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

     In doing science one is often forced to choose between providing precise answers to boring (or trivial) questions such as, How many cones are there in the human eye? or vague answers to big questions such as, What is consciousness? or, What is a metaphor? Fortunately, every now and then we get a precise answer to a big question and hit the jackpot (like DNA being the answer to the riddle of heredity). So far, synesthesia seems to lie halfway between those two extremes.

10. For up-to-date information, see the entry “Synesthesia,” by David Brang and me, at Scholarpedia (www.scholarpedia.org/article/Synesthesia). Scholarpedia is an open-access online encyclopedia written and peer-reviewed by scholars from around the world.

CHAPTER 4 THE NEURONS THAT SHAPED CIVILIZATION

1. A young orangutan in the London zoo once watched Darwin play a harmonica, grabbed it from him, and started to mime him; Darwin had already been thinking of the imitative capacities of apes in the nineteenth century.

2. Since their original discovery, the concept of mirror neurons has been confirmed repeatedly in experiments and has had tremendous heuristic value in our understanding the interface between structure and function in the brain. But it has also been challenged on various grounds. I will list the objections and reply to each.

     (a) “Mirroritis”: There is a great deal of media hype surrounding the mirror-neuron system (MNS), with anything and everything being attributed to them. This is true, but the existence of hype doesn’t by itself negate the value of a discovery.

     (b) The evidence for their existence in humans is unconvincing. This criticism seems odd to me given that we are closely related to monkeys; the default assumption should be that human mirror neurons do exist. Furthermore, Marco Iacoboni has shown their presence by directly recording from nerve cells in human patients (Iacoboni & Dapretto, 2006).

     (c) If such a system exists, why isn’t there a neurological syndrome in which damage to a small region leads to difficulty in BOTH performing and miming skilled or semiskilled actions (such as combing your hair or hammering a nail) AND recognizing the same action performed by someone else? Answer: Such a syndrome does exist, although most psychologists are unaware of it. It is called ideational apraxia and it’s seen after damage to the left supramarginal gyrus. Mirror neurons have been shown to exist in this region.

     (d) The antireductionist stance: “Mirror neurons” is just a sexy phrase synonymous with what psychologists have long called “theory of mind.” There’s nothing new about them. This argument confounds metaphor with mechanism: It’s like saying that, since we know what the phrase “passage of time” means, there is no need to understand how clocks work. Or that, since we already knew Mendel’s laws of heredity during the first half of the twentieth century, understanding DNA structure and function would have been superfluous. Analogously, the idea of mirror neurons doesn’t negate the concept of theory of mind. On the contrary, the two concepts complement each other and allow us to home in on the underlying neural circuitry.

     This power of having a mechanism to work with can be illustrated with many examples; here are three: In the 1960s, John Pettigrew, Peter Bishop, Colin Blakemore, Horace Barlow, David Hubel, and Torsten Wiesel discovered disparity-detecting neurons in the visual cortex; this finding alone provides an explanation for stereoscopic vision. Second, the discovery that the hippocampus is involved in memory allowed Eric Kandel to discover long-term potentiation (LTP), one of the key mechanisms of memory storage. And finally, one could argue that more was learned about memory in five years of research by Brenda Milner on the single patient “HM,” who had hippocampal damage, than in the previous hundred years of purely psychological approaches to memory. The falsely constructed antithesis between reductionist and holistic views of brain function is detrimental to science, something I discuss at length in Note 16 of Chapter 9.