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Phantom limb phenomena

Subjective sensations of a phantom limb following amputation were attributed by Descartes (1637) to stimulation of the remaining sensory nerves.

More recently the phenomena has been studied with brain imaging and it is commonly quoted as representing “dynamic neural remodelling in the mature nervous systems” (Agliotti et al 1994)

However, Doestch (1997) has pointed out that when the phantom phenomena remains stable over time it indicates that there is no functional respecification of the “phantom” area, even though it apparently responds to new inputs. It is usually suggested that the phantom sensations are transient, and do not occur with very early limb loss (Kew et al, 1994), but a paper in Brain (Melzack et al, 1997) surveys phantom limb reports in many individuals with congenital limb absence or amputation before age 6, concluding that there must therefore be some genetic specification of functional significance.

Florence et al (1997) review evidence suggesting that there are several different mechanisms of brain plasticity, and that — traumatic amputation in adult monkeys produces the sprouting of new sensory affertent axons from adjacent peripheral body areas, producing major subcortical changes of input into the remaining cortical representations of the lost body part. (Florence et al, 1997)

Ramachandran and Hirstein (1998) review recent studes of phantom limb phenomena and conclude that it involves nature/ nurture interactions.

It is a long paper covering a great deal of ground, but the conclusion is that a multifactorial theory is needed to account for the wide range of phenomena concerning phantom limbs that the authors review.

One factor they propose is a “remapping” (which sometimes very rapid) in which tactile inputs from body areas which are adjacent to those which have been lost in the sensory homunculus “take over” the areas of cortex formely innervated input from the limb which has been lost. Hovever this is not a complete take-over, otherwise one would expect to find, for instance that that was greater tactile sensitiviy in area area of the face which was making use of cortex orginally representing the hand, rather than that stimulating areas of the face produces subjective experience characteristic of a lost hand.

The nature and nurture of phantom limbs

Ramachandran and Hirstein (1998) end by asking the question “Do phantom limbs arise from experiential factors such as remapping or do they represent the ghostly persistence of a genetically specified body image” (p. 1624-5, paraphrased).

Their answer is that “the phantom emerges from a complex interaction between the two”

external link to Ramachandran and Hirstein, 1998]  

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