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How memories form and fade

記憶如何形成并消失

上海譯銳翻譯   2019-9-10     13:20

Strong memories are encoded by teams of neurons working together in synchrony

深刻的記憶由許多組協同工作的神經元進行編碼

Why is it that you can remember the name of your childhood best friend that you haven't seen in years yet easily forget the name of a person you just met a moment ago? In other words, why are some memories stable over decades, while others fade within minutes?

為什么即使多年沒有見面，你也能夠記得兒時最好朋友的名字？為什么和那個人見面才沒多久，你就已經忘了他叫什么？換句話說，為什么有些記憶能夠在數十年中留存，為什么有些記憶在幾分鐘內就會消失？

Using mouse models, Caltech researchers have now determined that strong, stable memories are encoded by "teams" of neurons all firing in synchrony, providing redundancy that enables these memories to persist over time. The research has implications for understanding how memory might be affected after brain damage, such as by strokes or Alzheimer's disease.

加州理工大學的研究人員通過利用老鼠模型發現，強烈持久的記憶由數組同時放電的神經元進行編碼，這一過程會提供冗余并能夠使這些記憶留存下來。研究有助于我們理解大腦在受到損害，比如中風或阿茲海默癥后，大腦中的記憶可能會受到影響。

The work was done in the laboratory of Carlos Lois, research professor of biology, and is described in a paper that appears in the August 23 of the journal Science. Lois is also an affiliated faculty member of the Tianqiao and Chrissy Chen Institute for Neuroscience at Caltech.

這項工作在生物研究教授Carlos Lois的實驗室內進行，研究工作將以論文形式在8月23日的期刊《科學》中發表。Lois同時也是加州理工大學陳天橋雒芊芊腦科學研究院的附屬教職人員。

Led by postdoctoral scholar Walter Gonzalez, the team developed a test to examine mice's neural activity as they learn about and remember a new place. In the test, a mouse was placed in a straight enclosure, about 5 feet long with white walls. Unique symbols marked different locations along the walls -- for example, a bold plus sign near the right-most end and an angled slash near the center. Sugar water (a treat for mice) was placed at either end of the track. While the mouse explored, the researchers measured the activity of specific neurons in the mouse hippocampus (the region of the brain where new memories are formed) that are known to encode for places.

在博士后Walter Gonzalez的帶領下，研究團隊制定出一個實驗，以測定老鼠在了解并記住一個新地方后的神經活動。在實驗中，老鼠被放在一個直筒型的圍場中，圍場四周為白色的墻，長度大約為5英尺。墻上獨特的符號代表不同的位置-比如，圍場最右邊的盡頭附近有粗體的加號，靠近中心位置則是帶一定角度的斜線。糖水（給老鼠享用）被放在通道的任何一頭。當老鼠開始探索時，研究人員測量老鼠海馬體（大腦中形成新記憶的區域并會對地點進行編碼）中特定神經元的活動。

hippocampus：海馬體

When an animal was initially placed in the track, it was unsure of what to do and wandered left and right until it came across the sugar water. In these cases, single neurons were activated when the mouse took notice of a symbol on the wall. But over multiple experiences with the track, the mouse became familiar with it and remembered the locations of the sugar. As the mouse became more familiar, more and more neurons were activated in synchrony by seeing each symbol on the wall. Essentially, the mouse was recognizing where it was with respect to each unique symbol.

當老鼠被第一次放入后，它并不知道要做什么，它會朝左邊和右邊漫無目的地亂走直到遇見糖水。在這種情況下，當老鼠注意到墻壁上的符號時，單個的神經元就被激活了。但是，隨著在通道內多次實踐，老鼠開始熟悉這里并記住了糖水的位置。隨著老鼠對周圍變得越來越熟悉，當老鼠看到墻壁上的每個符號后，越來越多的神經元被同時激活。最后，老鼠會根據單獨的符號認出自己所在的位置。

To study how memories fade over time, the researchers then withheld the mice from the track for up to 20 days. Upon returning to the track after this break, mice that had formed strong memories encoded by higher numbers of neurons remembered the task quickly. Even though some neurons showed different activity, the mouse's memory of the track was clearly identifiable when analyzing the activity of large groups of neurons. In other words, using groups of neurons enables the brain to have redundancy and still recall memories even if some of the original neurons fall silent or are damaged.

為了研究記憶如何隨著時間慢慢淡化，研究人員將老鼠與通道隔離多達20天。在經過隔離并再次返回通道后，老鼠憑借由更多神經元所編碼的深刻記憶迅速回憶起了任務。即使某些神經元顯出不同的活性，當分析大量神經元的活性時，老鼠有關通道的記憶很容易辨別。換句話說，使用大量神經元能使大腦產生冗余并仍舊能夠在某些原始神經元沉寂或受損后喚起記憶。

Gonzalez explains: "Imagine you have a long and complicated story to tell. In order to preserve the story, you could tell it to five of your friends and then occasionally get together with all of them to re-tell the story and help each other fill in any gaps that an individual had forgotten. Additionally, each time you re-tell the story, you could bring new friends to learn and therefore help preserve it and strengthen the memory. In an analogous way, your own neurons help each other out to encode memories that will persist over time."

Gonzalez解釋道：“設想你要講一個又長又復雜的故事。為了記住這個故事，你可以向5位朋友講這個故事，然后把5位朋友聚在一起并把這個故事再講一遍。當忘記故事中的某個細節時，朋友可以相互幫助把這個細節補充完整。此外，每次你重新講這個故事時，你可以請新朋友來聽這個故事并從而保留并強化記憶。通過類比的方式，你自己的神經元可以通過互相幫助來編碼記憶并使其長期保留下來。”

Memory is so fundamental to human behavior that any impairment to memory can severely impact our daily life. Memory loss that occurs as part of normal aging can be a significant handicap for senior citizens. Moreover, memory loss caused by several diseases, most notably Alzheimer's, has devastating consequences that can interfere with the most basic routines including recognizing relatives or remembering the way back home. This work suggests that memories might fade more rapidly as we age because a memory is encoded by fewer neurons, and if any of these neurons fail, the memory is lost. The study suggests that one day, designing treatments that could boost the recruitment of a higher number of neurons to encode a memory could help prevent memory loss.

記憶對于人類行為至關重要，以至于任何記憶損害都會嚴重影響我們的日常生活。隨著步入老年而出現的記憶缺失是老年人所面對的重大障礙。此外，由于數種疾病所導致的記憶缺失，最為人們熟知的就是阿茲海默癥，會產生破壞性極大的后果，這一后果會影響最基本的日常行為，包括認識家人或記住回家的路。這一方式表明，隨著我們步入老年，記憶可能會迅速衰退，因此對記憶進行編碼的神經元變得更少了。此外，如果任何神經元無法工作，記憶就丟失了。研究表明，某一天，通過一種可以促進大量神經元加入并對記憶進行編碼的治療能幫助人們預防失憶。

"For years, people have known that the more you practice an action, the better chance that you will remember it later," says Lois. "We now think that this is likely, because the more you practice an action, the higher the number of neurons that are encoding the action. The conventional theories about memory storage postulate that making a memory more stable requires the strengthening of the connections to an individual neuron. Our results suggest that increasing the number of neurons that encode the same memory enables the memory to persist for longer."

“多年來，人們已經知道，對于某件事，你做得越多，你就越有可能在后面也記得。現在，我們認為，這很可能是因為，當你做得越多，就會有越多的神經元對行為進行編碼。傳統的關于記憶存儲的理論假設，要想使記憶更加持久，需要加強與單個神經元之間的聯系。我們的結果表明，編碼相同記憶的神經元數量越多，記憶就會越持久。”

來源：科學日報   編輯：Susan