How does food's appearance or smell influence the way it tastes?

Dana M. Small, a neuroscientist at the John B. Pierce Laboratory and the Yale School of Medicine, replies:

What the brain perceives as flavor is actually a fusion of a food's taste, touch and smell into a single sensation—each not only influences flavor but is an integral part of it. Sight, though not technically part of this equation, certainly influences perception in its own way.

The flavor we sense from food or drink depends partly on which taste buds it activates: sweet, sour, salty, bitter, savory or, debatably, fat. Sensory cells, located side by side with the taste buds, allow us to perceive qualities such as temperature, spiciness and creaminess. Smells also seem to come from the mouth, even though there are no cells there responsible for detecting scents. Instead the sensation depends on activation of cells located at the end of the nasal passage. The information gathered by these cells is relayed to the mouth via a process called olfactory referral.

Acquiring information related to scent through the back of the mouth is called retronasal olfaction—via the nostrils it is called orthonasal olfaction. Both methods influence flavor; aromas such as vanilla, for example, can cause something perceived as sweet to taste sweeter.

To demonstrate the phenomenon of olfactory referral for yourself, hold your nose and chew on a strawberry jelly bean. You should detect sweetness and a little sourness, along with the hard (and then soft) feeling of the candy, but you will not detect the strawberry flavor. When you let go of your nose, though, the odor-bearing molecules can travel through the nasal cavity to the smell cells, and suddenly the flavor of the jelly bean reveals itself.

Although sight plays a less direct role than smell in the perception of flavor, it is the sense most often used to identify food, and it thereby affects expectations of the food's attributes. In a classic experiment, researchers in France colored a white wine red with an odorless dye and asked a panel of experts to describe its taste. The connoisseurs used typical red wine descriptors to characterize the colored wine rather than terms they would use to evaluate white wine, suggesting that the appearance of the beverage played a significant role in the way they perceived its flavor.

How do earthquakes come to a halt?

David D. Oglesby, a geophysicist at the University of California, Riverside, offers a response:

Simply put, an earthquake comes to a stop when the pent-up energy propelling the temblor can no longer overcome the friction holding it back.

To understand this braking process, it is important to comprehend first what makes an earthquake go. Rocks clustered around a fault—a fracture separating two blocks of rock—are held in place by friction. As time goes by, the movement of continent-size blocks of the earth's outer shell, known as tectonic plates, causes the rocks along the fault to bend and stretch, infusing them with energy like compressed springs.

When this pent-up force exceeds the friction holding the rocks, a crack on the fault forms and grows, and the fault begins to slip. This slippage releases part of the rocks' built-up energy, radiating seismic waves that travel to the earth's surface, where they can cause considerable damage.

Once the earthquake runs out of energy or encounters a sufficiently large increase in friction, it will stop. A quake can be halted in its tracks, for instance, if it hits material that does not slide as easily—that has more friction—like a skier gliding from snow onto dirt. Reaching a site where another earthquake recently occurred can also stop a temblor, because such areas do not have enough built-up energy. Or a fault might simply end; the amount of energy needed to cut a new fault through intact rock is far greater than that required to break an existing fault.

But earthquakes can also jump from one fault to another, often from as far as four kilometers away. So just as you never know when a big one will hit, it is hard to say when—or where—it might stop.

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