Passage 2        With every whiff you take as you walk by a bakery, a cloud of chemicals comes swirling upyour nose. Identifying the smell as freshly baked bread is a complicated process. But, compared tothe other senses, the sense of smell was often underappreciated. Recently, scientists studying olfactionhave shed new light on how our sense of smell works and provided compelling evidence that it’smore sophisticated than previously thought.        In a recent survey of 7,000 young people around the world, about half of those between theage of 16 and 30 said that they would rather lose their sense of smell than give up access to technologylike laptops or cell phones. So, what do we know about the sense of smell?  The Nose Knows        Smell begins at the back of nose, where millions of sensory neurons lie in a strip of tissuecalled the olfactory epithelium. The tips of these cells contain proteins called receptors that bind odormolecules. The receptors are like locks and the keys to open these locks are the odor molecules thatfloat past, explains Leslie Vosshall, a scientist who studies olfaction at Rockefeller University.         People have about 450 different types of olfactory receptors. Each receptor can be activatedby many different odor molecules, and each odor molecule can activate several different types ofreceptors. However, the forces that bind receptors and odor molecules can vary greatly in strength,so that some interactions are better “fits” than others.       “Think of a lock that can be opened by 10 different keys. Two of the keys are a perfect fit andopen the door easily. The other eight don’t fit as well, and it takes more jiggling to get the door open,”explains Vosshall.      The complexity of receptors and their interactions with odor molecules are what allow us todetect a wide variety of smells. And what we think of as a single smell is actually a combination ofmany odor molecules acting on a variety of receptors, creating an intricate neural code that we canidentify as the scent of a rose or freshly-cut grass.  Odors in the Brain       This neural code begins with the nose’s sensory neurons. Once an odor molecule binds to areceptor, it initiates an electrical signal that travels from the sensory neurons to the olfactory bulb, astructure at the base of the forebrain that relays the signal to other brain areas for additional processing.        One of these areas is the piriform cortex, a collection of neurons located just behind theolfactory bulb that works to identify the smell. Smell information also goes to the thalamus, a structurethat serves as a relay station for all of the sensory information coming into the brain. The thalamustransmits some of this smell information to the orbitofrontal cortex, where it can then be integratedwith taste information. What we often attribute to the sense of taste is actually the result of this sensoryintegration.       “The olfactory system is critical when we’re appreciating the foods and beverages weconsume,” says Monell Chemical Senses Center scientist Charles Wysocki. This coupling of smelland taste explains why foods seem lackluster with a head cold.       You’ve probably experienced that a scent can also conjure up emotions and even specificmemories, like when a whiff of cologne at a department store reminds you of your favorite uncle whowears the same scent. This happens because the thalamus sends smell information to the hippocampusand amygdala, key brain regions involved in learning and memory.  A Better Smeller        Although scientists used to think that the human nose could identify about 10,000 differentsmells, Vosshall and her colleagues have recently shown that people can identify far more scents.Starting with 128 different odor molecules, they made random mixtures of 10, 20, and 30 odormolecules, so many that the smell produced was unrecognizable to participants. The researchers thenpresented people with three vials, two of which contained identical mixtures while the third containeda different concoction, and asked them to pick out the smell that didn’t belong.Predictably, the more overlap there was between two types of mixtures, the harder they wereto tell apart. After calculating how many of the mixtures the majority of people could tell apart, theresearchers were able to predict how people would fare if presented with every possible mixture thatcould be created from the 128 different odor molecules. They used this data to estimate that theaverage person can detect at least one trillion different smells, a far cry from the previous estimate of10,000.The one trillion is probably an underestimation of the true number of smells we can detect,said Vosshall, because there are far more than 128 different types of odor molecules in the world.         No longer should humans be considered poor smellers. In fact, many recent studies haveshown that our noses can outperform our eyes and ears, which can discriminate between severalmillion colors and about half a million tones.
【題組】46. Which of the following statement is true?[!--empirenews.page--]
(A) The view that our noses may play a more important role than our ears and eyes has neverbeen empirically established by any research.
(B) All young people would undoubtedly prefer losing access to cellphones over losing theirsense of smell.
(C) Humans can detect different scents because we have a variety of odor receptors, throughwhich signals of the sensory neurons are transmitted to the base of the forebrain and thento other parts of the brain.
(D) We can compare odor molecules to a lock, and (odor) receptors can be referred to as keysthat are used to open the lock.