Monday, September 25, 2006

Can You Touch Your Jaw and Feel It in Your Hand?

"Phantom limb" phenomena have been well-known since at least the time of Descartes. People with missing limbs will report feeling sensations in the missing areas. In 1998, V.S. Ramachandran famously showed that people with missing arms can sometimes be induced to feel phantom sensations (as though in their missing hands) if gently stroked on the face. (See this article, for example, which is rich with interesting descriptions.) The reason for this, evidently, is that as the nerves from the phantom limb area provide no useful input, other nearby regions of the brain begin to recruit neurons from the areas formerly dedicated to input from the phantom limb; and the primary cortical region associated with tactile input from the face is adjacent to that associated with tactile input from the hand. Apparently, plasticity in input can sometimes outrun plasticity in the felt sensation, so that the relevant neurons that used to respond to stimulus from the hand and trigger (appropriately) a sensation subjectively located in the hand can come to respond to stimulus from the face while still triggering (now inappropriately) a sensation subjectively located in the hand.

Recent research -- for example by Peter Hickmott here at UC Riverside -- has shown that in animals whose nerves have been cut, one can start seeing neural plasticity within minutues. Cortical neurons near the border between forepaw and jaw which formerly acted in synchrony with other forepaw neurons start to act in synchrony with the jaw neurons.

This leads me to think of the following experiment. If we somehow induced in people cortical input from the hand similar to that one would get from denervation of the hand (by sensory deprivation? by anaesthesia?), and then one gently stroked the jaw, a la Ramachandran, might the person report a sensation in the hand?

If this has been done, I haven't heard of it.

2 comments:

Anibal said...

Or to complicated more, following your experiment, if we take into account some evidence that suggest that we share an interpersonal representation of ones and others body called "body neuromatrix", and also the reported cases of individuals with congenital absent limb that never will have an experience in their limbs but despite they feel their limbs, and neuropsychological cases in wich people believe that their hands belonged to other persons, or people which feel something in ruber hands (all of this potentially ocurring in the same person)...is it possible that plasticity happens, or "cortical deference" in terms of Nöe and Hurley´s philosophy, in a patient with those neurological deficits (which feels a limb not belonging to her, has congenital absent limb and is tested experimentally with ruber hands) when she sees the "delusional model" (the other person that the patient misidentified her hands with her hands) touching her jaw?
I don´t know if all of this is too much imagination.

Eric Schwitzgebel said...

Yes, that would be cool, if you could produce that!