phantomlimbs

Lost limbs reveal the brain's ghostly wiring
04 May 96

                                AMPUTEES who are touched on their intact arms can feel the
                                sensation in their missing limbs as well. This unexpected result
                                suggests that the two hemispheres of our brains are linked by
                                previously unnoticed sensory pathways.

                                Vilayanur Ramachandran and Diane Rogers-Ramachandran of the
                                University of California, San Diego, studied patients who had lost an
                                arm but could still feel it as a "phantom" limb. They describe their
                                results in the latest issue of the Proceedings of the Royal Society B
                                (vol 263, p 377).

                                The researchers blindfolded one patient, known as JP, and touched
                                him with a cotton bud. When they touched specific points on his
                                intact hand, he reported feeling the sensation at exactly the same
                                place on the phantom hand. Other touch stimuli, such as gentle
                                vibrations, gave similar results.

                                While the sensation of touch was easily transferred, however, other
                                sensations were not. When JP's fingers were dipped into icy cold or
                                very hot water, he reported feeling the dipping sensation in both
                                hands-but was only able to sense the temperature of the water in his
                                real fingers.

                                Another patient, called RL, could only experience sensation in his
                                phantom limb if the researchers were more devious. They positioned a
                                mirror so that the reflection of RL's good arm appeared where his
                                missing arm would have been. When they then touched his good arm,
                                he could see and feel the phantom being touched as well. "The visual
                                information feeds back to the areas concerned with touch," says
                                Ramachandran.

                                The researchers also found that when patients duped by the mirror
                                trick were asked to move their good arms, they felt their phantom
                                arms moving as well. For some patients, the trick even worked if the
                                moving arm seen in the mirror belonged to someone else. But the
                                sensation of touch could not be transferred by visual stimuli alone:
                                when the reflected arm belonged to a research assistant, poking the
                                assistant's hand did not produce a phantom sensation.

                                Neuroscience textbooks say that each half of the brain receives
                                sensory input from just one side of the body-the left hemisphere
                                handling information from the right half of the body, and vice versa.
                                Ramachandran, however, argues that the new results show that there
                                are connections between these sensory pathways that normally lie
                                dormant.

                                So why do these pathways exist? One possibility is that they are
                                among the many redundant neural connections which allow sensory
                                information to be rerouted if the normal pathways are disrupted.
                                "Maybe the brain has lots and lots of reserve troops," says
                                Ramachandran. This theory has also been used to explain the
                                phenomenon of "blindsight", the ability of some blind people to
                                subconsciously process a limited amount of visual information (see p
                                20, this issue).

                                But the pathways could also be a nonfunctional consequence of brain
                                geography. About a third of the amputees Ramachandran has studied
                                also seem to have connections between points on their faces and
                                points on their phantom hands. The brain regions that receive sensory
                                input from the hand are close to regions that receive information from
                                the face, he notes.

                                Alison Motluk
                                From New Scientist magazine, vol 150 issue 2028, 04/05/1996, page
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