Wednesday, June 08, 2005

Are there phase transitions in PDS?

One way to look for solutions to a problem is to put on different glasses i.e. to analyse a problem using different concepts/viewpoints. For example, I have talked about the sex ratio and used this concept to analyse PDS. The last posts were about the basal ganglia putting it in the spotlight and understanding PDS around it. Today, I want to put on yet other glasses and analyse PDS from the point of view of phase transitions.

What are phase transitions? It is a concept used in physics to describe phenomena where a small change of a parameter leads to a dramatic change in the properties of a system. I give an example: water. If water is at 0 Celcius and you decrease the temperature by a very small amount, it freezes to ice immediately thus dramatically changing the properties of the substance. But if you cool water from 10 Celcius by a very small amount, it is still liquid just a big colder thus the properties change a bit and in a continuous way. Another phase transition is at 100 Celcius where liquid water goes into vapour (i.e. adding a bit more temperature (=energy), the water molecules have suddenly enough energy to overcome attraction to other water molecules).

My question is: Are there phase transitions in PDS?

You can look for phase transitions in many areas of PDS. The most obvious is to look at the difference between people with PDS and without. Can you unambiguously put someone into either the PDS or non-PDS box? OR, is there is a continuum, a smooth transition, from a severe stutterer to a fluent speaker. The transition in disfluency rate (as a symptom of PDS) between stuttering and fluent speakers seems to be a continuous one, and you can find the whole range of disfluent speech in all degrees of severity. Even "fluent" speakers are disfluent to some degree and stuttering speakers very fluent sometimes. There does not seem to be an obvious boundary when just looking at stuttered syllables. Most studies define stuttering as above 3% of stuttered syllables, but this is a rough guideline.

On the other hand, you immediately know when someone has PDS when witnessing certain symptoms like loosing eye contact, word substitution or grimaces, etc. I once asked Luc de Nil whether if I gave him a brain scan, he would be able to say whether the person has PDS or not. And he said yes. There is some research by Neumann & Preibisch that claims that the more fluent a PDS subject the more atypical right-brain activation, but fluent speakers show no right-brain activation. This would mean that there is a clear cut-off, and distinction between fluent controls and fluent PDS subjects. If there is a gene (as seems to be the case for some people), then it's discontinuous again, you either have the stuttering gene or you dont. Compare this to a dysfunction of the basal ganglia, Per was talking about the relative number of dopamine receptors. This is continuous again, as the ratio can be changed slowly, and a higher ratio just means less/more activity and more dysfluency. But, there could also be a cut-off point, where you have a phase transition i.e. a small increase in dopamine receptors is the last drop that lets the barrel overflow?

I could go on... I think it is an interesting concept to ask new questions about (the findings on) PDS.

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