The Brown, Ingham, Ingham, Laird and Fox article is a good example of how this research area is slowly shifting focus. Over the last decade, brain research was rather "trivial": 1) find some people with PDS 2) put them in a brain scanner 3) look at the pretty pictures and use some basic statistical tests 4) write an article on what you find.
Their meta-analysis effectively summaries this empirical evidence from the last decade, in a sense drawing a line behind past imaging studies. But why is there overaction in motor areas plus one cerebellum area, atypical right hemispheric activation, and underactivation in auditory region?? Why does it lead to stuttering? And that's where the story becomes infinitely more complex: theory building.
Science really comes in two parts: collecting empirical evidence and constructing a theory that describes the patterns of the evidence and that predicts new empirical evidence which can be used to test the theory. There have been many theories on PDS around over the last century, but none made a decisive impact. Not surprisingly, some theories were not testable due to lack of empirical evidence e.g. it was not possible to look into the living brain 10-20 years ago. And most theories were pseudo-theories and not testable as they were only conceptual or made vague predictions like you are too nervous/have a conflict within you/had a bad experience. Thus many people were collecting experimental findings, and very few tried (or were unable either due to skill or lack of good evidence) to categorise them into a theory.
In 1999, at the IFA congress in Danemark, Prof. Conture , currently editor of The Journal of Fluency Disorders, complained in his keynote address that many researchers sofar gathered facts "like squirrels might gather nuts" rather than "develop and test theorectical explanations for facts and findings". But I have to slightly disagree. It is unfair as even the best scientists could in my opinion not have come up with a productive theory as no-one had access to the living brain 10 years ago. Or maybe 10-20 years ago, some good scientists left or never entered the field of PDS in the sense of "PDS must be in the brain, I cannot study the brain, so let's do research in another area"???
The meta-analysis puts some constraints on any theory of PDS:
1) The deficit happens INSIDE the brain, or up-stream from overt speech production.
2) Any complete theory on PDS MUST take into account the clear empirical evidence of differences in activation level in the PDS brain, or prove that the evidence of more than 10 brain studies is wrong.
Tomorrow more on why theory building is tricky and a discussion on the theory based on efference copy put forward by Ingham et al.