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

The surprise came when we showed the black-and-white displays to Mirabelle. Unlike the nonsynesthetes, she was able to identify the shape correctly on 80 to 90 percent of trials—just as if the numbers were actually colored differently! The synesthetically induced colors were just as effective as real colors in allowing her to discover and report the global shape.² This experiment provides unassailable proof that Mirabelle’s induced colors are genuinely sensory. There is simply no way she could fake it, and no way it could be the result of childhood memories or any of the other alternative explanations that have been proposed.

FIGURE 3.4 A cluster of 2s scattered among 5s. It is difficult for normal subjects to detect the shape formed by the 2s, but lower synesthetes as a group perform much better. The effect has been confirmed by Jamie Ward and his colleagues.

FIGURE 3.5 The same display as Figure 3.4 except that the numbers are shaded differently, allowing normal people to see the triangle instantly. Lower synesthetes (“projectors”) presumably see something like this.

Ed and I realized that, for the first time since Francis Galton, we had clear, unambiguous proof from our experiments (grouping and popout) that synesthesia was indeed a real sensory phenomenon—proof that had eluded researchers for over a century. Indeed, our displays could not only be used to distinguish fakes from genuine synesthetes, but also to ferret out closet synesthetes, people who might have the ability but not realize it or not be willing to admit it.

ED AND I sat back in the café discussing our findings. Between our experiments with Francesca and Mirabelle, we had established that synesthesia exists. The next question was, why does it exist? Could a glitch in brain wiring explain it? What did we know that could help us figure this out? First, we knew that the most common type of synesthesia is apparently number-color. Second, we knew that one of the main color centers in the brain is an area called V4 in the fusiform gyrus of the temporal lobes. (V4 was discovered by Semir Zeki, professor of neuroesthetics at University College of London, and a world authority on the organization of the primate visual system.) Third, we knew that there may be areas in roughly the same part of the brain that are specialized for numbers. (We know this because small lesions to this part of the brain cause patients to lose arithmetic skills.) I thought, wouldn’t it be wonderful if number-color synesthesia were simply caused by some accidental “cross-wiring” between the number and color centers in the brain? This seemed almost too obvious to be true—but why not? I suggested we look at some brain atlases to see exactly how close these two areas really are in relation to each other.

“Hey, maybe we can ask Tim,” Ed responded. He was referring to Tim Rickard, a colleague of ours at the center. Tim had used sophisticated brain-imaging techniques like fMRI to map out the brain area where visual number recognition occurs. Later that afternoon, Ed and I compared the exact location of V4 and the number area in an atlas of the human brain. To our amazement, we saw that the number area and V4 were right next to each other in the fusiform gyrus (Figure 3.6). This was strong support for the cross-wiring hypothesis. Can it really be a coincidence that the most common type of synesthesia is the number-color type, and the number and color areas are immediate neighbors in the brain?

FIGURE 3.6 The left side of the brain showing the approximate location of the fusiform area: black, a number area; white, a color area (shown schematically on the surface).