A phantom limb is the persistent sensation of an appendage, after it has been removed by amputation or lost in an accident. The onset of these sensations is fast, within weeks or even days and symptoms usually persist for years without diminishing (Shreeve 1993). A further disturbing aspect is that in many cases, not only is the limb missing, but it's also in constant pain, ranging from, 'burning' to, 'shooting' sensations. It is with this in mind, along with other sensations, that has lead researchers to develop explanations of the existence and pain of phantom limbs.For this essay, I will attempt to highlight the explanations given for this phenomenon and demonstrate the true nature of phantom limbs.
This will allow me to ask the deeper question of how the brain observes the world and the self that resides within it.The 'Neuromatrix'Ronald Melzack (1993) proposed that within the brain, lies a, 'neuromatrix'. This is a network of neurons that respond to sensory input and constantly produce patterns of impulses called a, 'neurosignature' which register that the body is as it should be - intact. The neuromatrix continues to operate even though there is no input from the amputated limb. This supposedly,'creates the impression of having a limb, even when that limb has been removed'(Melzack, 1992 pg. 120-126).
Phantom limb pain can be explained as follows: If the brain expects there to be a limb, it sends an output signal to it via the neural pathways in the neuromatrix. Because there isn't a limb there, the brain doesn't receive any sensory feedback. The brain thus increases the intensity of its signals to compensate, which causes the phantom pain. Such observations lead Melzack to believe that,'The body we perceive is in large part built in to our brain - it's not entirely learned. In fact, you do not need the body to feel the body'(Discover phantom limbs, brief article 1998)The neuromatrix is said to contain three brain circuits: the classic sensory pathway, the limbic system pathway (responsible for emotion and motivation) and the cortical pathway (essential for the recognition of self). These three components are said to account for the description of phantom limbs as being, 'exhausting', the pain sensations and the feeling that the limb pain is actually part of the patient.
Melzack deduces that this neuromatrix must be determined genetically, with, some sort of neurosignature assuming the human body is intact - whether or not a person does have all limbs in their body. The fact that people who are born without certain limbs, can still experience phantom limb pain seems to support this. This indicates the ever presence of the neurosignature, with the patterns highlighting the, 'normal' existence of the limbs.Melzack also proposes that this neuromatrix is modifiable with the observation that patients showing pain in the limb before deafferentation tend to show phantom limb pain, but those that have no pain previously, rarely show it.However, very little research has been carried out to test Melzack's ideas.
Many of the studies are methodologically flawed due to the samples used. They're usually very small, heterogeneous, are non-randomised and lack the appropriate control groups. They also have extremely short patient follow-up periods.Cortical remapping theoryInspired by the experiments carried out by Michael Merzenich in relation to the homunculus, V.S. Ramachandran (1993) formulated the cortical remapping theory.
Firstly, the homunculus is a blueprint representation of the whole surface of the body that identifies the locations of sensations felt on the skin. Features such as the lips and tongue, are exaggerated as attempts to indicate the overrepresentation of these parts. Merzenich's found that in amputated monkeys, despite the lack of feedback from the arm, the map of the arm region in the cortex still existed. Instead, signals from the face (which lie next to the arm on the map) had taken over for the phantom arm. The conclusion was that axon branches become unmasked when normal input disappears.Ramachandran wondered whether phantom pain in humans could be due to this rearranged body map and hence formulated his theory.
He examined patients who had limbs removed, paying close attention to the reorganised homunculus. Using q-tips to brush the faces of patients, he produced sensations in their phantom limb. When brushing the amputees chin with a q-tip, the subject would feel sensations in a specific area of the phantom limb. Ramachandran found a systematic on-to-one mapping between specific regions of the face and regions of the missing limb (i.e.
cheek to thumb, upper lip to index finger etc). The basis behind this was as follows: the map of the hand is right next to the face on the sensory homunculus in the cortex. The sensory input from the face may have found its way into the area of the homunculus that is normally for the hand. Therefore, the hand cortical area was taken over by the face cortical area because the hand wasn't receiving any sensory input due to the amputation.As mentioned, support for this theory comes from the many experiments done using a q-tip. These showed that upon touching specific regions in the face of a patient with an amputated arm, the patient felt tingling sensations in the fingers of the missing limb.
For example,Tom - "you are touching my cheek"V.S. - "anything else?"Tom - "hey, you know it's funny..
.your touching my missing thumb, my phantom thumb"(Phantoms in the brain, 1998, pg. 29)To prove that changes in the map are taking place, using an MEG, Ramachandran showed that the hand area in the right hemisphere was missing and was invaded by sensory input from the face and upper arm. This is a direct demonstration that large changes in human brain organisation could take place. Results such as these disprove ideas about phantom limbs occurring due to neuromas.
Its not the actual area of the limb which we should look into, but more central parts of the brain where remapping has occurred.Further studies showed patients reporting sensations they felt when certain areas of the thalamus were stimulated. These were areas in the thalamus that were previously innervated by neurons from the missing are. Similar sensations were also reported when stump areas were stimulated as these activated the reorganised areas of the brain.Ramachandran also noted that many phantoms stay fixated in a position whereas others can be moved voluntarily. This was observed when a patient yelled, 'ouch!' after a cup was yanked away from his reach.
In many cases, patients who feel their phantom limb frozen, have had a paralysed limb before amputation. It seems that memory of the paralysis may have been passed over to the phantom limb.The idea behind movement of phantom limbs or lack of, is that remapping and motor sensory systems are involved. Every time the motor command centre sends a signal to the missing arm, this information is also sent to the parietal lobe which contains our body image.
The coming together of information from these two sources gives us a picture of the phantom arm at any time. Because there are no joints, ligaments in the phantom arm, it doesn't receive any information regarding them. In addition to this, the brain doesn't receive proper visual feedback and so realises that the limb isn't moving and almost, 'learns paralysis'. It seems that vision plays a crucial role in sustaining the phantom limb experience.To support this, Ramachandran reasoned that it might be possible to unlearn the paralysis. If his views concerning the role of motor systems and vision are correct, then ideally the process can be reversed to an extent whereby the phantom limb may move.
Allowing patients to see the movements they are making, would theoretically open the doors of visual feedback, allowing systems to work in unison without contradiction. He developed an ingenious method using mirrors that provided visual stimulation. A mid-vertical mirror was placed in front of the patient whereby they placed their remaining limb in a hole in an exact mirror symmetric location opposite their phantom limb. The idea is that two hands are now created instead of one. This way, the brain receives visual feedback confirming movement in response to its commands.
This is exactly what was found. Patients who had a frozen phantom limb, were able to feel movement in the limb once again, which froze again once the eyes were closed. This indicates the importance of visual feedback, and lends Ramachandran's theory immense support regarding his ideas about the mechanisms behind phantom limb paralysis and the role of visual feedback.Ramachandran also noted the pain experienced in the phantom soon after amputation.
Though he considered that it may be due to irritation of nerve tissues in the clump, or to the persistence of the memory of the pain felt before the amputation, his focus was remapping. Fibres that deal with each sense have a way of knowing where to find their targets. That is, touch follows touch pathways and warmth follows warmth pathways etc. An error may occur in the mapping process whereby a particular input (say touch) may be connected to pain receptors. As fibres are now in the wrong place doing wrong tasks, immense pain may be felt when the face is touched.
With this abnormal mapping, the volume control mechanism, which heightens or reduces pain, may have gone wayward in amputees and hence amplified pain. Similarly it may be that as a result of remapping, the touch synapses aren't wired properly, eliciting chaotic activity. The brain thus interprets this as junk, which is seen as pain.With the virtual reality box, Ramachandran has shown that many patients can get rid of their pain temporarily once they put their good hand in the box.
Examples are those who feel the pain because their fingers are clenched into their hands. The mirror allows them to see the unclenching of the good hand resulting in the phantom being unclenched. Once again, visual feedback plays a crucial role for allowing this to happen, as not using the box meant that they couldn't unclench their fists. The pain felt by amputees may have a simple mechanism. When we make a fist, at one stage the brain will tell us to stop, as any more pressure will lead to pain. However, as the limb is missing, the brain doesn't receive any signals back and so continuously commands the clenching resulting in pain.
The pain is so brutal because the amputee doesn't have a hand. Unlike those with hands, amputees can't receive signals from the palm to stop stored memories of pain coming in.These experiments have shown that, far from being a physical dysfunction, phantom limbs and pain are involved in the bigger picture of the brain. The ability that neurons have to adapt to change, questions the brains plasticity. It is more malleable than previously thought with dynamic connections at different levels of hierarchy and across different senses.
Considering the importance of vision in eliminating pain, there appear to be many interactions taking place.Phantoms limb occurrences bring about questions regarding the I-function. What happens to the persons sense of self when their homunculus is disrupted by loss of a limb? Accurate information from the body is required to maintain a correct concept of the self. With amputees, many of the commands and signals are contradictory.
With this information not being accurate, the concept of the self is inaccurate. It seems the contradictions, abnormal functioning and abnormal interactions of the systems of amputees have skewed their sense of self. The effect thus, is the pain that is felt by patients. Ramachandran has shown the importance of the visual system in maintaining the body image and how the brain does so, even in the face of abnormalities or error.
