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.
13 Responses to “Crossing the Midline”
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thanks for explaining this — the topic has fascinated me from the perspective of atypical development. I don’t know if there has been a good study on the topic, but many with ASD have trouble crossing the midline (i.e., if you fix their trunk and one arm and require them to reach across midline with their opposite hand to acquire a desired object, then cannot figure it out). It is helpful to understand this as it relates to ‘typical’ sensory processing.
This reminds me of the other childhood trick where you cross your fingers and then stimulate them with something between the tips. The sensation is weird and I assume it is from a similar mapping problem.
What kind of dysfunction is being tested for when a patient reaches across their midline? I would estimate that 50% or more of the time when an object is on my left side I will still reach for it with my right hand. This may just be my epilepsy since most of my seizures are localized in my left hand, but what conditions normally cause this?
That reminds me of a little Feldenkrais trick where you reverse the way you fold your hands, ie. left index finger on top if you’re right-handed, or else the reverse. I’d quote my Feldenkrais book (Awareness Heals – awesome book) if I hadn’t left it with somebody the other day.
reminds me of this “super brain yoga”
http://www.youtube.com/watch?v=KSwhpF9iJSs&feature=player_embedded#!
great fun!
I always found it bizarre that your brain does not incorporate your conscious knowledge into its ownership model–the subjects in the experiment you described are at some level fully aware that one arm is underneath the table and that the arm to their right is, in fact, a fake arm. Yet for some reason this high-level knowledge does not get used by the part of the brain that is deciding whose arm it is.
The same goes for phantom limbs and the studies about how amputees with phantom limbs are able to un-clench their phantom fists using a mirror and the opposite hand. Obviously, part of the brain knows which hand is being signaled, yet somehow this information is not used when the phantom limb feels like it was unclenched. Furthermore, the amputee can look at his right arm and see a stump, yet once again this information is not used by the brain to cancel the feeling of a clenched phantom fist.
Just how independent are the parts of the brain that control all these things?
This article reminds me of another trick you can do with the crossed arms, clasped fingers etc. Once the person is in position you grab their hands and pull down inducing instant excruciating pain in your victim resulting in a chase around the playground.
Ah, childhood.
“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?”
I wish I could figure this out.
Is there a video?
fmts, I noticed that too, because it’s never occurred to me that reaching across the midline is anything other than normal. I routinely reach for objects on one side of me with the other hand. I do it equally often with both hands, and on occasion with both hands at once – for instance when sitting down at the table and finding the knife and fork are on the wrong sides of my plate when I’ll cross my hands in front of me to pick up each implement in the hand I wish to use. I haven’t any dysfunctions that I’m aware of – unless not knowing my left from my right is (a) a dysfunction or (b) the reason I do this. I’m also almost completely ambidextrous but have never thought of that as a dysfunction.
“I wish I could figure this out.
Is there a video?”
I tried to do a google search, but I got mostly yoga and religious prayer images. =(
I’ll try my best to describe.
1. With your arms both straight out in front of you, cross them so that one is on top of the other.
2.Now position your palms so that they are facing each other (arms still straight out) and
3. Clasp your hands so that your fingers cross in an alternating fashion (left thumb, right thumb, left pointer, right pointer, etc.)
4. With your hands clasped, pull your hands toward you (you’ll only be able to bend them in one direction due to your elbows) until they reach near your body then you will be able to bring them up towards the chin and in front of your face
From this position it may be difficult to determine which finger is from which hand
I think a great prank would be to use sensory stimulation to trick someone into believing a fake arm is sending them info….then once you had started the process you could whip out a huge knife and slam it down on the fake arm!
I am officially no longer the weirdest commenter here.
ccbowers,
Thanks for that.
Interestingly when I tried it out on my wife, she moved the wrong finger but my daughter moved the correct finger.