Graphesthesia is the ability to recognize writing on the skin purely by the sensation of touch.
An examiner writes single numbers or simple letters on the skin (usually the palm) with something that will provide a clear stimulus, such as a broken tongue depressor, pen cap etc.
[2] Loss of graphesthesia indicates either parietal lobe damage on the side opposite the hand tested or damage to the dorsal columns pathway at any point between the tested point and the contralateral parietal lobe.
The major clinical utility of the test in the 21st century is in the condition, cortico-basal ganglionic degeneration, where, in addition to evidence of basal ganglia dysfunction, the presence of cortical sensory loss is likely to have reasonably high specificity for the diagnosis.
[2] Graphesthesia can be considered as a type of synthetic sensation as it involves a complex interaction between three neural components i.e. Tactile sensation, two point discrimination and cortical component that stores infirmation about the symbol/letter that is being traced in the skin and was learned through some previous experience.