Your Beautiful Brain

Dispatches from the frontiers of neuroscience.

by Bill Retherford '14JRN Published Winter 2016
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In some ways, human brains and bird brains are unnervingly alike. “As we study the auditory cortex of the zebra finch, we find similarity after similarity after similarity after similarity,” Woolley says. If she can identify those neurons in the baby zebra finch, then Woolley can predict the approximate location of the comparable human neurons. “I can map my bird’s neurons onto a mammal’s neurons,” she says, “and thus onto a human’s neurons.” Someday, Woolley’s displaced baby songbirds might help millions of autistic children reconnect.

Charles Zuker and his research lab plan to map the taste and thirst neurons in the human brain. / Photograph by John Abbott

For decades, scientists readily swallowed the notion that a “taste map” partitions your tongue
— sweet at the front, salty at the sides. “It’s all incorrect,” says Charles Zuker, the Columbia neuroscientist, who has spent the last fifteen years studying how we perceive taste. “There’s no taste map.” Instead, he says, thousands of taste buds are scattered around your tongue, with sweet, salty, sour, bitter, and umami receptor cells throughout.

Nor do our taste buds actually decide how the food tastes. They do the detection work, definitely, but they serve primarily as relays, dispatching signals directly to the brain. “Sweet taste cells in the tongue talk to sweet neurons,” says Zuker. “Salty to salty. Bitter to bitter.” Within those micro-groups of neuronal constellations, the taste is given a definition. That’s how you know the difference between strudel and sauerkraut.

When humans are hungry, or thirsty, those neurons will ping us to eat something, or to get a glass of water. “Evolution is smart. Clean, clear, and simple,” says Zuker. “This is what innate hardwired circuits are all about.” Now Zuker and his lab of twenty-two researchers want to map precisely where the taste and thirst neurons are located in the human brain. Finding them could lead to clues in controlling our cravings. In research with mice, Zuker’s team shined a fiber-optic light over their thirst neurons. The mice instantly sprinted to the water spout. “Even if the mouse is not thirsty, the mouse will think it’s thirsty, and look for water to drink,” he says. “Isn’t that remarkable?”

The same seems to apply to taste. The messages from the mouse tongue travel directly to its taste neurons. Just as in humans, those nerve cells are dedicated strictly to the five basic taste qualities. Activate the bitter neurons while a mouse drinks regular water, and it’s repelled. (The mouse squints, shudders, and jiggles its head, just like someone who bit into a lemon.) But silence the bitter neurons, and the mouse will slurp bitter liquid.

The inferences are dumbfounding. Could physicians someday manipulate neurons to regulate diet, consumption, and sugar cravings — perhaps with a pill? “There are amazing implications,” says Zuker. “I think the field is poised to do something very special.” Then, reining it in: “There are challenges — making sure [a pill] acts on the right group of cells, that it targets the right circuit.” And a reminder: “We are still doing basic neuroscience. We are still at the stage of uncovering fundamental logic and principles.” Yet from his lab’s ever-accumulating data, one can extrapolate the prospective human applications — controlling anorexia, obesity, and diabetes.

More than one-third of adult Americans today are obese, and at increased risk for heart disease, stroke, and cancer. Thirty million Americans have diabetes, and three hundred thousand die from it annually. Overeating and excessive sugar consumption are the causes of both obesity and diabetes. Finding a way to govern them with pharmaceuticals would be a miracle. “And now we can begin to ask,” says Zuker, “if we can control feeding and sugar craving to make a meaningful difference. I believe the answer will be yes.”

The decades-old “left brain–right brain” paradigm,
although not completely discarded by researchers, now survives considerably diminished, a moldy scientific chestnut (left-brainers, supposedly, are analytical and good at math; right-brainers, emotional and hyper-imaginative). “There is some truth to it,” says Randy Bruno, a CUMC associate professor of neuroscience. “But not all functions are completely one side or another. Some things are not lateralized at all.” Instead, Bruno’s research reveals something much more tantalizing: “What we’re working on now is top brain and bottom brain.”

For more than twenty years, Bruno has been investigating the cerebral cortex, an outer sliver of brain barely thicker than a credit card and critical for higher-order functions like perception and attention. In mammals, the cortex envelops nearly the entire organ, and divides into “upper” and “deeper” layers. Our deeper layers, evolutionarily older, faintly evoke the reptilian brain. Indeed, today’s alligators, turtles, and snakes have only the lower layers. “There’s a really good reason for why mammals developed the upper layers,” says Bruno. “But I don’t know what the answer is.”

Randy Bruno is investigating why mammals have "upper" brain layers that reptiles lack. / Photograph by John Abbott

Neuroscientists long assumed the upper cortex transmitted its sensory data — that’s everything you see, hear, smell, taste, and feel — directly to the deeper cortex. Without that, researchers believed, the lower region in mammals would never detect an outside world. But in 2013, Bruno and his team shut off the upper cortex in a mouse. What happened (or what didn’t happen) startled everyone, not just in Bruno’s lab, but in the scientific community worldwide.

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