Approach middle age, and it's hard not to notice that your recall is flick-ering. This, we're reassured, is per-fectly normal--all your friends are complaining about the same thing, aren't they--and yet it doesn't feel normal. Nothing threatens your well-being so much as the feeling that it's at risk. What's more, while most memory loss is normal, at least some people must be part of the un-lucky minority that develops Alzheimer's disease or other forms of dementia. Why not you

人近中年，很難不注意到你的記憶力閃爍不定。我們再三得到保證，這十分正常，幾乎你所有朋友都抱怨同樣的事，不是嗎？但感覺上還是不太正常。沒有任何東西對你的健康所構成的威脅，比覺得你的心智已經亮起紅燈還大。尤有甚者，雖然大部分記憶流失是正常的，但至少有些人是不幸得到老人失智症或其他形式癡呆症的少數人。為什麼不是你？

Alzheimer's is expected to strike 34 million people globally by 2025 and 14 million in the U.S. alone over the next 40 years. Half of all people who reach age 85 will exhibit symptoms of the disease. That, however, means that the other half won't. And since average U.S. life expectancy currently tops out at 80.4 for women and only 75.2 for men, by the time your 85th birthday rolls around, you're not like-ly to be troubled by Alzheimer's dis-ease--or anything else.

到2025年，預料全球會有3千4百萬人得老人失智症，光是在美國，未來40年就有1千4百萬失智症患者。到85歲時，有一半人口會出現這種病症，但這至少意味另一半人不會罹病。而由於美國平均預期壽命（或譯平均餘命）目前女性超過80.4歲，男性只有75.2歲。所以等你要過85大壽時，已經不可能被老人失智症或任何其他疾病困擾了。

Still, that doesn't make it any easier when you forget to pick up the dry cleaning or fumble to recall familiar addresses. The good news is, science is as interested in what's going on as you are. With better scanning equipment and knowledge of brain structure and chemistry, investigators are steadily improving their understanding of how memory works, what makes it fail, how the problems can be fixed--and when they can't.

但當你忘了拿乾洗衣物或七拼八湊想不起來熟悉的地址時，還是讓人很不安。好消息是，科學跟你一樣，對你到底出了什麼問題很感興趣。有了更好的掃描儀和有關腦結構及化學作用的知識，研究人員正不斷改善他們對記憶如何運作的了解，是什麼使它失靈，問題如何解決，及他們能做到什麼地步。

For most people, all this will mean reassurance as worrisome symptoms turn out to be nothing at all. "Normal is the new frontier," says Mony de Leon, director of the Center for Brain Health at New York University Tisch Hospital. And for those who do drift beyond that frontier, the same re-search may offer new hope for treat-ments and even cures.

對大部分人，所有這一切意味再度獲得保證，那些令人憂心的徵狀，結果根本沒什麼大不了。「正常是新疆界，」紐約大學提許醫院腦健康中心主任莫妮．德里昂說。對那些已經漂泊遊蕩過那道邊界的病患，同樣的研究可能可以提出新療法，甚至療程。

Consider, for a moment, how mem-ory is supposed to operate. Consider, that is, the hippocampus. A cashew-shaped node of tissue, the hip-pocampus sits deep in the temporal lobe of the brain, near the amygdala, which is the seat of emotions. If the brain has a gatekeeper of sensory information, the hippocampus is it. The aroma and sizzle of bacon frying, the smooth finish of polished granite, a phone number you need to call--all must pass through the hippocampus. Only if information gets in can it be moved along to the pre-frontal cortex, where it will be held briefly in what is called working--or short-term--memory. When you look up the phone number, dial it and promptly forget it, that's your prefrontal cortex working in tandem with your hippocampus.

暫時想想，記憶應該如何運作。想想海馬迴，一個形似腰果的組織結，這個海馬迴坐在腦顳葉深處，靠近杏仁核，主管情緒的區塊。如果腦部對感覺訊息有個守門人，則非海馬迴莫屬。不管是煎烤煙燻豬肉的香味和嘶嘶聲，光亮圓滑的花崗石，或一個你必須要打的電話號碼，統統都必須通過海馬迴。唯有當資訊進門後，才能進繼續前進到前額葉皮質，它會短暫留在這裡，這就叫工作記憶或短期記憶。當你找到電話號碼，撥打後很快又忘了，那就是你的前額葉皮質正和海馬迴協同運作。

But let's say you hang on to the number 10 minutes or even 10 months later. Why Because that bit of information has gone through a chemical process called long-term potentiation (LTP) that strengthens the synapses. You need LTP to form long-term memories. And LTP takes place in the hippocampus.

但如果你記得這個電話號碼十分鐘，甚至十個月，那又是為什麼？那是因為那一部分訊息已經經過一個化學進程叫「長期增益作用」（簡稱LTP），可以強化神經元的軸突接觸。要形成長期記憶，一定要有「長期增益作用」，而這個作用就發生在海馬迴。

The hippocampus begins to mal-function early in Alzheimer's disease. Imaging studies have shown that peo-ple with Alzheimer's typically have smaller than average hippocampi. Meanwhile, as the hippocampus is shrinking, the pathway between it and the prefrontal cortex also begins to de-grade. Signals peter out and fade away, and questions take their place: Do I know you Who am I But it's not just with Alzheimer's: the hippocampus also goes at least somewhat awry in normal memory loss. "It's relatively stable in volume till about 60," Har-vard neuroscientist Randy Buckner explains, "and then begins to change. People with Alzheimer's disease, though--they slide off the cliff."

海馬迴在羅患老人失智症初期，就開始機能不全。腦部影像的研究顯示，得到老人失智症的人通常海馬迴會小於平均值。同時，隨著海馬迴縮小，它和前額葉皮質之間的通道也開始故障弱化。訊息漸漸耗損消逝，取而代之的是一大堆問題：我認識你嗎？我是誰？但這不只發生在老人失智症上：海馬迴在正常記憶流失上，多少有點未能正常運作。「通常到大約60歲，腦容量還相當穩定正常，」哈佛腦神經科學家巴克納解釋說，「但隨後就開始改變。得到老人失智症者，則有如掉落懸崖，急轉直下。」

But don't pin all the blame for your memory loss on your hippocampus. As people get older, their attention starts to flicker, and that plays a role of its own. The prefrontal cortex, which controls planning, organization, ab-straction and forethought, is the same region that allows us to concentrate, and it starts to diminish in size well before middle age. It also begins to use the brain's fuel, glucose, less effi-ciently and loses about half the neu-rotransmitter dopamine it once had. The result of all this, says Amy Arn-sten, a neurobiologist at Yale Medical School, is that as we get older, we get "ADHD, but it's attention-deficit hypoactivity--not hyperactivity."

但不要把記憶流失全怪到你的海馬迴上。人越老，注意力越不容易集中，這本身也會產生影響。控制計畫、組織、抽象概念和深謀遠慮的前額葉皮質，也讓我們得以集中注意力，但它早在人到中年之前就開始變小。它使用腦部燃炓葡萄醣的效能也開始降低，並損失約一半過去原本有的神經傳導物質多巴胺。結果，誠如耶魯大學醫學院神經生物學家艾美．阿尼斯登所說，這一切導致我們越老，越容易羅患ADHD，這可不是通常所說的「注意力不足過動症」，而是「注意力不足不動症」。

（取材自紐約時報）