Tuesday, May 20, 2008

Ludo Max's talk in Paris

I am going to write short summaries of the conference talks so everyone gets an idea what was talked about. It would be shame if the non-attendants missed out on a good conference. Of course, I have to simplify and my notes are definitely not perfect. And I did not understand each statement, question or answer! But I am sure that this applies to all attendants! I will give my opinion at the end.

Ludo Max gave the first talk. He is a professor at the University of Connecticut, south of Boston. I visited him in October of last year. He is originally from Belgium. Check out his webpage with all relevant references to his work: here.

He didn't really say anything revolutionary new. His approach to stuttering is still the following:

1) Use the most plausible flow diagram of the processes that generate (speech) motor control.
2) Test the various components for people who stutter.

He argues that this is what real science should be about: namely testing theories. A key component of the model is the efference copy concept. In order to correct for imprecise muscle movement, the brain needs sensory feedback. For example, if I reach for the glass and I miss it by 1 cm, my finger and my visual cortex generate a "not touching by a bit" information. Then the brain can correct for this. However, this is quite slow as the brain needs to wait for the sensory feedback from my finger and my eyes, then combine and process the information, and then send the "not touching by a bit" information to my motor regions. This mechanism will fail if the brain has to do too many movements in a short period of time like in speech, and therefore it has invented the efference copy, which is effectively parallel processing. So once the motor command is sent to the muscles, a copy of the comman is sent to a region that predicts what the sensory feedback should be at the same time as the muscles are executing the command. So the expected sensory feedback is ready and waiting for the actual sensory feedback. Time is saved, because the actual feedback does not need to be analysed any more but only compared to the predicited sensory feedback.

Ludo showed results from his experiments. Actually, I participated in one of them. It is a nonspeech experiment that involved making reaching movements in the presence of a visuomotor rotation. You have to move your finger towards a target, which is easy enough. However after a while they manipulated the visual field by a rotation and you are actually missing the target by a bit. After a few minutes of irritation your motor control system gets used to this shift and automatically corrects for the rotation. The experiment tests how fast you can adapt, which depends on your inverse internal model (I think) which is part of the motor control framework. Ludo found a noticeable but not dramatic group difference between controls and people who stutter. He also talked about an experiment involving a formant shift, but I do not have anything written down on it. He also said that the visuomotor rotation experiment is currently being repeated with children, but the boring targets being replaced by basketball baskets. Ludo also talked about a new set of experiments he is conducting to find out whether differences in the control of orofacial movements lie in the sensory and/or the motor domain. The experimental setup involves being stuck into an apparatus that monitors your orofacial movements, which looks a bit like a chastity belts for your mouth!

Christy Ludlow asked a methological question. I did not really follow on what, but the gist of it is that the interpretation of the experiment is not completely clear-cut and other effects could distort results allowing for alternative interpretation. I asked Ludo whether he had tried to correlate the group difference in the visuomotor experiment with stuttering severity. I suggested he could divide the sample into three: controls, mild stutterers, and severe stutterers, and see whether the controls-to-mild difference was in between the controls-to-severe group. He replied that he hadn't done this but might do.

In my opinion, Ludo is a good scientist and in principle doing the absolutely scientifically correct thing, namely to start out with an established or candidate model and then test the different components. However, in the given circumstances his approach is probably not leading to significant results in the near term, because the setup, the result and the interpretation of the experiments is very sensitive to all kinds of experimental and theoretical issues. But he can prove me wrong! First, experiments like the visuo-rotation experiment seem to get only group difference results, i.e an average difference, because people who stutter do not have failures but only defficiencies. There is a high overlap between control's and pws's performance: some stutterers have in fact better skill than many fluent controls! Only their group average is lower. So deficiency is not a sufficient condition for stuttering. Second, there might well be sub-types distorting results. Third, the model is a functional model but processes of speech motor control might well be implemented differently in different people. So they might have the same functional ability but the brain actually uses different pathways. Fourth, in my opinion, most of the differences we see in stutterers across several areas are simply consequences or a side-effect in a stuttering brain, and not directly involved in causing stuttering. Fifth and probably most importantly, the experiments are quite hard work. You need to recruit the subjects, setup the experiments, be scared of all kinds of experimental artefacts. To me, his approach makes more sense from the perspective of a researcher interested in speech motor control with a focus on stuttering as opposed to a researcher only interested into stuttering.


Anonymous said...

Tom, the one thing that I have to disagree with (simply because the data show otherwise) is your argument that experiments like the visuomotor rotation one can show only group average differences. In a poster (Progress in Motor Control in Brazil, August 2007) and talk
(Society for Neuroscience, San Diego, November 2007) that we have already presented and the manuscript that is in preparation (to be submitted this Summer) we did/do include individual subject data which demonstrate that this is not just a difference that is only noticeable
in the group averages. Of course, there are many variables that can be measured as the dependent variables of interest and the actual measurements that we derive are not actually measures of "skill." Rather we measure things such as the abruptness of the subjects'
initial adaptation (how quickly and how much do they adapt initially when the sensorimotor environment is first changed) and how slow or how fast learning still continues after that. If we look at the measurements that reflect the initial abruptness of the subjects's
adaptation, then your argument that some of the stuttering subjects do better than the nonstuttering subjects does not hold. In this particular experiment (reaching movements with a visuomotor rotation), 7 of the 10 nonstuttering subjects show a larger/faster initial adaptation than do
_ALL 10_ the stuttering subjects. In other words, all 10 stuttering subjects have low values (for that particular measure) that are in the same range as those of the three lowest nonstuttering subjects. Thus, even in behavioral experiments, the amount of overlap in the
individual subject data can be extremely small. In fact, it is a great advantage of behavioral experiments that it is possible to examine individual subject data.

Tom Weidig said...

Thx Ludo. I wasn't aware that the overlap was quite small. Glad to have clarified this issue! I guess a bigger sample would give us more confidence on the real overlap.

Best wishes.