Saturday, May 07, 2005

How tickling yourself helps to understand PDS

In the last part of their paper, Brown, Ingham, Ingham, Laird and Fox give a possible explanation of their meta-analysis' findings ( i.e motor overactivity and atypical right-hemispheric activation, auditory underactivity, and cerebellar overactivity) using a theory at least partially based on the concept of efference copy. Here is how I understand it. I hope I dont mess it up!!

They suggest that the "abnormality is... not likely a defect of motor programming per...nor an incorrect mental model of the desired movement... nor an abnormality of the motor execution system...The problem is limited to successful initiation of the motor program."

Then they go about explaining the differences in activation. To this aim, they introduce the concept of efference copy. Roughly, the theory goes that every motor plan (ie. the plan on how to move the muscles) is copied and sent to a predictor system that predicts the expected sensory feedback from the motor action. This copy is called the efference copy, and allows the brain to do self-monitoring by comparing the expected and the actual sensory feedback.

One application of efference copy concept is in the context of Why-Cant-You-Tickle-Yourself?? !!! :-) Answer: Because the efference copy is used to generate an expected sensory feedback that attenuates the tickling sensation.

Concerning lack of auditory activation, they speculate that every unsuccessful initiation of the motor plan sends an efference copy to auditory regions which attentuate their activity due to the expected sensory feedback. As there are many initiations, many efference copies are sent and the regions are constantly attentuated.

Concerning the cerebellar overactivation, the cerebellum is expected to be involved in the comparison of the expected and the actual sensory feedback. So each time an efference copy is created the cerebellum might prepare or perform this comparison task.

Concerning the overactivation in motor region and atypical hemispheric activation, they propose two effects. First, unsucessful initiation of a motor plan likely results in overactivation in motor regions that prepare or perform necessary tasks. Second, the lower skilled the brain is on a specific type of motor plan, the more active the regions involved are. This comes in handy, as they refer to Sommer et al. and Foundas et al. who found left-hemispheric structural defficiences, which suggest lower skills/ability of this region. (Actually there is now a THIRD article out by Jancke et al. ) This would also explain why there is some atypical right-hemispheric activation. The right brain might help out but is lowly skilled in the tasks.

That's roughly their theory, as I understand it. In my next post, I will comment on it...


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