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

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 naïve question, “How do I know that your red isn’t my blue?” Synesthesia reminds us that this question may not be that naïve 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

Even when we are alone, how often do we think with pain and pleasure of what others think of us, or their imagined approbation or disapprobation; and this all follows from sympathy, a fundamental element of the social instincts.

—CHARLES DARWIN