原文链接：How humans—and other mammals—might have gotten their night vision
发表时间：2018年9月13日
作者：Emily Underwood
分类：生物学 脑与行为
On a moonless night, the light that reaches Earth is a trillion–fold less than on a sunny day. Yet most mammals still see well enough to get around just fine—even without the special light-boosting membranes in the eyes of cats and other nocturnal animals. A new study in mice hints at how this natural night vision works: Motion-sensing nerve cells in the retina temporarily change how they wire to each other in dark conditions. The findings might one day help visually impaired humans, researchers say.
在没有月亮的夜晚，到达地球的光比晴天要少万亿倍。然而即使没有像猫和其他夜行动物那样眼中有特殊增光膜，大多数哺乳动物仍能看得很清楚。针对小鼠的新研究揭示了这种自然夜视能力的工作模式：视网膜上的运动感知神经系统在黑暗环境中临时改变了它们的连接方式。研究人员表示，这个成果有可能会帮助到视力受损的人们。
Scientists already knew a bit about how night vision works in rabbits, mice, humans, and other mammals. Mammalian retinas can respond to “ridiculously small” numbers of photons, says Joshua Singer, a neuroscientist at the University of Maryland in College Park who was not involved in the new study. A single photon can activate a light-sensitive cell known as a rod cell in the retina, which sends a minute electrical signal to the brain through a ganglion cell.
科学家们已经对兔子、小鼠、人类和其他哺乳动物的夜视工作模式有了一定的了解。马里兰大学帕克分校的神经科学家Joshua Singer说，哺乳动物的视网膜可以感应到相当小的光子数量。单个光子可以激活视网膜中称为视杆细胞的光敏细胞，并通过神经节细胞向大脑发送微小的电信号。
One kind of ganglion cell specializes in motion detection—a vital function if you’re a mouse being hunted by an owl, or a person darting to avoid oncoming traffic. Some of these direction-selective ganglion cells (DSGCs) get excited only when an object is moving upward. Others fire only when objects are moving down, or to the left or right. Together, the cells decide where an object is headed and relay that information to the brain, which decides how to act.
其中有一种神经节细胞是专门用来进行检测物体的运动的，如果你是被猫头鹰猎杀的老鼠或是在快速躲避迎面而来的汽车，这是一个至关重要的功能。这些方向选择性神经节细胞（DSGC）有些在物体向上移动时被激发，而其他一些会在物体向下，向左或向右移动时相应的被激发。这些细胞共同确定物体的朝向位置，大脑据此来决定如何行动。
DSGCs “stand out as one of the few places in the brain” where neuroscientists feel pretty confident they know what neurons are doing, Singer says. But the cells behave in surprising ways when the lights go down.
Singer表示方向选择性神经节细胞（DSGC）是大脑中科学家明确其神经元工作模式的为数不多的结构。但是当光线变暗时，这种细胞的行为模式令人吃惊。
To find out how DSGCs adapt to the dark, neuroscientist Greg Field and colleagues at Duke University in Durham, North Carolina, examined slices of mouse retinas by laying them on tiny glass plates embedded with an electrode array. Each array includes about 500 electrodes, but is so small that it spans just a half-millimeter, Field says. Bathed in an oxygenated solution, the mouse retinas can still function and “see” while the array records electrical activity from hundreds of neurons.
为了解方向选择性神经节细胞（DSGC）是如何适应黑暗的，神经科学家Greg Field和同事们在北卡罗来纳州达勒姆的杜克大学，通过将小鼠视网膜的切片放置在嵌有电极阵列的小型玻璃板上进行检查。据Field介绍每个电极阵列包括大约500个大约只有0.5毫米大小的电极。在含氧溶液中进行实验时，电极阵列中的数百个神经元记录到电活动后，小鼠的视网膜仍然可以发挥作用并且“看到”。
The team showed the dissected retinas a simple movie—bands moving across a contrasting background—then turned the light down by a factor of 10,000, going from typical office-level lighting to a more moonlit scene. Three of the four directional DSGCs remained “rock solid” in their response to the motion when the lights went down, Field says. But the fourth type, which usually responds to upward motion, now responded to a much broader range of motion, including down and sideways, they report today in Neuron.
研究团队向解剖后的视网膜展示了一段简单的影片——乐队在对比鲜明的背景中移动——然后将灯光调暗一万倍，从典型的办公室级照明变为月光下的场景。Field介绍说，当灯光变暗时，四个起定向作用的方向选择性神经节细胞（DSGC）中有三个在对运动的反应中“坚如磐石”。他们今天发表在《神经元》杂志上的发布的文章称，第四种即通常对向上运动做出反应的神经节细胞，现在对于运动反应的范围明显扩大，包括了向下和侧向的运动。
Field and his colleagues then analyzed why the “up” cells were acting oddly. Using a computer model of all four directional cells’ activity, they concluded that when the “up” cells sacrificed some of their preference for one direction, they improved the performance of the group as a whole, boosting DSGCs’ ability to detect motion in low light.
Field和他的同事们之后分析了为什么“向上”细胞会表现得如此奇怪，通过使用电脑模拟所有四种定向细胞的活动，他们提出了结论：当这种“向上”细胞牺牲了对于一个方向的偏好时，它们协同其他细胞共同会改善定向细胞的整体表现，提高了方向选择性神经节细胞（DSGC）在弱光下对于运动的检测能力。
To find out how the “up cells” had switched their function, scientists genetically engineered mice that lacked intracellular connections called gap junctions in their upsensing neurons. Such protein channels allow chemical signals to pass from one neuron to another and have previously been linked to night vision. Field’s team found that in retinal tissue from mice without the gap junctions, upsensing cells didn’t adapt to the dark. That means that at least some of the “up” cells’ ability to boost motion detection in low light depends on gap junctions, the authors say.
科学家们通过遗传工程改造了缺乏细胞内连接的小鼠，这些小鼠的向上感知神经元中存在连接间隙。这些被证明与夜视有关的蛋白质间隙允许化学信号从一个神经元传递到另一个神经元。Field的团队发现对于视网膜组织没有这种连接间隙的小鼠，其向上感知细胞并不能够适应黑暗的环境。这也就意味着至少某些“向上”细胞在弱光下提升运动检测的能力取决于连接间隙。
Whether this holds true in people as well is unclear, but the rodent insight might still be applied to artificial vision efforts. Even though DSGCs make up just 4% of ganglion cells in humans, compared with about 20% in mice, many new retinal prosthetics for visually impaired people rely in large part on electrically stimulating ganglion cells. Studies like this could help fine-tune those technologies, Field says. “If you’re going to stimulate ganglion cells, you need to get them to send the right signals to the brain.”
这种情况是否也适用于人类尚不清楚，但啮齿类动物的视觉能力仍可能适用于人工视觉工作。相较于方向选择性神经节细胞（DSGC）占小鼠神经节细胞的20％，人类的比例仅为4％，对于视力受损人群的许多新的视网膜假体在很大程度上都依赖于电刺激神经节细胞。Field表示，这样的研究可以帮助微调这些技术，“如果你想要刺激神经节细胞，你就需要让它们向大脑发送正确的信号。”