時尚雙語:我們爲什麼總是健忘

The brain can store a vast number of memories, so why can't we find these memories when we need to? A new study provides insights into this question.

By Edward K. Vogel and Trafton Drew

Our brains are crammed with a massive amount of memories that we have formed over a lifetime of experiences. These memories range from the profound (who am I and how did I get here?) to the most trivial (the license plate of the car at a stoplight). Furthermore, our memories also vary considerably in their precision. Parents, for instance, often know the perils of a fuzzy memory when shopping for a birthday gift for their child: remembering that their son wanted the G.I. Joe with Kung Fu Grip rather than the regular G.I. Joe could make an enormous difference in how well the gift is received. Thus, the “fuzziness” of our memory can often be just as important in our daily lives as being able to remember lots and lots of information in the first place.

Different Levels of Detail for Different Types of Memory?
In the past several decades, cognitive psychologists have determined that there are two primary memory systems in the human mind: a short-term, or “working,” memory that temporarily holds information about just a few things that we are currently thinking about; and a long-lasting memory that can hold massive amounts of information gained through a lifetime of thoughts and experiences. These two memory systems are also thought to differ in the level of detail they provide: working memory provides sharp detail about the few things we are presently thinking about, whereas long-term memory provides a much fuzzier picture about lots of different things we have seen or experienced. That is, although we can hold lots of things in long-term memory, the details of the memory aren’t always crystal-clear and are often limited to just the gist of what we saw or what happened.

A recently published study by Timothy F. Brady, a cognitive neuroscientist at the Massachusetts Institute of Technology, and colleagues suggests that these long-term memories may not be nearly as fuzzy as once thought, however. In their work, the researchers asked subjects to try to remember 3,000 pictures of common objects—including items such as backpacks, remote controls and toasters—that were presented one at a time for just a few seconds each. At the end of this viewing phase, the researchers tested subjects’ memory for each object by showing them two objects and asking which one they had seen before. Not surprisingly, subjects were exceptionally good (more than 90 percent correct) even though there were thousands of objects to remember. This high success rate attests to the massive storage ability of long-term memory. What was most surprising, however, was the amazing level of detail that the subjects had for all of these memories. The subjects were just as good at telling the difference between two pictures of the same object even when the objects differed in an extremely subtle manner, such as a pair of toasters with slightly different slices of bread.

If It’s Not Fuzzy, Why Do We Still Forget Things?
This new work provides compelling evidence that the enormous amount of information we hold in long-term memory is not so uncertain after all. It seems that we actually hold representations of things we’ve seen in a fairly detailed and precise form.

Of course, this finding raises the obvious question: if our memories aren’t all that fuzzy, then why do we often forget the details of things we want to remember? One explanation is that, although the brain contains detailed representations of lots of different events and objects, we can’t always find that information when we want it. As this study reveals, if we’re shown an object, we can often be very accurate and precise at being able to say whether we’ve seen it before. If we’re in a toy store and trying to remember what it was that our son wanted for his birthday, however, we need to be able to voluntarily search our memory for the right answer—without being prompted by a visual reminder. It seems that it is this voluntary searching mechanism that’s prone to interference and forgetfulness. At least that’s our story when we come home without the Kung Fu Grip G.I. Joe.
Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist.

大腦能夠儲存巨大數量的記憶，那麼爲什麼在需要的時候我們會回憶不起來呢？ 一項新的研究爲這一問題提供了見解。

作者Edward K. Vogel and Trafton Drew

我們的大腦裏填滿了在我們整個人生經歷中形成的大量記憶。 這些記憶可以很深奧（我是誰還有我怎麼來到這的？）也可以是最瑣碎的（紅燈停車時看到的某輛車的牌照）。 此外，我們記憶的準確度在相當程度上也存在着分別。 打個比方，父母們大多都明白在給他們的孩子買一份生日禮物時，如果記憶不清楚會有什麼不良後果。 記得他們的孩子更想要的是耍功夫把式的G.I. Joe（眼鏡蛇部隊），而非普通的G.I. Joe，會顯著影響孩子接受禮物的樂意程度。 因此，在日常生活的大多數情況下，我們記憶的“模糊性”與一開始就能夠記住很多很多信息是同等重要的。

不同類型的記憶有不同的細節水平？

過去的幾十年中，認知心理學家們已經確定，在人類大腦中主要存在着兩種記憶系統。 一種短期的，或者“工作”的記憶短暫地儲存了我們正在想着的少數事情的信息；另一種長期的記憶能夠儲存通過一生思考和經歷得到的海量信息。 在提供細節的水平上，這兩種記憶也被認爲是不同的。 工作記憶給出的是有關我們當前正在思考的少數事情的清晰細節，而長期記憶給出的則是有關我們曾經見過或經歷過的大量事情的，更加模糊的圖像。 這就是說，雖然我們在長期記憶中儲存了很多東西，但其記憶細節並不總是清晰透徹，而是經常只限於我們所見或已發生事情的要點梗概。

然而，在最近發表的一項研究中，麻省理工學院認知神經學家蒂莫西·布萊迪及其同事們提出，這些長期記憶並非如之前所認爲的那麼模糊。 研究中，研究人員要求實驗者們嘗試記住3000幅普通物件的圖片——包括如揹包、遙控器和烤麪包片機等物品——一張每次就看幾秒鐘。 觀看環節結束時，研究人員考察了實驗者的記憶：向他們展示了兩個物品，並問哪一個是之前見過的。 毫無意外，實驗者們的表現格外優秀（超過90%正確），儘管要記住的有數千件物品。 如此高的成功率應證了長期記憶的巨大存儲能力。 然而，最出乎意料的是，實驗者們的這些記憶中表現出來的令人驚異的細節水平。 實驗者們在區分兩張同種物品的圖片之間的差別上做的相當不錯，即便這種區別極其細微，例如兩個只在所放麪包薄片上差別微小的烤麪包片機。

如果並不模糊，爲什麼我們仍然忘記事情？

我們在長期記憶中儲存的大量信息歸根到底並不是那麼不確定的，這項新研究爲此提供了引人注目的證據。 看起來，我們對曾見過事情的再現實際上是相當詳細和精確的。

當然，這項發現也提出了顯而易見的問題。 如果我們的記憶並非都那麼模糊，那爲什麼我們老是忘記我們所要記住事物的細節呢？ 一個解釋是，儘管大腦保存了大量不同事件和物品的細節表象，但我們並不是總能在需要的時候把那信息找出來。 就如這次研究所顯示的，如果給我們看一個物品，我們大多能夠非常精確、準確無誤地說出我們之前是否見過它。 然而如果當我們身在玩具店，試着記起兒子想要的生日禮物是什麼樣的時候，我們就需要能夠自動地在記憶中搜索正確答案——而不是受到視覺上能提醒你的物品的推動。 看來，似乎是這個自動的搜索機制使我們易於受到干擾和健忘。 至少這也是我們沒有帶着耍功夫的眼鏡蛇部隊回家的一個藉口。