Although there is a vast clinical literature on phantom limbs, there has been almost no experimental work on them. The somatosensory cortex of adult primates is capable of a surprising degree of functional reorganization. After twelve years of deafferentation of 1 upper limb, for example, the area in 3b corresponding to that hand gets """"""""taken over"""""""" by sensory input from the face, i.e., it can be activated by touching the face. It is remarkable that although somatosensory """"""""plasticity"""""""" in S1 has been known for over a decade there have been no systematic experiments designed specifically to explore the behavioral consequence of these changes. Our main goal in this proposal is to look for such behavioral effect by studying human patients after amputation of an arm or a single digit. The observation predicts, for example, that after amputation of 1 upper limb in a human subject, touching the face should evoke sensations not only in the face but also in specific parts of the missing phantom limb. We will confirm our preliminary observation that this does indeed happen. Furthermore since the """"""""remapping"""""""" observed in monkeys is somatotopically organized we will test the prediction that the pints that elicit referred sensations should also exhibit topography. Remapping increases the cortical magnifications: the amount of cortex devoted to a given sensory surface such as the face. We will therefore look for improvements in grating acuity and tactile hyperacuity thresholds in these areas and try to correlate this with """"""""maps"""""""" of referred sensations. To determine whether the changes arise from """"""""sprouting"""""""" or from unmasking of preexisting connections we will also study patients immediately after amputation and then follow the time course of these effects. We will also perform 3 additional experiments. First, we will examine patients after section of the trigeminal nerve (ganglion) to find out if they refer sensations from the hand to the face. Second, we will use MEG recordings to track both the perceptual and the physiological changes over time in the same patient. Third, in order to explore the effects of visual feedback on phantom limbs, we will optically superimpose the mirror reflection of the normal hand on the phantom. We recently showed that visually perceived movements in the phantom are felt as hepatic movements. We will confirm this effect and exploit its theoretically and clinical implications. These experiments may help provide powerful links between physiology and behavior and would have obvious clinical relevance for the relief of phantom-limb pain.
| Hubbard, Edward M; Manohar, Sanjay; Ramachandran, Vilayanur S (2006) Contrast affects the strength of synesthetic colors. Cortex 42:184-94 |
| Hubbard, Edward M; Arman, A Cyrus; Ramachandran, Vilayanur S et al. (2005) Individual differences among grapheme-color synesthetes: brain-behavior correlations. Neuron 45:975-85 |
| Hubbard, Edward M; Ramachandran, V S (2005) Neurocognitive mechanisms of synesthesia. Neuron 48:509-20 |
| Ramachandran, V S; Hubbard, E M (2001) Psychophysical investigations into the neural basis of synaesthesia. Proc Biol Sci 268:979-83 |
