My current research involves developing and testing an account of why stuttering persists in some speakers whilst other speakers recover. EXPLAN theory maintains that there are two distinct components involved when producing speech: linguistic processing (called planning) and motor programming (termed execution).

According to the theory speakers start an utterance by planning the linguistic component of the first element in the intended sequence and, once this is complete, they pass it on to motor programming after which speech output results (execution). Planning the next element takes place concurrent with execution of the current plan. Speech is fluent when there is sufficient time during motor programming of one plan for the next one to be generated. Speech is dysfluent if the next plan is not ready. This is most likely to happen when the previous word takes a short time to execute and the next word takes a long time to plan. Consequently, fluency problems arise at the junction of easy/ hard words (function/content, unstressed/stressed, high frequency/low frequency etc.). The relative difficulty of the material at these points determine fluency. According to EXPLAN there are two ways of dealing with this situation: One is to repeat already-completed motor plans or hesitate speech at the juncture (called collectively stalling). This results in pausing or whole word repetition on one or more words (when they are called phrase repetitions) before a difficult word. Stalling slows speech execution rate. The other way to deal with the problem at the juncture is to generate speech output using the completed part of the linguistic plan on the difficult word. As only part of the plan is available, this leads to breakdown when the plan runs out. This brief overview indicates three elements are important in EXPLAN - planning, execution and the interface between them.

Some of the main achievements with respect to stuttering in addition to the model are: discovery of the fluency-enhancing effects of frequency altered feedback that is used in prosthetic devices with people who stutter; a model that predicts which children will persist and which children will recover from stuttering, a major longitudinal project on children who stutter which identified several previously unknown risk factors for stuttering, the world’s first online archive of stuttered speech, estimates of heritability of stuttering, scanning work to identify functional and structural factors in stuttering and child and parent expectations about treatment for stuttering.

My current research on stuttering is focusing on motor learning in people who stutter, development of a model for screening unselected school children for stuttering and other communication disorders and the impact of language usage factors on stuttering. The motor learning work involves training English speakers on tone contrasts and seeing how brain activity changes during this process. Control work for this (including scanning) as well as parallel studies training Chinese speakers on Western speech sounds they do not know is taking place in Beijing with Dr Chunming Lu (I am a visiting professor at Beijing Normal University).