The Curious Case of the Phantom Limb

After the unfortunate event of having a limb amputated two thirds of patients continue to feel the vivid sensation of a limb that is no longer there. 50-80% of patients experience incredibly painful burning, crushing or cramping in their phantom limb, leading many to depression and suicide. The phenomenon of a phantom limb is one that usually diminishes in time; but for some these phantom limbs stay present for years. Now a neuroscientist from San Diego has come up with a novel way of relieving people of their phantom.

V. S. Ramachandran’s first patient had his phantom arm for 11 years, before which his arm was in a paralysed and sling after one of the nerves was severed. After the amputation he had a phantom arm that was still chronically painful and he believed to be paralysed. Ramachandran placed him in front of a upright, side on mirror, and told him to place his left phantom arm behind the mirror and his right normal hand in front of the mirror. He wriggled his fingers on both hands and watched the reflection in the mirror. To his surprise he could see, and for the first time in 11 years, feel his phantom arm moving, and the pain was instantly reduced. After three weeks of daily practicing with this mirror box, his phantom arm disappeared and there was a remarkable reduction of pain.

The fact that sensations of paralysis and pain were perceived as being in the arm before and after the amputation is key to understanding what is going on here. Prior to the amputation the brain sent signals to move the arm, but it received back sensory input that the arm would not move. From this feedback the brain learnt the paralysis. So when the arm was amputated and the brain was not receiving any input from it, the brain assumed it was still paralysed.

However when in front of the mirror, the brain was receiving a flurry of new mixed messages about the arm: the muscle cells in his stump was telling the brain there was no arm, and in front of the mirror box his motor command and vision told him there suddenly was a fully functioning arm. Eventually the brain gave up on the arm, and learnt that it was just a phantom.

It all comes down to a part of the brain called the somatosensory cortex which processes all tactile sensations received from the skin. Sensory nerves from all of the skin have been mapped onto the somatosensory cortex, and this enabled us to create a fetching map of sensory input called Penfield Homunculi. Absence of sensory input from an arm for example does not mean that part of the somatosensory cortex is necessarily idle but continuously active, which is where the bizarre sensation of a phantom limb comes from.

The somatosensory complex tends to start reorganising itself directly after amputation and for several years after, to try and compensate for the loss of sensory input. Another patient of Ramachandran’s had a painful left phantom arm, and he discovered that when touched on the right side of his face he felt the sensation in his arm. They were able to map sensations for each of his fingers onto his face. The Penfield Homunculi shows the face representation is next to the hand representation, so it was concluded likely that when the cortex received no sensory input from the arm, the area of the cortex representing the face began to invade the hand region; creating the misplaced sensations in the phantom limb.

Considering the plastic nature of the brain as demonstrated here, it would be reasonable to say that in those who had a phantom limb which disappeared in time, their brain accepts the visual information of the limb not being there, and somatosensory complex reorganises itself to not have an area for the limb which is no longer there.

An article I wrote for the second issue of QM Sci.

Stuff of Christmas Cards

Low Tide

Found these photos from Summer '09.

Love at First.. Sniff?

The smelling stimuli we pick up from each other plays a bigger role than we think. By attracting us to our most genetically different and  most compatible partners; it just proves there is a lot to be said for relationship “chemistry”.

We have all heard how pheromones secreted in our sweat can supposedly attract the opposite sex, and certain pheromones have been proven to alter levels of hormones in the recipient smeller. But studies have shown that we produce a different molecule, which rather than causing a hormonal change, plays a significant role in deciding how attractive and compatible you find a person.

The “chemistry” that attracts a lover, friend, or foe is something distinct to each individual and can be traced right down to your genes. It reaches beyond physical traits and personality, to subconsciously influence behaviour. These chemical signals that are in your sweat, tears and saliva are directly influenced by your immune system.

MHC molecules (major histocompatabiliy complexes) are important in identifying invaders in our bodies, such as viruses and bacteria. The range of MHC molecules varies from person to person and the larger the diversity of MHC genes, the better adapted the individual is to fighting off disease. These molecules have been shown to influence a person’s natural odour.

A study into mating preferences in humans by the biologist Claus Wedekind, had six males wear T-shirts for two consecutive nights, the odours of which were then rated by females in degrees of pleasantness. Men with MHC genes that differed most to the females scored much more attractive than those with similar gene ranges. In fact, scents from MHC-dissimilar men actually reminded the women of their current or past mates more than the MHC-similar men, suggesting the subconscious analysis of MHC genes in humans plays a large part in our partner choices. Studies have suggested men are less sensitive to smelling stimuli than women.

The way we respond to chemical signals differs depending on one’s sexual orientation; heterosexual and homosexual women respond differently to male sweat. Smell stimuli can also be a turn off; the scent of female tears has been proven to lower sexual arousal in men. 

 These olfactory signals that encourage mating between those with dissimilar genes is biology’s way of trying to mix up the human gene pool and produce more virile offspring. Parents with more diversity in their MHC genes would have offspring with stronger immune systems, who will therefore be better adapted to fight the pathogens the world throws at them. It also deters inbreeding and close family relations. Evolution has even taken it a step further, to the point that parents with MHC similar genes are more likely to lose a child in the early stages of pregnancy than another couple with MHC similar genes.

An article I wrote last year for QMessenger online.

QM Sci: January

The first science magazine at QMUL, created by students (one of which was me!).