Neuroscientist Omar Ghaffar of the University of Toronto and his colleagues scanned the three patients while a vibrating pad was attached to their numb body parts as well as their still sensate counterparts. With a click of a button, the researchers could stimulate the body part and see what region of the brain "lit up," or benefited from increased blood flow as it dealt with new input. When they stimulated a numb appendage, the region of the brain (the primary somatosensory cortex) responsible for processing sensory information from the body failed to light up.
When the researchers stimulated the appendage's counterpart, the brain functioned normally and, intriguingly, when the researchers stimulated both body parts the somatosensory cortex also did its job. "This suggests that what have until now been, quote-unquote, 'unexplained neurological symptoms' do have a neurological underpinning that is demonstrable by fMRI," Ghaffar says. But it remains unclear why their brains work this way; "one possible mechanism is that bilateral stimulation acts as a distractor which overcomes the inhibition [in the brain] that occurs with unilateral stimulation," the researchers write in the December 12 issue of Neurology.
This finding also bridges the gap between conflicting earlier reports on whether such phenomena had a brain basis: The studies that said "yes" had only used stimulation of either the functioning or the nonfunctioning appendage, while the "no" studies primarily used stimulation of both. "We did both," Ghaffar notes. "And our findings are consistent with both, though it's a tentative conclusion from our small sample size."
The work is part of a longer study assessing whether paying attention to the stimulation plays a role in the disorder, along with a larger cohort to cancel out confounding factors such as other psychiatric disorders. But it adds credence to the growing roster of patients complaining of such symptoms without any conventional diagnosis; they are not hypochondriacs but they may be hysterical--in a nonpejorative sense.