Health Talk: Phantom Limbs
Amputees often report that they can still sense and feel pain in their lost appendages. These symptoms can continue for months or even years after surgery and their onset is unpredictable.
A May 2006 article in The Clinical Journal of Pain explored the incidence of phantom limb phenomena in patients six months post-amputation. The authors reported that nearly 85 of the patients complained of phantom limb pain.
Silas Weir Mitchell was the first to use the term “phantom limb” in 1871. Scientists often regard Weir Mitchell as the first physician to treat the disorder as a real condition rather than simply hallucinations.
For years after his publication, many scientists offered theories as to why phantom limb sensations and pain develop.
It was only in the late 1990s that new imaging techniques enabled neuroscientists to confirm the cause of phantom limb phenomena. To understand the cause is to explore the basis of human sensory perception.
All sensory and motor signals from appendages pass through a network of specialized tissue known as the nervous system. The nervous system can be generalized into two components: the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which can be generalized as the nerves connected to the rest of the body.
When a finger is burned, for example, the message is picked up by receptors on the skin and travels up a nerve pathway to the brain. Once in the brain, this signal is brought to the somatosensory area in the cerebral cortex, which has cells responsible for processing information including temperature, position, pain, and pressure.
In the 1950s, Canadian neurosurgeon Wilder Penfield compiled a physical representation of the cells on the primary somatosensory cortex in humans.
He gathered his data by triggering small groups of cells with electrodes on the brains of epileptic patients. His experiments told him how the cells of the somatosensory cortex were connected to the various parts of the body. His data is now drawn as a Penfield map.
The map shows the relative percentage and placement of cells that process the sensations of extremities.
The skewed representation of the human figure along the cortex is referred to as the sensory homunculus, affectionately depicted as a small man with large hands, lips, and ears. After an amputation, inflamed scar tissue and severed nerve endings, from the area of the amputation, form a bulb known as a “traumatic neuroma.” In the past, scientists observed that these unfinished nerve pathways caused random signals that created sensations of pain at the surgical site or stump.
In his book, Phantoms in the Brain, neurologist V.S. Ramachandran couples the neuroma’s random signal generation with his understanding of the primary somatosensory cortex.
He argues that activation of nerve endings in a neuroma travel up to the somatosensory area, where they are interpreted as originating from the missing limb.
Additionally, Ramachandran worked further with other neuroscientists to show that the brain can reallocate cells in the somatosensory cortex after injury.
The hand and face are neighbors on the Penfield map and provide a clear example of this neural plasticity. After a patient’s hand is amputated, the “hand” cells on his or her somatosensory cortex may begin processing signals from the cheek instead. That is, if the cheek is stimulated, the brain will sometimes perceive the signal as originating from the missing hand.
Ramachandran worked extensively with magnetoencephalography (MEG), a brain activity imaging technique, to verify the reorganization of cells in the cortex.
Ramachandran’s two theories, of neuromas sending aberrant signals to the cortex and of cortex remapping, have led to widespread acceptance of phantom limb phenomena.
He has even gone so far as to tie his research to the underlying cause of foot fetishes. He mentions foot fetishes in his book, saying, “I suggest that the reason is quite simply that in the brain the foot lies right next to the genitalia. Maybe even many of us so-called normal people have a bit of cross-wiring, which would explain why we like to have our toes sucked.”