A PET study of the neural systems of stuttering

Abstract

THE cause of stuttering is unknown¹. Failure to develop left-hemispheric dominance for speech is a long-standing theory¹ although others implicate the motor system more broadly², often postulating hyperactivity of the right (language non-dominant) cerebral hemisphere³. As knowledge of motor circuitry has advanced⁴, theories of stuttering have become more anatomically specific, postulating hyperactivity of premotor cortex, either directly⁵ or through connectivity with the thalamus and basal ganglia⁶. Alternative theories target the auditory⁷ and speech production^8,9 systems. By contrasting stuttering with fluent speech using positron emission tomography combined with chorus reading to induce fluency, we found support for each of these hypotheses. Stuttering induced widespread over-activations of the motor system in both cerebrum and cerebellum, with right cerebral dominance. Stuttered reading lacked left-lateralized activations of the auditory system, which are thought to support the self-monitoring of speech, and selectively deactivated a frontal–temporal system implicated in speech production. Induced fluency decreased or eliminated the overactivity in most motor areas, and largely reversed the auditory-system underactivations and the deactivation of the speech production system. Thus stuttering is a disorder affecting the multiple neural systems used for speaking.