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Figure 6-1. Tom tries out the crossed hands illusion

 

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.

The parietal cortex [Hack #8] contains areas that are concerned with visual and spatial representation. The premotor cortex is involved in representing and selecting movements.

 

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?

End Notes

1. Maravita, A., Spence, C., & Driver, J. (2003). Multisensory integration and the body schema: Close to hand and within reach. Current Biology, 13, R531-R539.

2. Yamamoto, S., & Kitazawa, S. (2001). Reversal of subjective temporal order due to arm crossing. Nature Neuroscience 4, 759-765.

3. Shore, D. I., Spry, E., & Spence, C. (2002). Confusing the mind by crossing the hands. Cognitive Brain Research, 14, 153-163.

4. Kitazawa, S. (2002). Where conscious sensation takes place. Consciousness and Cognition, 11, 475-477.

5. Spence, C. J., Kingstone, A., Shore, D. I., & Gazzaniga, M. S. (2001). Representation of visuotactile space in the split brain. Psychological Science, 12, 90-93.

Ellen Poliakoff

 

 


 

 


Date: 2015-12-11; view: 565


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