Though stuttering is manifest in its motor characteristics, the cause of stuttering may not relate purely to impairments in the motor system as stuttering frequency is increased by linguistic factors, such as syntactic complexity and length of utterance, and decreased by changes in perception, such as masking or altering auditory feedback. Using functional and diffusion imaging, we examined brain structure and function in the motor and language areas in a group of young people who stutter. During speech production, irrespective of fluency or auditory feedback, the people who stuttered showed overactivity relative to controls in the anterior insula, cerebellum and midbrain bilaterally and underactivity in the ventral premotor, Rolandic opercular and sensorimotor cortex bilaterally and Heschl’s gyrus on the left. These results are consistent with a recent meta-analysis of functional imaging studies in developmental stuttering. Two additional findings emerged from our study. First, we found overactivity in the midbrain, which was at the level of the substantia nigra and extended to the pedunculopontine nucleus, red nucleus and subthalamic nucleus. This overactivity is consistent with suggestions in previous studies of abnormal function of the basal ganglia or excessive dopamine in people who stutter. Second, we found underactivity of the cortical motor and premotor areas associated with articulation and speech production. Analysis of the diffusion data revealed that the integrity of the white matter underlying the underactive areas in ventral premotor cortex was reduced in people who stutter. The white matter

Abstract – The EXPLAN model of speech control is outlined and it is shown how this accounts for fluent speech and different forms of dysfluency. Assessments of the model are reported. Index Terms – Speech Development, Stuttering, EXPLAN model. I.

several psycholinguistic-oriented theories of stuttering, including EXPLAN (Howell, 2004). Although the study was not explicitly designed to test EXPLAN, it was generally concluded that the evidence does not provide appreciable support for EXPLAN. Howell (2010) objected to this conclusion on several grounds, claiming that the findings do, in fact, support EXPLAN. Anderson responds to Howell’s objectionsinthisletter. Method: Background and perspective on the original study were presented. Howell’s comments on EXPLAN’s predictions concerning stuttering on “easy ” and “difficult” words and the relationship between speech errors and stuttering are disputed in this reply to Howell’s commentary. Results: Howell’s claims that the findings of Anderson support EXPLAN are largely refuted on the basis that (a) the data had not been analyzed by grammatical class and (b) there are no clear predictions regarding the association between variables affecting speech errors and stuttering in EXPLAN. Conclusions: Howell’s commentary seemingly represents an attempt to provide confirmatory evidence for EXPLAN using data that are simply not suitable for this purpose.