Mental Rotation and Focal Dystonia, or why Scott Adams doesn't draw Dilbert upside-down

There was a fascinating article in the Washington Post last May about Dilbert creator Scott Adams' battle with focal dystonia. Though the symptoms of this disorder are involuntary muscle contractions (in Adams' case, his right pinky finger), the root of the problem is in the brain. For Adams, it has meant suspending his cartooning career more than once. The first time, he taught himself to draw with his left hand, only to see the symptoms reappear there. He's also tried grueling physical therapy regimens. His most recent effort to battle dystonia has been drawing his cartoons using a computer graphics tablet.

So what's different about brains with focal dystonia? Neuroimaging has found that the basal ganglia is the affected area of the brain -- not surprisingly, this is the area responsible for motor control. People suffering from focal dystonia are also limited in their ability to plan and execute muscle movements. So though the most common form of focal hand dystonia is simply called "writer's cramp," clearly there's much more going on here.

If focal dystonia affects not only our ability to control muscles, but also planning muscle movement, maybe it affects how we think about movement as well. Remember, we've discussed how memory for visual images appears to make use of the visual system. Perhaps thinking about moving body parts also relies on the basal ganglia. In a pioneering experiment in 1971, Roger Shepard and Jacqueline Metzler found that when people try to determine if two objects are the same, the time the decision takes corresponds to how far the objects are rotated. If one object is rotated, say 90 degrees compared to the other, then the comparison takes less time than if the object is rotated by 180 degrees. They concluded that people must mentally rotate the objects in order to make the comparison.

Mirta Fiorio, Michele Tinazzi, and Salvatore Aglioti realized that they could apply Shepard and Metzler's method to people with focal dystonia. Perhaps focal dystonia also affects the rate of mental rotation. But Fiorio's team took this analysis one step further: is it possible, since focal dystonia only affects certain regions of the body, that mental rotation is only impaired for those body parts? They worked with 15 people who suffered from writer's cramp in their right hands, and 15 normal individuals.

Participants sat at a computer screen with their own hands hidden from view. A photo of a hand or a foot appeared on the screen, and viewers responded "right" or "left" by speaking into a microphone. The body parts were displayed in standard (0-degree) orientation, and also rotated in 60-degree increments. Here are the results:

Participants with hand cramps mentally rotated photos of hands more slowly than normal participants. It appears that focal dystonia has an impact not only on muscle movement, but also thinking about moving body parts. But now look at the data for rotating feet:

Though there is a trend towards rotating feet more slowly as well, this result failed to achieve statistical significance. So focal dystonia really appears to affect thinking only on the specific muscle groups affected by the condition. Interestingly, however, even though these patients only showed symptoms of focal dystonia on their right hands, they were slower at mental rotations for both hands. Fiorio et al. argue that this may suggest that focal dystonia actually affects the mental representation of hands in general, not the particular afflicted hand.

Many focal dystonia patients -- including Scott Adams -- train themselves to use their opposite hand, only to find the symptoms recurring in the new hand a few years later. Adams' current practice of using a large digital tablet may be less vulnerable to this problem, since he is using the larger muscle groups of his arm to draw with, instead of his hand. Dilbert fans the world over certainly must hope this is the case!

Fiorio, M., Tinazzi, M., & Aglioti, S.M. (2006). Selective impairment of hand mental rotation in patients with focal hand dystonia. Brain, 129, 47-54.