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

Another brain region, the insula, maps internal feelings from your body. Your insula receives continuous streams of sensation from receptor cells in your heart, lungs, liver, viscera, bones, joints, ligaments, fascia, and muscles, as well as from specialized receptors in your skin that sense heat, cold, pain, sensual touch, and perhaps tickle and itch as well. Your insula uses this information to represent how you feel in relation to the outside world and your immediate environment. Such sensations are fundamentally internal, and comprise the primary ingredients of your emotional state. As a central player in your emotional life, your insula sends signals to and receives signals from other emotional centers in your brain including the amygdala, the autonomic nervous system (powered by the hypothalamus), and the orbitofrontal cortex, which is involved in nuanced emotional judgments. In normal people these circuits are activated when they touch certain emotionally charged objects. Caressing, say, a lover, could generate complex feelings of ardor, intimacy, and pleasure. Squeezing a lump of feces, in contrast, likely leads to strong feelings of disgust and revulsion. Now think of what would happen if there were an extreme exaggeration of these very connections linking S2, the insula, the amygdala, and the orbitofrontal cortex. You would expect to see precisely the sort of touch-triggered complex emotions that Francesca experiences when she touches denim, silver, silk, or paper—things that would leave most of us unmoved.

Incidentally, Francesca’s mother also has synesthesia. But in addition to emotions, she reports taste sensations in response to touch. For example, caressing a wrought-iron fence evokes an intense salty flavor in her mouth. This too makes sense: The insula receives strong taste input from the tongue.

WITH THE IDEA of cross-activation we seemed to be homing in on a neurological explanation for number-color and textural synesthesia.³ But as other synesthetes showed up in my office, we realized there are many more forms of the condition. In some people, days of the week or months of the year produced colors: Monday might be green, Wednesday pink, and December yellow. No wonder many scientists thought they were crazy! But, as I said earlier, I’ve learned over the years to listen to what people say. In this particular case, I realized that the only thing days of the week, months, and numbers have in common is the concept of numerical sequence or ordinality. So in these individuals, unlike Becky and Susan, perhaps it is the abstract concept of numerical sequence that evokes the color, rather than the visual appearance of the number. Why the difference between the two types of synesthetes? To answer this, we have to return to brain anatomy.

After the shape of a number is recognized in your fusiform, the message is relayed further on to your angular gyrus, a region in your parietal lobes involved, among other things, in higher color processing. The idea that some types of synesthesia might involve the angular gyrus is consistent with an old clinical observation that this structure is involved in cross-sensory synthesis. In other words, it is thought that this is a grand junction where information about touch, hearing, and vision flow together to enable the construction of high-level percepts. For example, a cat purrs and is fluffy (touch), it purrs and meows (hearing), and it has a certain appearance (vision) and fishy breath (smell)—all of which are evoked by the memory of a cat or the sound of the word “cat.” No wonder patients with damage here lose the ability to name things (anomia) even though they can recognize them. They have difficulty with arithmetic, which, if you think about it, also involves cross-sensory integration: in kindergarten you learn to count with your fingers, after all. (Indeed, if you touch the patient’s finger and ask her which one it is, she often can’t tell you.) All of these bits of clinical evidence strongly suggest that the angular gyrus is a great center in the brain for sensory convergence and integration. So perhaps it’s not so outlandish, after all, that a flaw in the circuitry could lead to colors being quite literally evoked by certain sounds.

According to clinical neurologists, the left angular gyrus in particular may be involved in juggling numerical quantity, sequences, and arithmetic. When this region is damaged by stroke, the patient can recognize numbers and can still think reasonably clearly, but he has difficulty with even the simplest arithmetic. He can’t subtract 7 from 12. I have seen patients who cannot tell you which of two numbers—3 or 5—is larger.