Why can touching the face of someone who lost an arm be felt in the missing hand? · Grey Matter

On the somatosensory map the hand area lies next to the face area.

Why can touching the face of someone who lost an arm be felt in the missing hand?

On the somatosensory map the hand area lies next to the face area. After an amputation the hand area stops receiving its own input, and inputs from its neighbour, the face, spread into it. The rest of the brain still reads activity in that patch as coming from the hand, so a touch on the cheek is felt both on the cheek and on the phantom hand, sometimes in a point-by-point layout, as Ramachandran and colleagues reported in 1992. It is one of the clearest demonstrations that adult cortical maps keep reorganising.