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

In the spirit of taking a speculative leap, I would like to argue that further specialization might have occurred in our space-mapping parietal lobes. The left angular gyrus might be involved in representing ordinality. The right angular gyrus might be specialized for quantity. The simplest way to spatially map out a numerical sequence in the brain would be a straight line from left to right. This in turn might be mapped onto notions of quantity represented in the right hemisphere. But now let’s assume that the gene that allows such remapping of sequence on visual space is mutated. The result might be a convoluted number line of the kind you see in number-space synesthetes. If I were to guess, I’d say other types of sequence—such as months or weeks—are also housed in the left angular gyrus. If this is correct, we should expect that a patient with a stroke in this area might have difficulty in quickly telling you whether, for example, Wednesday comes after or before Tuesday. Someday I hope to meet such a patient.

ABOUT THREE MONTHS after I had embarked on synesthesia research, I encountered a strange twist. I received an email from one of my undergraduate students, Spike Jahan. I opened it expecting to find the usual “please reconsider my grade” request, but it turned out that he’s a number-color synesthete who had read about our work and wanted to be tested. Nothing strange so far, but then he dropped a bombshell: He’s color-blind. A color-blind synesthete! My mind began to reel. If he experiences colors, are they anything like the colors you or I experience? Could synesthesia shed light on that ultimate human mystery, conscious awareness?

Color vision is a remarkable thing. Even though most of us can experience millions of subtly different hues, it turns out our eyes use only three kinds of color photoreceptors, called cones, to represent all of them. As we saw in Chapter 2, each cone contains a pigment that responds optimally to just one color: red, green, or blue. Although each type of cone responds optimally only to one specific wavelength, it will also respond to a lesser extent to other wavelengths that are close to the optimum. For example, red cones respond vigorously to red light, fairly well to orange, weakly to yellow, and hardly at all to green or blue. Green cones respond best to green, less well to yellowish green, and even less to yellow. Thus every specific wavelength of (visible) light stimulates your red, green, and blue cones by a specific amount. There are literally millions of possible three-way combinations, and your brain knows to interpret each one as a separate color.

Color blindness is a congenital condition in which one or more of these pigments is deficient or absent. A color-blind person’s vision works perfectly normally in nearly every respect, but she can see only a limited range of hues. Depending on which cone pigment is lost and on the extent of loss, she may be red-green color-blind or blue-yellow color-blind. In rare cases two pigments are deficient, and the person sees purely in black and white.

Spike had the red-green variety. He experienced far fewer colors in the world than most of us do. What was truly bizarre, though, was that he often saw numbers tinged with colors that he had never seen in the real world. He referred to them, quite charmingly and appropriately, as “Martian colors” that were “weird” and seemed quite “unreal.” He could only see these when looking at numbers.

Ordinarily one would be tempted to ignore such remarks as being crazy, but in this case the explanation was staring me in the face. I realized that my theory about cross-activation of brain maps provides a neat explanation for this bizarre phenomenon. Remember, Spike’s cone receptors are deficient, but the problem is entirely in his eyes. His retinas are unable to send the full normal range of color signals up to the brain, but in all likelihood his cortical color-processing areas, such as V4 in the fusiform, are perfectly normal. At the same time, he is a number-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!