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How adult brains learn the new without forgetting the old
成人大脑如何学新而不忘旧
Learning new things is hard. Remembering what has already been learned is harder.
学习新知识不易，记住旧知识就更难了。
Any successful learning system, be it a brain or a piece of artificial-intelligence software, must strike the right balance between stability and flexibility.
无论是大脑还是人工智能软件，任何成功的学习系统都必须在稳定性和灵活性之间取得恰到好处的平衡。
It must be stable enough to remember important old things yet flexible enough to learn new ones without destroying old memory traces--preferably for as long as it exists.
这样的学习系统必须足够稳定，能够记住重要的旧知识，同时也需要相当灵活，以便在不破坏旧记忆痕迹的情况下学习新知识----最好在整个生命周期都能保持这种平衡。
Learning is a result of changes in the pattern of neural connectivity in the brain. Each connection between nerve cells, called a synapse, is a tiny gap between the ends of branches ramifying from such cells.
学习是大脑神经连接模式变化的结果。神经细胞之间的各个连接被称为突触，也就是这些细胞分枝末端之间的微小间隙。
Messages jump across these gaps in the form of molecules called neurotransmitters. Current estimates suggest there are 600 trillion synapses in a human brain. How, then, to deal with the stability-plasticity dilemma--particularly as brains age and, as it were, fill up?
信息以一种被称为"神经递质"的分子形式穿越这些间隙。目前据估计，人类大脑中有600万亿个突触。那么，我们该如何处理大脑稳定性和可塑性之间的两难呢----特别是在大脑不断老化，也可以说是被逐渐填满时？
Research by Dimitra Vardalaki, Kwanghun Chung and Mark Harnett at the Massachusetts Institute of Technology, just published in Nature, suggests one way is to preserve into adulthood a type of memory-forming synapse found in children.
麻省理工学院的迪米特拉·瓦达拉基、钟光勋和马克·哈内特近期在《自然》杂志上发表的研究提出，一种方法是将儿童时期一种形成记忆的突触一直保存到成年。
These are called silent synapses. Silent synapses - which, as their name suggests, transmit no signal from one nerve cell to another - are often found on the ends of slender, immature protrusions from nerve cells, called filopodia.
这些被保留下来的突触名为沉默突触。顾名思义，沉默突触不在神经细胞间传递信号，它们通常存在于神经细胞伸出的细长而未成熟的突起（称为丝状伪足）的末端。
Until now, it had been thought that these disappeared as a brain matured. But Drs Vardalaki, Chung and Harnett have shown not only that they are present in adulthood, but also that they are common, at least in mice.
在此之前，人们一直认为沉默突触会随着大脑的成熟而消失。但瓦达拉基、钟光勋和哈内特博士已经证明，至少在小鼠体内，它们不仅存在于成年期，而且还很常见。
Just over a quarter of the connections they sampled in adult mouse visual cortices were silent synapses on filopodia. And murine and human brains are sufficiently alike that something similar almost certainly applies to people.
他们从成年小鼠的视觉皮层中采样的连接中，略多于四分之一属于丝状伪足上的沉默突触。根据鼠脑和人脑之间的相似程度，几乎可以肯定人类大脑也有相似的现象。
