雅思阅读 72: The Chemical of Wanting（渴望的化学物质）

改编参考 Neuroscience News / ScienceDaily（2025年）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://neurosciencenews.com/dopamine-motivation-29964/

Reading Passage

A. If you ask a person on the street what dopamine does, the answer is almost predictable: it is the "pleasure chemical", the brain's reward juice, released when we eat good food, fall in love or take drugs. This popular image is not entirely wrong, but it is misleading in a way that has distorted how people understand motivation, addiction and even their own behaviour. For the last twenty years, neuroscientists have steadily dismantled the simple story. The neurotransmitter that surges when a reward is anticipated turns out to be less about the enjoyment of the reward itself and more about the drive to pursue it. Dopamine, in the modern view, is less the chemical of pleasure than the chemical of wanting — the signal that tells an animal how hard to work, how eagerly to approach, and when to keep going in the face of distraction. Reframing it this way has changed not only how we explain addiction but how we think about why we persist at anything.

B. The classic experiment that shaped the field goes back to rats that had been taught that a tone predicts the arrival of sugar. If dopamine neurones in a region called the ventral tegmental area were recorded, they did not fire when the rat actually tasted the sugar — they fired when the tone first sounded, the moment the reward was promised. Later work refined the picture. Dopamine signals track the difference between what was expected and what actually arrives: a reward that arrives as expected produces little extra signal, a reward that is better than expected produces a burst, and a reward that fails to arrive produces a dip. This "reward prediction error" became the central theory of dopamine, explaining how animals learn which actions lead to what. It also mapped neatly onto behaviour: actions that consistently over-deliver are repeated, while those that under-deliver fade away. Yet even this elegant model has recently been challenged. Some 2025 studies argue that dopamine's real job is less to report on learning than to control the vigour of behaviour — how hard the animal pushes, how fast it moves — while it is performing a task, rather than while it is learning one.

C. The anatomy matters here. Dopamine neurones originate in the ventral tegmental area and project heavily into a small structure near the front of the brain called the nucleus accumbens, the hub of the so-called reward circuit. Within that hub, two kinds of dopamine-receiving cell play opposing roles. One type, carrying the D1 receptor, tends to drive approach and action, firing more strongly when the animal decides to start a trial and when it expects a big reward. The other, carrying the D2 receptor, seems to fire later, especially when the reward actually arrives. In a striking recent finding, researchers studying fruit flies showed that the very same D2 mechanism long blamed for drug addiction also governs a natural decline in motivation: as rewarding experiences are repeated, the D2 receptors become less sensitive, dopamine acts less effectively, and the animal gives up more easily when distracted. The "addiction" receptor, it turns out, evolved first to make repeated rewards gradually feel less compelling — a built-in way of stopping an animal from endlessly repeating the same act. Without such a dampening mechanism, an animal might fixate on a single easy reward at the expense of everything else it needs to survive.

D. This reframing explains a great deal that the "pleasure chemical" story could not. If dopamine were simply pleasure, then blocking it should feel like cutting off fun; instead, it reduces motivation — the person can still enjoy a meal when it is placed in front of them, but they no longer feel inclined to get up and cook it. Addictive drugs flood the system with unnatural surges, so the brain down-regulates its receptors to compensate, and ordinary rewards — a conversation, a walk, a book — no longer register enough wanting to act on. the ordinary world has gone quiet. The addict does not necessarily take drugs because they are uniquely pleasurable; he takes them because the machinery of wanting has been hijacked, and the ordinary world has gone quiet. This distinction — between liking and wanting — is the single most useful idea the new neuroscience has handed to the treatment room. A 2025 rodent study added another layer: when levels of a protein called KCC2 fall, dopamine neurones fire more rapidly and new reward associations form more readily, suggesting that the brain tunes its own willingness to learn new habits through this delicate molecular dial.

E. The practical stakes are high. If dopamine is about wanting rather than liking, then treatments for addiction, depression and procrastination should target motivation and drive, not the simple search for pleasure. It also offers a gentler way to understand our own lapses: the person who scrolls a phone at midnight is not, in this view, abnormally gluttonous for fun, but caught in a loop where dopamine cues promise a reward that the brain has already grown slightly numb to. The brain's reward system, refined over millions of years to push animals toward scarce food and mates, was never designed for a world engineered to trigger it constantly. The old image of dopamine as a fountain of joy is being replaced by something more sober: a chemical of striving, that pushes us toward what we have learned to want, and that goes quiet when wanting itself becomes too familiar. Understanding this does not make the cravings easier to resist, but it does make them less mysterious: the same circuit that once drove an animal toward scarce food now drives us toward a screen, and it does so without ever asking whether we actually enjoy what it pushes us toward.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. The popular myth versus the modern view ii. How the classic prediction-error model was challenged iii. The two receptor types and a surprising natural mechanism iv. Why the reframing explains addiction and apathy v. The practical stakes for motivation and health vi. The history of sugar in rats vii. How dopamine produces pleasure

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. According to the modern view, dopamine is mainly about A. physical pleasure itself. B. the drive to pursue and obtain rewards. C. digesting food. D. remembering old memories.

6. In the classic rat experiment, dopamine neurones fired most strongly when A. the rat actually tasted the sugar. B. the tone promised the sugar. C. the rat was asleep. D. the sugar was removed.

7. What did the fruit-fly study reveal about D2 receptors? A. They make repeated rewards feel less compelling over time. B. They are unique to humans. C. They increase motivation forever. D. They block all dopamine.

8. Why does the writer say blocking dopamine reduces motivation rather than pleasure? A. A person can still enjoy food already offered, but won't bother to cook it. B. It destroys the sense of taste entirely. C. It makes food taste better. D. It only affects memory.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. The ventral tegmental area projects heavily into the nucleus accumbens.
10. D1 receptors tend to drive approach and action.
11. Addictive drugs make ordinary rewards more rewarding over time.
12. The KCC2 protein has no effect on dopamine neurones.
13. Dopamine neurones are found only in the human brain.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

Dopamine signals a (14) ________ between expectation and reality, but newer work suggests its real role is to control how (15) ________ an animal behaves while performing a task.

答案与解析

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