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Rationalists complain that most people are too willing to make excuses for their positions, and too unwilling to abandon those positions for ones that better fit the evidence. And most people really are pretty bad at this. But certain stroke victims called anosognosiacs are much, much worse.
Anosognosia is the condition of not being aware of your own disabilities. To be clear, we're not talking minor disabilities here, the sort that only show up during a comprehensive clinical exam. We're talking paralysis or even blindness1. Things that should be pretty hard to miss.
Take the example of the woman discussed in Lishman's Organic Psychiatry. 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".
Why won't these patients admit they're paralyzed, and what are the implications for neurotypical humans? Dr. Vilayanur Ramachandran, leading neuroscientist and current holder of the world land-speed record for hypothesis generation, has a theory.
One immediately plausible hypothesis: the patient is unable to cope psychologically with the possibility of being paralyzed, so he responds with denial. Plausible, but according to Dr. Ramachandran, wrong. He notes that patients with left-side strokes almost never suffer anosognosia, even though the left side controls the right half of the body in about the same way the right side controls the left half. There must be something special about the right hemisphere.
Another plausible hypothesis: the part of the brain responsible for thinking about the affected area was damaged in the stroke. Therefore, the patient has lost access to the area, so to speak. Dr. Ramachandran doesn't like this idea either. The lack of right-sided anosognosia in left-hemisphere stroke victims argues against it as well. But how can we disconfirm it?
Dr. Ramachandran performed an experiment2 where he "paralyzed" an anosognosiac's good right arm. He placed it in a clever system of mirrors that caused a research assistant's arm to look as if it was attached to the patient's shoulder. Ramachandran told the patient to move his own right arm, and the false arm didn't move. What happened? The patient claimed he could see the arm moving - a classic anosognosiac response. This suggests that the anosognosia is not specifically a deficit of the brain's left-arm monitoring system, but rather some sort of failure of rationality.
Says Dr. Ramachandran:
The reason anosognosia is so puzzling is that we have come to regard the 'intellect' as primarily propositional in character and one ordinarily expects propositional logic to be internally consistent. To listen to a patient deny ownership of her arm and yet, in the same breath, admit that it is attached to her shoulder is one of the most perplexing phenomena that one can encounter as a neurologist.
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".
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.
This divorce between the apologist and the revolutionary might also explain some of the odd behavior of split-brain patients. Consider the following experiment: a split-brain patient was shown two images, one in each visual field. The left hemisphere received the image of a chicken claw, and the right hemisphere received the image of a snowed-in house. The patient was asked verbally to describe what he saw, activating the left (more verbal) hemisphere. The patient said he saw a chicken claw, as expected. Then the patient was asked to point with his left hand (controlled by the right hemisphere) to a picture related to the scene. Among the pictures available were a shovel and a chicken. He pointed to the shovel. So far, no crazier than what we've come to expect from neuroscience.
Now the doctor verbally asked the patient to describe why he just pointed to the shovel. The patient verbally (left hemisphere!) answered that he saw a chicken claw, and of course shovels are necessary to clean out chicken sheds, so he pointed to the shovel to indicate chickens. The apologist in the left-brain is helpless to do anything besides explain why the data fits its own theory, and its own theory is that whatever happened had something to do with chickens, dammit!
The logical follow-up experiment would be to ask the right hemisphere to explain the left hemisphere's actions. Unfortunately, the right hemisphere is either non-linguistic or as close as to make no difference. Whatever its thoughts, it's keeping them to itself.
...you know, my mouth is still agape at that whole cold-water-in-the-ear trick. I have this fantasy of gathering all the leading creationists together and squirting ice cold water in each of their left ears. All of a sudden, one and all, they admit their mistakes, and express bafflement at ever having believed such nonsense. And then ten minutes later the effect wears off, and they're all back to talking about irreducible complexity or whatever. I don't mind. I've already run off to upload the video to YouTube.
This is surely so great an exaggeration of Dr. Ramachandran's theory as to be a parody of it. And in any case I don't know how much to believe all this about different reasoning modules, or how closely the intuitive understanding of it I take from his paper matches the way a neuroscientist would think of it. Are the apologist and the revolutionary active in normal thought? Do anosognosiacs demonstrate the same pathological inability to change their mind on issues other than their disabilities? What of the argument that confabulation is a rather common failure mode of the brain, shared by some conditions that have little to do with right-hemisphere failure? Why does the effect of the cold water wear off so quickly? I've yet to see any really satisfying answers to any of these questions.
But whether Ramachandran is right or wrong, I give him enormous credit for doing serious research into the neural correlates of human rationality. I can think of few other fields that offer so many potential benefits.
1: See Anton-Babinski syndrome
2: See Ramachandran's "The Evolutionary Biology of Self-Deception", the link from "posits two different reasoning modules" in this article.
3: For Ramachandran's thoughts on REM, again see "The Evolutionary Biology of Self Deception"