Normal speech production involves a balanced coordination between muscle movements involving breathing, phonation (voice production), and articulation (movement of the throat, palate, tongue, and lips). The speech cycle essentially incorporates input (hearing, vision, proprioception), central processing (receptive and expressive speech areas of the brain) and output (phonation, articulation and resonation).
The precise mechanism of what led to stuttering is unknown but has long been thought to be disorder of the central speech processing. The advent of advanced imaging techniques like PET Scan and functional MRI in investigation of stuttering revealed interesting facts that people who stutter show hypoactivity in cortical areas associated with language processing, such as Broca's area, but hyperactivity in areas associated with motor function.
A puzzling aspect of stuttering is the various conditions which can temporarily alleviate dysfluency in most cases: the rhythm effect (speaking to the pace of a metronome), singing (remember the dude at just-ended American Idol season?), chorus speech, and altered auditory feedback. Another interesting aspect of stuttering is that in some sufferers, there is spontaneous resolution either during childhood, or even during adult life! But we currently have no clue as to in which subject stuttering will spontaneously resolve. Another interesting fact is that this recovery can be induced by therapy in few cases. Both spontaneous and assisted recoveries also appear to be heritable. More interesting still is that children with early onset of stuttering tend to show precocious language development.
Stuttering, long thought to be largely psychogenic is now believed to consist of 3 types - developmental, neurogenic and functional.
Further Readings:
1. Journal of Communication Disorders 37 (2004) 325–369
2. Stuttering
3. How the brain repair stuttering. http://m.brain.oxfordjournals.org/content/132/10/2747.long
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