It's not my arm!
Fascinating article by Yvain on Less Wrong:
After a right-hemisphere stroke, she lost movement in her left arm but continuously denied it. When the doctor asked her to move her arm, and she observed it not moving, she claimed that it wasn't actually her arm, it was her daughter's. Why was her daughter's arm attached to her shoulder? The patient claimed her daughter had been there in the bed with her all week. Why was her wedding ring on her daughter's hand? The patient said her daughter had borrowed it. Where was the patient's arm? The patient "turned her head and searched in a bemused way over her left shoulder".
So what's Dr. Ramachandran's solution? He posits two different reasoning modules located in the two different hemispheres. The left brain tries to fit the data to the theory to preserve a coherent internal narrative and prevent a person from jumping back and forth between conclusions upon each new data point. It is primarily an apologist, there to explain why any experience is exactly what its own theory would have predicted. The right brain is the seat of the second virtue. When it's had enough of the left-brain's confabulating, it initiates a Kuhnian paradigm shift to a completely new narrative. Ramachandran describes it as "a left-wing revolutionary".There's more.
Normally these two systems work in balance. But if a stroke takes the revolutionary offline, the brain loses its ability to change its mind about anything significant. If your left arm was working before your stroke, the little voice that ought to tell you it might be time to reject the "left arm works fine" theory goes silent. The only one left is the poor apologist, who must tirelessly invent stranger and stranger excuses for why all the facts really fit the "left arm works fine" theory perfectly well.
It gets weirder. For some reason, squirting cold water into the left ear canal wakes up the revolutionary. Maybe the intense sensory input from an unexpected source makes the right hemisphere unusually aroused. Maybe distoring the balance sense causes the eyes to move rapidly, activating a latent system for inter-hemisphere co-ordination usually restricted to REM sleep3. In any case, a patient who has been denying paralysis for weeks or months will, upon having cold water placed in the ear, admit to paralysis, admit to having been paralyzed the past few weeks or months, and express bewilderment at having ever denied such an obvious fact. And then the effect wears off, and the patient not only denies the paralysis but denies ever having admitted to it.