Monday, February 6, 2012

Deep, elegant, beautiful, and not there.

Each year, the people at craft a question that would terrify whole classrooms of students if it appeared, say, as an essay prompt on the SAT’s. Lucky for generations of college-bound young people, they save it for “the most complex and sophisticated minds” out there. In practice, this translates to a whole slew of top scientists, writers, philosophers, economists, and whoever else might fit the bill.

Past questions have left respondents adrift in a sea of ambiguity (the college essay writers are uneasy now). Things like, “What now?”, “What will change everything?” and “What do you believe is true even though you cannot prove it?” Part of the fun is seeing how the participants steer through all that wide-open space.

The question slated for 2012 is, “What is your favorite deep, elegant, or beautiful explanation?” 

There are few stipulations: “The only requirement is that some simple and non-obvious idea explain some diverse and complicated set of phenomena.” Some great answers have already been posted by people like Carl Zimmer, Sean Carroll, Irene Pepperberg and Steven Pinker.

For his pick, neuroscientist V.S. Ramachandran calls the discovery of DNA’s twisting, spiral structure “surely the most obvious.” The double helix model is brilliantly elegant, but to a modern biology student like me it is elegant in an abstract, faraway sense; that puzzle’s final piece lives half a century back in a fuzzy X-ray image.
Happily, there have been plenty of sophisticated ideas in my lifetime to latch onto for inspiration. And some of the best have come from Ramachandran himself.

V.S. Ramachandran, from Neurophilosophy
The Phantom Limb

In 1797, Lord Horatio Nelson led the Royal Navy in an attack against the Spanish. At the end of it, Nelson was left with one fewer arm and the sting of defeat. But Nelson felt a sting in his missing arm as well. For the rest of his life, he experienced a curious sensation that his arm was still around despite the fact that, clearly, it was not. To him, this proved the existence of the soul.

Nelson was not the first to note the strange phenomenon of a ghostly extremity. In his Meditations on First Philosophy (1641), Rene Descartes writes that post-amputation feeling is reason for epistemic doubt towards bodily sensations. About 200 years later, the phantom limb makes another literary cameo in Moby Dick after the white whale takes off with Captain Ahab’s leg.

It took until 1872 for Silas Weir Mitchell to officially coin the term ‘phantom limb,’ and it has become a catchphrase of recent times as scientists like Ramachandran try to tease out an explanation.

Most patients notice phantom sensation immediately after the loss of a body part, and phantom limbs result from most amputations. The feeling may pass after a few months, but sometimes it lasts for decades. Sometimes, the feeling is painful –very painful. An unseen limb may hang helplessly, forever stuck in one position and unresponsive to the will to move it around. Or a hand may clench tightly into itself, as one of Ramachandran’s patients explains in the 1998 book Phantoms in the Brain. That particular patient felt as though his nails were constantly digging hard into palm flesh.

The potential hopelessness of this situation is enormous. You know that you have lost your arm because there is just stunning empty space where it should rest. The problem is that you can still feel stiff, ghostly fingertips squeezing somewhere in the ether. How do you fight something that you cannot see?

Before we can answer that, we have to look further into the brain.

How the Brain Sees the Body

When Ramachandran began his work on phantom pain, the only explanation out there was that maybe tangled stumps of nerve endings become irritated enough to shoot deceptive signals up to the brain. There were many problems with this hypothesis, of course. It was too simple, and not in the elegant and beautiful way. So Ramachadran set out to find the old map of the brain’s sensory system that he remembered from his medical school coursework.

Penfield’s cortical map loosely divides a strip of the brain by the body part that each segment controls. As you can see, some areas are overrepresented: lips, hands, feet. If you scale the information into a figure sketch (called a homunculus), you get a funny little man with exaggerated features that spill over his frame.  

From University of Nebraska Medical Center

The homunculus; a 3D representation of the Penfield map shown above

The gist of a few follow-up experiments that I'll omit here for space* was that when one area of the Penfield map is knocked out by loss of limb (say, the arm), adjacent regions creep over into that space and take some of it over (so, the face region bleeds into the arm region). Movements of the face, then, could create the sensation of a phantom limb.

*I encourage you to check out the book, because it's an interesting story

An Elegant Solution

So far, the most confounding element of this whole phantom limb situation has been the disparity between what a patient sees and what he feels. We’re about to jump into the subtle hint of that visual cue, as Ramachandran did in the mid-nineties.

Let’s return, for a moment, to the immobile phantom arm. If this were a real arm, we would call it paralyzed. How can a phantom limb become paralyzed? We don’t really know, and this is part of the mystery Ramachandran still grapples with.

Maybe in the injured moments before amputation, the brain sends signals to move the limb. The limb doesn’t move after several sucessive commands, so the brain gives up and moves on. After the limb is gone, the brain --bitter about being ignored the first time --figures that the limb is still there and just as useless as ever. “Learned paralysis,” Ramachandran calls it.

This is an interesting start. If your brain can learn paralysis, maybe it can it unlearn it too? What if you could have that arm back just long enough to provide the visual feedback that says, “Hey look, brain, the arm’s working again!”? The trick that accomplishes this has got to be very convincing. Your brain is not an easy thing to fool. Ramachandran had a very interesting trick in mind, indeed.

He headed down to the hardware store and built this:

As you can see, it’s a simple mirror-box. The patient slips both arms (one real, one phantom) into either side of the box. He is instructed to send synchronized commands to the arms; say, to move them up and down in unison. The mirrored image of the real arm should neatly overlap the perceived location of its phantom twin. In the confusion of the moment, the phantom appears to be real again and you get genuine (sort of) visual feedback telling the brain the good news. Like a reset button, the paralysis is effectively “unlearned”. With eyes closed, the effect disappears --more evidence for the involvement of visual feedback cues.  

Surprisingly, some patients’ phantom limbs disappear altogether after several weeks of the box’s use (the therapy in general is successful for about half of those who try it). Ramachandran calls this successful “amputation of a phantom limb.” For one patient, it meant that the arm was gone but also that a phantom hand was left dangling from his shoulder. (He didn’t mind so much, since his pain had dissipated along with the arm).

What about the patient who could not unclench his phantom fist? Ramachandran offers a potential answer for this too. When you make a fist a little too eagerly, motor signals relay back to the brain to say, “Stop! This hurts, and you should let up a bit.” This is a problem for amputees, because no arm means no motor signals. It’s possible that the brain doesn’t know to stop in the case of a phantom limb and so keeps pressing forward with the command to clench tighter. As for the specific sensation of sharp nails, it could be a remembered sort of pain. The brain associates a clenched fist with the feeling of nails digging. All of these ideas are mostly Ramachandran’s speculation, but they are clever ideas.

Deep, elegant, beautiful?

At this point, my mind is swimming in phantom-related thoughts.

Our minds can do this to us! What a trick!

The story of the phantom limb offers less explanation than the Edge prompt invites because there is still so much left to understand about the brain’s inner workings. The most exciting explanations are the slightly messy ones, which I suppose are neither beautiful nor elegant.

But the lack of neat, tied-up explanation here is not a copout. The beauty and elegance of the phantom limb in the mirror-box, I think, is in the notion that our brains harbor complex images of our bodies that extend beyond the standard motor response interaction. These images depend upon a maze of somatosensory data (sight of the arm moving in the mirror-box, for instance), and they are perhaps even slightly malleable (as with the Penfield map).

Ramachandran’s ideas have nothing to do with proving the existence of the soul as Lord Nelson would have it, but they are almost as noble in implication. Phantom limbs and mirror-boxes reveal the quiet ingenuity of the brain as it coordinates with the body. And we just hum along unknowingly, limbs and all.  

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