快猫短视频

A WOMAN who 鈥渟enses鈥� limbs she never had is forcing a rethink of theories
about how the nervous system develops. Tests by a Swiss team show that her brain
has a sensory map of the missing limbs.

鈥淢ost people think sensory representations of ourselves do not develop in the
brain unless there is sensory input,鈥� says Peter Brugger of University of Zurich
Hospital. If we鈥檝e never had an arm, goes the theory, we never develop the
neurons that 鈥渇eel鈥� an arm.

Nearly all amputees have 鈥減hantom limbs鈥�: they experience detailed sensations
that appear to come from the limb they have lost. Neuroscientists believe this
is because neurons in their brain that used to receive sensation from the limbs
are still firing.

Some people born without limbs also say they feel phantoms, but until now
these claims have been dismissed as fantasy. But a 44-year-old woman born with
no legs or forearms has disproved this.

The woman experiences detailed phantom arms and legs, reports Brugger. When
he and his colleagues magnetically stimulated parts of the brain that normally
sense limb movement, she felt movement in the corresponding places in her
phantom limbs. When she said she was 鈥渕oving鈥� her phantom limbs, magnetic
resonance imaging detected neuronal activity in parts of the brain, the premotor
and parietal cortex, where we prepare or visualise movement鈥攂ut not in the
motor cortex where we execute it.

鈥淲hat really shows that our subject鈥檚 phantom hands and feet are not mere
products of wishful thinking,鈥� says Brugger, 鈥渋s a cognitive test.鈥� If you are
shown a picture of a hand or foot and asked to say if it is left or right, you
do it by mentally superimposing your mental image of your right or left hand or
foot. If the picture is upside down compared to the way you usually see your
hand or foot, this takes longer. It took Brugger鈥檚 subject longer, too.

He suggests that despite her missing limbs, the woman鈥檚 cortex did receive
some sensory input to shape her phantom sensations, from neurons in her premotor
cortex that fire as though they are copying movement seen in other people. This
contradicts current dogma, which insists that she should have had no innate
sensory image of her missing limbs to shape.

鈥淭his is an intriguing observation,鈥� says Vilayanur Ramachandran of the
University of California, San Diego, a leading researcher on phantom limbs. It
shows that the brain鈥檚 map is genetically specified, he says. So even though the
鈥渉and鈥� area gets input from an arm stump, when it fires, you feel a
hand鈥攅ven if you never had one.