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

This was starting to look too much like nineteenth-century phrenology, but maybe it was true! Since the nineteenth century a debate has raged between phrenology—the notion that different functions are sharply localized in different brain areas—versus holism, which holds that functions are emergent properties of the entire brain whose parts are in constant interaction. It turns out this is an artificial polarization to some degree, because the answer depends on the particular function one is talking about. It would be ludicrous to say that gambling or cooking are localized (although there may be aspects of them that are) but it would be equally silly to say that the cough reflex or the pupils’ reflex to light is not localized. What’s surprising, though, is that even some nonstereotyped functions, such as seeing colors or numbers (as shapes or even as numerical ideas), are in fact mediated by specialized brain regions. Even high-level perceptions such as tools or vegetables or fruits—which border on being concepts rather than mere perceptions—can be lost selectively depending on the particular small region of the brain that is damaged by stroke or accident.

So what do we know about brain localization? How many specialized regions are there, and how are they arranged? Just as the CEO of a corporation delegates different tasks to different people occupying different offices, your brain parcels out different jobs to different regions. The process begins when neural signals from your retina travel to an area in the back of your brain where the image gets categorized into different simple attributes such as color, motion, form, and depth. After that, information about separate features gets divvied up and distributed to several far-flung regions in your temporal and parietal lobes. For example, information about the direction of moving targets goes to V5 in your parietal lobes. Color information gets sent mainly to V4 in your temporal lobes.

The reason for this division of labor is not hard to divine. The kinds of computation you need for extracting information about wavelength (color) is very different from the computations required for extracting information about motion. It may be simpler to accomplish this if you have separate areas for each task, keeping the neural machinery distinct for economy of wiring and ease of computation.

It also makes sense to organize specialized regions into hierarchies. In a hierarchical system, each “higher” level carries out more sophisticated tasks but, just like in a corporation, there is an enormous amount of feedback and crosstalk. For example, color information processed in V4 gets relayed to higher color areas that lie farther up in the temporal lobes, near the angular gyrus. These higher areas may be concerned with more complex aspects of color processing. The eucalyptus leaves I see all over campus appear to be the same shade of green at dusk as they do midday, even though the wavelength composition of light reflected is very different in the two cases. (Light at dusk is red, but you don’t suddenly see leaves as reddish green; they still look green because your higher color areas compensate.)

Numerical computation, too, seems to occur in stages: an early stage in the fusiform gyrus where the actual shapes of numbers are represented, and a later stage in the angular gyrus concerned with numerical concepts such as ordinality (sequence) and cardinality (quantity). When the angular gyrus is damaged by a stroke or a tumor, a patient may still be able to identify numbers but can no longer divide or subtract. (Multiplication often survives because it is learned by rote.) It was this aspect of brain anatomy—the close proximity of colors and numbers in the brain in both the fusiform gyrus and near the angular gyrus—that made me suspect that number-color synesthesia was caused by crosstalk between these specialized brain areas.

But if such neural cross-wiring is the correct explanation, why does it occur at all? Galton observed that synesthesia runs in families, a finding that has been repeatedly confirmed by other researchers. Thus it is fair to ask whether there is a genetic basis for synesthesia. Perhaps synesthetes harbor a mutation that causes some abnormal connections to exist between adjacent brain areas that are normally well segregated from each other. If this mutation is useless or deleterious, why hasn’t it been weeded out by natural selection?

Furthermore, if the mutation were to be expressed in a patchy manner, it might explain why some synesthetes “cross-wire” colors and numbers whereas others, like a synesthete I once saw named Esmerelda, see colors in response to musical notes. Consistent with Esmerelda’s case, hearing centers in the temporal lobes are close to the brain areas that receive color signals from V4 and higher color centers. I felt the pieces were starting to fall into place.