Hack 63. Keep Hold of Yourself
How do we keep the sensations on our skin up to date as we move our bodies around in space?
When an insect lands on your skin, receptors in that area of skin fire and a signal travels up to your brain. The identity of the receptor indicates which part of your skin has been touched. But how do you know exactly where that bit of your body is so you can swat the fly? As we move our bodies around in space we have to remap and take account of our changes in posture to understand the sensations arriving at our skin; very different movements are required to scratch your knee depending on whether you're sitting down or standing up. This might seem like a trivial problem, but it is more complex than it seems at first. We have to integrate information from our joints and muscles about the current position of our bodyproprioceptive informationas well as touch and vision, for example, to gauge that the sight of a fly landing and the sensation of it contacting your finger are coming from the same place.
6.3.1. In Action
Try closing your eyes and feeling an object on a table in front of you with the fingers of both hands. Now, cross your hands and return your fingers to the object. Despite swapping the point of contact between your two hands, you do not feel that the object has flipped around. The next two illusions attempt to make this remapping fail.
First, try crossing your index finger and middle finger and run the gap between them along the ridge and around the tip of your nose (make sure you do this quite slowly). You will probably feel as if you have two noses. This is because your brain has failed to take account of the fact that you have crossed your fingers. Notice that you are unable to overcome this illusion even if you consciously try to do so. This is sometimes called Aristotle's Illusion, as he was apparently the first person to record it.
Now, try out the crossed hands illusion. You'll need a friend to help. Cross your hands over in front of your chest, at arm's length. Then turn your palms inward, so your thumbs point downward and clasp your hands together, so your fingers are interleaved. Next, rotate your hands up toward your chest, until your thumbs are pointing away from you, as shown in Figure 6-1. Now, if a friend points to one of your fingers and asks you to move it, you will probably fail to move the correct finger and instead move the same finger but on the opposite hand. Again, you have failed to take account of your unusual posture; you assume that the finger you see corresponds to the finger that would be in that position if you had simply clasped your hands, without crossing them over. You may find that you are able to overcome the illusion if your friend indicates which finger he wants you to move by touching it. This can help you to remap and take your posture into account.
Figure 6-1. Tom tries out the crossed hands illusion
6.3.2. How It Works
Charles Spence and colleagues1 have shown that we can update how we bind together vision and touch when we cross our hands over. They asked people to attend to and make judgments about vibrations that they felt on their hands, while ignoring lights presented at the same time. When feeling a vibration on their right hand, the lights on the right sideclosest to their right handinterfered much more (made people slower to carry out the task), than lights on their left side. That is, we tend to bind together vision and touch when they come from the same part of the outside world. So what happened when they crossed their hands over? The interaction between vision and touch changed over: lights over on the left side of their body were now closest to their right hand and interfered more with the right hand than the lights over on the right side. So, when we change where our hands are in space, we integrate different sets of visual and tactile signals.
But remapping can sometimes fail, even without intertwining our fingers. Two recent experiments2,3 have shown that we are particularly bad at dealing with information in quick succession. If your hands are in their usual uncrossed position and you are asked to judge which hand is touched first, it is relatively easy. On the other hand, if your hands are crossed, the same task becomes much more difficult. This difficulty in coping with stimuli presented in quick succession, suggests that remapping can be a time-consuming process. Shigeru Kitazawa4 has suggested we do not become conscious of a sensation on a particular part of our skin and then attribute it to a particular location in space. Rather, our conscious sensation of touch seems to be delayed until we can identify where it's coming from.
So where in the brain do we remap and update our connections? Some clues have come from investigating the monkey brain. Cells that respond to both vision and touch have been found in the parietal and premotor cortexhigher areas, upstream of the somatosensory [Hack #12] and visual areas, which deal mainly with touch and vision alone.
These cells usually respond to stimuli coming from the same region of space: a cell might respond to a finger being touched and to a light close to that finger. The most fascinating thing about some of these cells is that when the monkey moves its arm around, the region of visual space to which the cell responds also moves. Such cells are thought to represent the space that is close to our bodies. It is particularly important for us to merge together information from our different senses about this, our peripersonal space, which is within our immediate reach.
Spence and colleagues5 gave a patient with a split brain (whose left and right hemispheres were disconnected [Hack #69] ) the same touch and vision distraction task as described earlier. The patient behaved as normal with his right hand in the right side of space. That is, the lights on the right side produced the greatest interference. In this case, both touch and vision arrived first at the left hemisphere of his brain. When he moved his right hand over to the left side of space, we would now expect his right hand to be disrupted most by the nearby lights on the left side. However, the lights on the right side still interfered most with touches to the right hand (despite being on the opposite side of space to his hand). In this case, the lights on the left arrived first at the right hemisphere and touches to the right hand at the left hemisphere, and without connections between the two halves of his brain, he was unable to update. This shows how important the long-range connections between distant cortical areas of the brain are for remapping.
The fact that the updating of our posture and remapping of our visual-tactile links appears to occur before conscious awareness could explain why we take them for granted in our everyday lives. Some people seem to find such processing easier than others. Could experience affect these abilities? Might drummers who spend many hours playing with their arms crossed find remapping easier?
6.3.3. End Notes