May 24 2011

Crossing the Midline

Have you ever played the game as a child that involves crossing your arms, clasping your hands, then pulling them in towards you and back up and around? The result is that your hands are clasped backwards to their normal position. Then someone else points at one of your fingers (without touching it) indicating which finger for you to move. It is common on your first attempt to move the corresponding finger on the opposite hand. You are then delightfully freaked out by how easy it is to trick your brain into getting its wires crossed.

This simple schoolyard demonstration reveals interesting neuroscience – a principle which has recently been exploited in a pilot study of pain perception. Researchers induced pain with a laser (so no visual cues) in one hand or another of 20 subjects. They found that if the subjects crossed their arms past the midline then their pain perception was decreased.

Both effects likely result from the organization of the brain. Our brain integrates the various sensory modalities with our map of the world, our map of ourselves, and feedback from our muscles. This information works together to create our internal sense of ourselves and the world. When the various sensory inputs match, then everything is hunky dory. When they do not match, our brains gets confused and generate anomalous manifestations.

We are used to seeing our right hand on the right side of the our body and the world, and the left on the left. We rarely, under normal circumstances, cross the midline with one hand. In fact, neurologists use this as a test of function. If a patient will cross the midline with one hand to reach for an object, that implies a dysfunction of the other hand (the one they should have used).

A slight decrease in pain perception is actually at the mild end of the spectrum of what happens when spatial information does not integrate properly – provoked by simply seeing your hand on the wrong side of the world.

There are more fascinating manifestations of this – phantom limbs and alien hands. We can trick the brain into thinking that a fake arm is part of its body. In experiments, subjects have had one arm placed beneath a table so that they could not see it, while a fake arm is placed so that it comes from their shoulder and is in view on top of the table. Then their real arm is touched (which they can feel but not see) while the fake arm is also touched in the same place (which they can see but not feel). The brain struggles to integrate these two sensory modalities. The result is that a part of the brain known as the ownership module concludes that the subject owns the arm they see being touched. In some cases the subjects are made to feel as if the fake arm is part of their body – that they own it.

The ownership module can be tricked (or malfunction) in other ways. If the sensory information from our real arm does not match the information from the motor part of the brain – that part that plans and executes movements – then we can be made to feel as if we do not own our arm. This can occur with strokes or other brain damage that disrupts these pathways. Moment to moment pathways in the brain compare our intention to move each part of our body in a certain way, and the signals that execute the movement with sensory feedback about where our limbs are in three dimensional space, as well as feedback from the muscle about their state of contracture. This creates the sensation, moment to moment, that we own and control each of our body parts – something we take for granted, until the process breaks down.

In cases where these pathways are disrupted the result can be alien hand syndrome – the sensation that we do not control a body part, that it is operating on its own as if controlled by an alien.

Alien hand syndrome is a more elaborate manifestation of the same principle as this simple pain study. The brain integrates multiple streams of information to create our sense of ourselves, what parts belong to us and the sense that we feel and control them.

It’s funny that this is something many of us learn as schoolchildren (or should learn) with the crossed-arm trick. Most, however, do not extrapolate from this to the underlying lesson – our reality is constructed by our brains in an imperfect process that can (and does) break down.

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