Research shows that cluttering speakers have a so-called inhibition deficit: once they have started speaking, it can be difficult for them to keep their speech rate in check, and even to stop speaking. There is evidence that this inhibition problem originates in the brainstem, the part of the brain that controls the central nervous system.
The inhibition deficit can manifest itself in a too high or an irregular speech rate. At an irregular rate of speech, sudden accelerations in the speech start to emerge. These spurts arise because cluttering speakers underestimate the time they need to express their thoughts. This disrupts their language planning and makes it harder for them to get their words out of their mouths smoothly. Typical speakers and people who stutter do not suffer from this inhibition deficit, which improves their ability to continue speaking at a more constant pace, and explains why they normally do not experience language planning problems.