficient communication with other regions of the brain that are important for fluent speech production.

Grey matter

Chang and colleagues also found differences in grey matter volume in parts of the cortex involved in speech production and perception in children who stammer. They found less grey matter close to the ventral premotor cortex where we found reduced activity. They also found less grey matter in the regions that are normally activated when listening to speech (the temporal lobes). These differences in volume were present even in children who had recovered from stammering.

Unlike studies of adults who stammer, however, no increases in grey matter volume were found in the right hemisphere of the children. This difference suggests that in adults who stammer, some of the brain differences may have resulted from a lifetime of stammering.

It is important to understand more about the causes of the differences in brain structure and function associated with stammering. These differences might reflect compensatory strategies that make use of another function to control speech, or that use other brain regions outside of the disrupted system that are not normally used in non-stammering brains; this idea is supported by Chang?s findings on the grey matter volume. Alternatively, differences could cause stammering and may relate to either genetic variation or events in early development. Further research, particularly longitudinal studies starting early in life may help shed light on these questions.

Kate Watkins is a lecturer in Experimental Psychology at the University of Oxford and the Centre for Functional Magnetic Imaging of the Brain (FMRIB).

References:
Chang, SE et al (2008). Brain anatomy differences in childhood stuttering, NeuroImage 39(3):1333-44. doi:10.1016/j.neuroimage.2007.09.067, PMID: 18023366
Watkins, KE et al (2008). Structural and functional abnormalities of the motor system in developmental stuttering. Brain 131(Pt 1):50-9. doi:10.1093/brain/awm241, PMID: 17928317

On the basal ganglia and dopamine, see e.g. www.stammering.org/peralm.html

From the Spring 2008 issue of 'Speaking Out', pages 14 & 15.