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

Here we have the perfect arrangement for another type of cross-wiring. The angular gyrus is involved in color processing and numerical sequences. Could it be that, in some synesthetes, the crosstalk occurs between these two higher areas near the angular gyrus rather than lower down in the fusiform? If so, that would explain why, in them, even abstract number representations or the idea of a number prompted by days of the week or months will strongly manifest color. In other words, depending on which part of the brain the abnormal synesthesia gene is expressed, you get different types of synesthetes: “higher” synesthetes driven by numerical concept, and “lower” synesthetes driven by visual appearance alone. Given the multiple back-and-forth connections between brain areas, it is also possible that numerical ideas about sequentiality are sent back down to the fusiform gyrus to evoke colors.

In 2003 I began a collaboration with Ed Hubbard and Geoff Boynton from the Salk Institute for Biological Studies to test these ideas with brain imaging. The experiment took four years, but we were finally able to show that, in grapheme-color synesthetes, the color area V4 lights up even when you present colorless numbers. This cross-activation could never happen in you or me. In recent experiments carried out in Holland, researchers Romke Rouw and Steven Scholte found that there were substantially more axons (“wires”) linking V4 and the grapheme area in lower synesthetes compared to the general population. And even more remarkably, in higher synesthetes, they found a greater number of fibers in the general vicinity of the angular gyrus. This all is precisely what we had proposed. The fit between prediction and subsequent confirmation rarely proceeds so smoothly in science.

The observations we had made so far broadly support the cross-activation theory and provide an elegant explanation of the different perceptions of “higher” and “lower” synesthetes.⁴ But there are many other tantalizing questions we can ask about the condition. What if a letter synesthete were bilingual and knew two languages with different alphabets, such as Russian and English? The English P and the Cyrillic represent more or less the same phoneme (sound) but look completely dissimilar. Would they evoke the same or different colors? Is the grapheme alone critical, or is it the phoneme? Maybe in lower synesthetes it’s the visual appearance that drives it whereas in higher synesthetes it’s the sound. And what about uppercase versus lowercase letters? Or letters depicted in cursive writing? Do the colors of two adjacent graphemes run or flow into each other, or do they cancel each other out? To my knowledge none of these questions have been adequately answered yet—which means we have many exciting years of synesthesia research ahead of us. Fortunately, many new researchers have joined us in the enterprise including Jamie Ward, Julia Simner, and Jason Mattingley. There is now a whole thriving industry on the subject.

Let me tell you about one last patient. In Chapter 2 we noted that the fusiform gyrus represents not only shapes like letters of the alphabet but faces as well. Thus, shouldn’t we expect there to be cases in which a synesthete sees different faces as possessing intrinsic colors? We recently came across a student, Robert, who reported experiencing exactly that. He usually saw the color as a halo around the face, but when he was inebriated the color would become much more intense and spread into the face itself.⁵ To find out if Robert was being truthful we did a simple experiment. I asked him to stare at the nose of a photograph of another college student and asked Robert what color he saw around the face. Robert said the student’s halo was red. I then briefly flashed either red or green dots on different locations in the halo. Robert’s gaze immediately darted toward a green spot but only rarely toward a red one; in fact, he claimed not to have seen the red spots at all. This provides compelling evidence that Robert really was seeing halos: On a red background, green would be conspicuous while red would be almost imperceptible.