JCPSLP vol 14 no 3 2012

Research update

Developmental stuttering A paediatric neuroimaging study Libby Smith

Libby Smith

Libby and a research participant prepare for a magnetic resonance imaging (MRI) scan

N euroimaging studies conducted over the last decade have consistently found differences in brain anatomy and brain activation patterns during speech between people who stutter and fluent speakers (Beal, Gracco, Lafaille, & De Nil, 2007; Cykowski, Fox, Ingham, Ingham, & Robin, 2010; Foundas, Bollich, Corey, Hurley, & Heilman, 2001; Fox et al., 1996; Neumann et al., 2003; Sommer, Koch, Paulus, Weiller, & Buchel, 2002; Watkins, Smith, Davis, & Howell, 2008). It is likely that a complex interaction of genetic and environmental factors influence the development of brain structure and function in children who stutter, altering the normal functioning motor speech networks in the brain (Watkins, Gadian, & Vargha- Khadem, 1999). Brain activation studies (using positron emission tomography [PET] or functional magnetic resonance imaging [MRI]) reveal that people who stutter use the speech motor areas in the left side of their brain less than fluent speakers and use their right side more than fluent speakers (Brown, Ingham, Ingham, Laird, & Fox, 2005; Watkins et al., 2008). These findings suggest that people who stutter may use a compensatory network for speech due to inadequate function in the normal speech areas in the left hemisphere of the brain (Preibisch et al., 2003; Sommer et al., 2002). Recent research using diffusion tensor imaging (a type of MRI) has found that a particular white matter fibre tract

(a bundle of nerve fibres that carries messages from one part of the brain to another) in the left hemisphere differs in people who stutter compared to fluent speakers (Chang, Erickson, Ambrose, Hasegawa-Johnson, & Ludlow, 2008; Cykowski et al., 2010; Sommer et al., 2002; Watkins et al., 2008). Researchers are not yet sure what causes tracts to differ in these images, but it may be due to abnormalities in the protective sheath (myelin) that helps nerve fibres carry messages (Cykowski et al., 2010). This leads to the intriguing conclusion that stuttering might be caused by a problem with the formation of the myelin sheath during brain development (myelogenesis) (Cykowski et al., 2010). Unlike much of the brain that develops before birth, the particular fibre tract implicated in these studies undergoes myelination during the first two years of life (Yakovlev & Lecours, 1967). It connects brain areas important for speech which integrate auditory and speech motor information (Cykowski et al., 2010). Impaired myelination would interrupt the normal functioning of this connection. Despite stuttering being a developmental disorder, neuroimaging research has so far predominantly involved adults who stutter and they participate many years after stuttering onset. There remains a possibility that the brain differences reported in neuroimaging studies involving adults may be a consequence of stuttering behaviour of the individuals over time, rather than a result of abnormal development in the early post-natal period. By including

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JCPSLP Volume 14, Number 3 2012

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