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.

大脑能够储存巨大数量的记忆，那么为什么在需要的时候我们会回忆不起来呢？ 一项新的研究为这一问题提供了见解。

作者Edward K. Vogel and Trafton Drew

我们的大脑里填满了在我们整个人生经历中形成的大量记忆。 这些记忆可以很深奥（我是谁还有我怎么来到这的？）也可以是最琐碎的（红灯停车时看到的某辆车的牌照）。 此外，我们记忆的准确度在相当程度上也存在着分别。 打个比方，父母们大多都明白在给他们的孩子买一份生日礼物时，如果记忆不清楚会有什么不良后果。 记得他们的孩子更想要的是耍功夫把式的G.I. Joe（眼镜蛇部队），而非普通的G.I. Joe，会显著影响孩子接受礼物的乐意程度。 因此，在日常生活的大多数情况下，我们记忆的“模糊性”与一开始就能够记住很多很多信息是同等重要的。

不同类型的记忆有不同的细节水平？

过去的几十年中，认知心理学家们已经确定，在人类大脑中主要存在着两种记忆系统。 一种短期的，或者“工作”的记忆短暂地储存了我们正在想着的少数事情的信息；另一种长期的记忆能够储存通过一生思考和经历得到的海量信息。 在提供细节的水平上，这两种记忆也被认为是不同的。 工作记忆给出的是有关我们当前正在思考的少数事情的清晰细节，而长期记忆给出的则是有关我们曾经见过或经历过的大量事情的，更加模糊的图像。 这就是说，虽然我们在长期记忆中储存了很多东西，但其记忆细节并不总是清晰透彻，而是经常只限于我们所见或已发生事情的要点梗概。

然而，在最近发表的一项研究中，麻省理工学院认知神经学家蒂莫西·布莱迪及其同事们提出，这些长期记忆并非如之前所认为的那么模糊。 研究中，研究人员要求实验者们尝试记住3000幅普通物件的图片——包括如背包、遥控器和烤面包片机等物品——一张每次就看几秒钟。 观看环节结束时，研究人员考察了实验者的记忆：向他们展示了两个物品，并问哪一个是之前见过的。 毫无意外，实验者们的表现格外优秀（超过90%正确），尽管要记住的有数千件物品。 如此高的成功率应证了长期记忆的巨大存储能力。 然而，最出乎意料的是，实验者们的这些记忆中表现出来的令人惊异的细节水平。 实验者们在区分两张同种物品的图片之间的差别上做的相当不错，即便这种区别极其细微，例如两个只在所放面包薄片上差别微小的烤面包片机。

如果并不模糊，为什么我们仍然忘记事情？

我们在长期记忆中储存的大量信息归根到底并不是那么不确定的，这项新研究为此提供了引人注目的证据。 看起来，我们对曾见过事情的再现实际上是相当详细和精确的。

当然，这项发现也提出了显而易见的问题。 如果我们的记忆并非都那么模糊，那为什么我们老是忘记我们所要记住事物的细节呢？ 一个解释是，尽管大脑保存了大量不同事件和物品的细节表象，但我们并不是总能在需要的时候把那信息找出来。 就如这次研究所显示的，如果给我们看一个物品，我们大多能够非常精确、准确无误地说出我们之前是否见过它。 然而如果当我们身在玩具店，试着记起儿子想要的生日礼物是什么样的时候，我们就需要能够自动地在记忆中搜索正确答案——而不是受到视觉上能提醒你的物品的推动。 看来，似乎是这个自动的搜索机制使我们易于受到干扰和健忘。 至少这也是我们没有带着耍功夫的眼镜蛇部队回家的一个借口。