Second, you may see the hand as not belonging to you. Your brain is then ignoring its proprioceptive (muscle and joint feedback) congruence with the visual image of your hand. It is as if the brain is concluding that “because I see the touch but don’t feel it, that hand must be someone else’s.” Sometimes you will “see” the hand as a cadaver’s hand or a realistic plastic dummy. Interestingly, the brain does not settle on “halfway” ambiguities—at any given time you clearly experience one of the percepts.
Last, if you are lucky, you will actually feel some tingling touch sensations in the left hand—even though nothing is being done to it. This effect is a striking example of the brain “filling in” the missing information. Two sources of information (proprioception and vision) are internally consistent in telling you that it is your hand. But the third piece of information—that the hand looks like it is being stroked—is inconsistent with lack of touch sensations. So the brain “flags” the discrepancy as tingling—as if to say, “I’m feeling something odd.” Very infrequently, you may actually feel the touch—as though the brain fills in the blanks to create an internally consistent package to higher centers. We call this phenomenon intermanual touch referral.
Clues to Managing Pain
Try the following experiments. Before the stroking begins, look into the mirror and wiggle the fingers of your two hands in perfect synchrony. Nothing odd so far. Now have a friend deliver strokes, taps or pinches as before, but this time to the visible hand only. All of a sudden you start feeling intermanual referral (that is, feeling the actual touch in the hidden hand) much more vividly and less fleetingly than when your hands were stationary. Why?
In constructing a picture of the world, the brain assigns various weights to different sensory inputs based on a lifetime’s experience of their statistical reliability, as well as ongoing patterns of activation. In short, the brain does not average the signals—it looks for improbable internal consistencies.
When you start wiggling the fingers synchronously, the brain suddenly gets extra information that the hand is really yours. These data force your brain to accept the hand as your own, so you lean toward experiencing intermanual referral with or without tingling. The flood of proprioceptive signals coming in from the hidden hand vetoes any attempt by your brain to engage in disownership. So your brain adopts the next available strategy: accept the hand and feel intermanual referral.
The same effect occurs if you wiggle right- and left-hand fingers nonsynchronously. This time the tendency to think of the reflection as your own left hand is slightly mitigated by the incongruity between vision and proprioception. (The sight of wriggling is somewhat desynchronized from the felt position of the fingers.) Consequently, the intermanual referral is halfway between our previous two experiments.
One last experiment you—the reader—can do. Drop some itching powder on the (hidden) left hand so that it begins to itch. Next have the right hand vigorously stroked and scratched while wiggling both hands synchronously (that is, generate intermanual referral). Question: Does the illusory stroking and scratching felt in the left hand relieve the real itch? It worked better on one of us (Ramachandran) than the other (Rogers-Ramachandran), but you should try it on yourself. If it can be replicated on a large number of subjects, it would be the first example of a purely visual input (which creates an illusory touch) relieving a real itch in a normal hand. Write to us (firstname.lastname@example.org or email@example.com).
These effects are more than amusing curiosities; they may be clinically useful for treating pain and paralysis in existing limbs as well as phantom ones, as we discovered in the early 1990s.