The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

     Unfortunately, for many physiologists reductionism becomes an end in itself, a fetish almost. An analogy to illustrate this comes from Horace Barlow. Imagine that an asexual (parthenogenetic) Martian biologist lands on Earth. He has no idea what sex is since he reproduces by dividing into two, like an ameba. He (it) examines a human and finds two round objects (which we call testes) dangling between the legs. Being a reductionist Martian, he dissects them and, looking through microscope, finds them swarming with sperms; but he wouldn’t know what they were for. Barlow’s point is that no matter how meticulous the Martian is at dissection and how detailed an analysis he performs on them he will never truly understand the function of the testes unless he knew about the “macroscopic” phenomenon of sex; he may even think the sperm are wriggling parasites. Many (fortunately not all!) of our physiologists recording from brain cells are in the same position as the asexual Martian.

     The second, related point is that one must have the intuition to focus on the appropriate level of reductionism for explaining a given higher-level function (such as sex). If Watson and Crick had focused on the subatomic level or atomic level of chromosomes instead of the macromolecular level (DNA), or if they had focused on the wrong molecules (the histones in the chromosomes instead of DNA) they would have made no headway in discovering the mechanism of heredity.

18. Even simple experiments on normal subjects can be instructive in this regard. I will mention an experiment I did (with my student Laura Case) inspired by the “rubber hand illusion” discovered by Botvinick and Cohen (1998) and by the dummy-head illusion (Ramachandran and Hirstein, 1998). You, the reader, stand about a foot behind a bald-headed manikin looking at its head. I stand on the right side of you both and randomly tap and stroke the back of your head (especially ears) with my left hand (so you can’t see my hand) while simultaneously doing the same thing on the plastic head with my right hand, in perfect synchrony. In about two minutes you will experience that the stroking and tapping on your head is emerging from the dummy you are looking at. Some people develop the illusion of a twin or phantom head in front of them, especially if they get it going by “imagining” their head displaced forward. The brain regards it as highly improbable that the plastic head is seen to be tapped in the same precise sequence as you feel on own head by chance and so is willing to temporarily to project your head on the manikin’s shoulder. This has powerful implications since, contrary to recent proposals, it rules out simple associative learning as the basis of the rubber hand illusion. (Every time you saw your hand touched you felt it touched as well.) After all, you have never seen the back of your head being touched. It is one thing to regard your hand sensations as being slightly out of register with your real hand but quite another to project them to the back of a dummy head!

     The experiment proves that your brain has constructed an internal model of your head—even unseen parts—and used Bayesian inference to experience (incorrectly) your sensations as arising from the dummy’s head even though it is logically absurd. Would doing something like this help alleviate your migraine symptoms (“the dummy is experiencing migraine; not me”)? I wonder.

     Olaf Blanke and Henrik Ehrsson of the Karolinska Institute in Sweden have shown that out-of-body experiences can also be induced by having subjects watch video images of themselves moving or being touched. Laura Case, Elizabeth Seckel, and I found that such illusions are enhanced if you wear a Halloween mask and introduce a tiny time delay together with a left-right reversal in the image. You suddenly start inhabiting and controlling the “alien” in the video image. Remarkably, if you wear a smiling mask you actually feel happy because “you, out there” look happy! I wonder if you could use it to “cure” depression.

EPILOGUE

1. These two Darwin quotes come from the London Illustrated News, April 21, 1862 (“I feel most deeply…”), and Darwin’s letter to Asa Gray, May 22, 1860 (“I own that I cannot see…”).

BIBLIOGRAPHY

Entries marked with an asterisk are suggestions for further reading.

Aglioti, S., Bonazzi, A., & Cortese, F. (1994). Phantom lower limb as a perceptual marker of neural plasticity in the mature human brain. Proceedings of the Royal Society of London, Series B: Biological Sciences, 255, 273–278.

Aglioti, S., Smania, N., Atzei, A., & Berlucchi, G. (1997). Spatio-temporal properties of the pattern of evoked phantom sensations in a left index amputee patient. Behavioral Neuroscience, 111, 867–872.