color synesthete. Thus number shapes are processed normally all the way up to his fusiform and then, due to cross-wiring, produce cross-activation of cells in his V4 color area. Since Spike has never experienced his missing colors in the real world and can do so only by looking at numbers, he finds them incredibly strange. Incidentally, this observation also demolishes the idea that synesthesia arises from early-childhood memory associations such as having played with colored magnets. For how can someone “remember” a color he has never seen? After all, there are no magnets painted with Martian colors!
It is worth pointing out that non-color-blind synesthetes may also see “Martian” colors. Some describe letters of the alphabet as being composed of multiple colors simultaneously “layered on top of each other” making them not quite fit the standard taxonomy of colors. This phenomenon probably arises from mechanisms similar to those observed in Spike; the colors look weird because the connections in his visual pathways are weird and thus uninterpretable.
What is it like to experience colors that don’t appear anywhere in the rainbow, colors from another dimension? Imagine how frustrating it must be to sense something you cannot describe. Could you explain what it feels like to see blue to a person who has been blind from birth? Or the smell of Marmite to an Indian, or saffron to an Englishman? It raises the old philosophical conundrum of whether we can ever really know what someone else is experiencing. Many a student has asked the seemingly naive question, “How do I know that your red isn’t my blue?” Synesthesia reminds us that this question may not be that naive after all. As you may recall from earlier, the term for referring to the ineffable subjective quality of conscious experience is “qualia.” These questions about whether other people’s qualia are similar to our own, or different, or possibly absent, may seem as pointless as asking how many angels can dance on the head of a pin—but I remain hopeful. Philosophers have struggled with these questions for centuries, but here at last, with our blooming knowledge about synesthesia, a tiny crack in the door of this mystery may be opening. This is the way science works: Begin with simple, clearly formulated, tractable questions that can pave the way for eventually answering the Big Questions, such as “What are qualia,” “What is the self,” and even “What is consciousness?”
Synesthesia might be able to give us some clues to these abiding mysteries9,10 because it provides a way of selectively activating some visual areas while skipping or bypassing others. It is not ordinarily possible to do this. So instead of asking the somewhat nebulous questions “What is consciousness?” and “What is the self?” we can refine our approach to the problem by focusing on just one aspect of consciousness—our awareness of visual sensations—and ask ourselves, Does conscious awareness of redness require activation of all or most of the thirty areas in the visual cortex? Or only a small subset of them? What about the whole cascade of activity from the retina to the thalamus to the primary visual cortex before the messages get relayed to the thirty higher visual areas? Is their activity also required for conscious experience, or can you skip them and directly activate V4 and experience an equally vivid red? If you look at a red apple, you would ordinarily activate the visual area for both color (red) and form (apple-like). But what if you could artificially stimulate the color area without stimulating cells concerned with form? Would you experience disembodied red color floating out there in front of you like a mass of amorphous ectoplasm or other spooky stuff? And lastly, we also know that there are many more neural projections going backward from each level in the hierarchy of visual processing to earlier areas than there are going forward. The function of these back-projections is completely unknown. Is their activity required for conscious awareness of red? What if you could selectively silence them with a chemical while you looked at a red apple—would you lose awareness? These questions come perilously close to being the kind of impossible-to-do armchair thought experiments that philosophers revel in. The key difference is that such experiments really can be done—maybe within our lifetimes.
And then we may finally understand why apes care about nothing beyond ripe fruit and red rumps, while we are drawn to the stars.
CHAPTER 4
The Neurons That Shaped Civilization
—CHARLES DARWIN
A FISH KNOWS HOW TO SWIM THE INSTANT IT HATCHES, AND OFF it darts to fend for itself. When a duckling hatches, it can follow its mother over land and across the water within moments. Foals, still dripping with amniotic fluid, spend a few minutes bucking around to get the feel of their legs, then join the herd. Not so with humans. We come out limp and squalling and utterly dependent on round-the-clock care and supervision. We mature glacially, and do not approach anything resembling adult competence for many, many years. Obviously we must gain some very large advantage from this costly, not to mention risky up-front investment, and we do: It’s called culture.
In this chapter I explore how a specific class of brain cells, called mirror neurons, may have played a pivotal role in our becoming the one and only species that veritably lives and breathes culture. Culture consists of massive collections of complex skills and knowledge which are transferred from person to person through two core mediums, language and imitation. We would be nothing without our savant-like ability to imitate others. Accurate imitation, in turn, may depend on the uniquely human ability to “adopt another’s point of view”—both visually and metaphorically—and may have required a more sophisticated deployment of these neurons compared with how they are organized in the brains of monkeys. The ability to see the world from another person’s vantage point is also essential for constructing a mental model of another person’s complex thoughts and intentions in order to predict and manipulate his behavior. (“Sam thinks I don’t realize that Martha hurt him.”) This capacity, called theory of mind, is unique to humans. Finally, certain aspects of language itself—that vital medium of cultural transmission— was probably built at least partly on our facility for imitation.
Darwin’s theory of evolution is one of the most important scientific discoveries of all time. Unfortunately, however, the theory makes no provision for an afterlife. Consequently it has provoked more acrimonious debate than any other topic in science—so much so that some school districts in the United States have insisted on giving the “theory” of intelligent design (which is really just a fig leaf for creationism) equal status in textbooks. As has been pointed out repeatedly by the British scientist and social critic Richard Dawkins, this is little different from giving equal status to the idea that the sun goes around Earth. At the time evolutionary theory was proposed—long before the discovery of DNA and the molecular machinery of life, back when paleontology had just barely begun to piece together the fossil record—the gaps in our knowledge were sufficiently large to leave room for honest doubt. That point is long past, but that doesn’t mean we have solved the entire puzzle. It would be arrogant for a scientist to deny that there are still many important questions about the evolution of the human mind and brain that remain unanswered. At the top of my list would be the following:
1. The hominin brain reached nearly its present size, and perhaps even its present intellectual capacity, about 300,000 years ago. Yet many of the attributes we regard as uniquely human—such as toolmaking, fire building, art, music, and perhaps even full-blown language—appeared only much later, around 75,000 years ago. Why? What was the brain doing during that long incubation period? Why did it take so long for all this latent potential to blossom, and then why did it blossom so suddenly? Given that natural selection can only select expressed abilities, not latent ones, how did all this latent potential get built up in the first place? I shall call this “Wallace’s problem” after the Victorian naturalist Alfred Russel Wallace, who first proposed it when discussing the origins of language:
