The puzzle was why.

In the latest study, the new advances in diffusion imaging allowed them to see something for the first time. They saw that there were two places along the motor pathway that seemed to stop the flow of neural signals from one part of the circuit to the other. Those with only one lesion in the circuit developed the debilitating movements and those with two lesions did not. "There is something about this second lesion that is protective," the authors concluded. "We found a consistent cerebellar pathway problem in all DYT1 carriers. When we went back and looked at those without symptoms, we saw that they had an additional lesion downstream in the portion of the pathway connecting directly to the motor cortex." This second area of pathway disruption abrogated the effects of the first lesion.

Normally, the cerebellum (a region that controls movement) puts the breaks on the motor cortex by potentiating inhibition at the cortical level. It is likely that mutation carriers have a developmental problem in the flow of neural signals along this circuit such that the brain can't inhibit an unwanted movement. With the second pathway lesion, Dr. Eidelberg explained, "the flow is shut off and the abnormal activity stops."

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