How Do Dogs See the World?狗眼中的世界

Dog vision is very different from human vision. Dogs see the world in fewer hues than we do， but this doesn’t mean our canine companions are completely colorblind. But even if dogs’ visual worlds are not as clear or as colorful as ours， their ability to see motion is superior.

狗的视界跟人类看到的大不相同。它们看到的世界虽不如我们所看到的那般色彩斑斓，但这并不是说我们的汪汪队友是完完全全的色盲。哪怕狗的视界比不得我们的那么清晰、那么缤纷，可它们捕捉动作的本领比我们高出一大截。

What colors can dogs see？

狗能看见哪几种颜色？

The human eye works thanks to three kinds of color-detecting cells called cones. By comparing the way each of these cones is stimulated by incoming visible light， our brains distinguish red wavelengths from green wavelengths and blue wavelengths from yellow wavelengths. Dogs’ eyes， like those of most other mammals， contain just two kinds of cones. These enable their brains to distinguish blue from yellow， but not red from green.

人的肉眼能看见东西得益于三种叫视锥细胞的色彩感知细胞。通过对比入射可见光刺激每种视锥细胞的方式，我们的大脑就能区分开红、绿波长，分得清蓝、黄波长。而犬类的眼睛跟多数哺乳动物一样，只有两种视锥细胞，能让大脑区分开蓝黄，却分不清红绿。

Dogs are not completely colorblind， but their eyes are structured in a similar way to those of people with red-green color blindness， whose eyes also lack the third kind of cone normally present in humans， Jay Neitz， a color vision scientist at the University of Washington who conducted many of the modern experiments on color perception in dogs， told Live Science.

狗并非全色盲，只不过其眼部构造与红绿色盲人群相类似，后者同样缺少正常人眼所具备的第三种视锥细胞，进行过不少犬类色彩感知现代实验的华盛顿大学色彩视觉科学家杰伊·奈茨对《生活科学》杂志如此说。

We can get an idea of what dogs see， Neitz said， if we assume their brains interpret signals from their cone cells much like the brains of people with colorblindness do.

奈茨表示，不妨设想一下，狗大脑处理视锥细胞信号的方式与色盲人群非常相似，如此一来，就能对狗的视界略知一二。

To see blue and yellow， dogs and humans rely on neurons inside a part of the eye called the retina. These neurons are excited in response to yellow light detected in the cone cells （which are also inside the retina）， but the neurons’ activity gets suppressed when blue light hits the cones. A dog’s brain interprets the excitation or suppression of these neurons as the sensation of yellow or blue， respectively. However， in dogs and in people who are colorblind， both red light and green light have a neutral effect on the neurons. With no signal to interpret these colors， the dogs’ brains don’t perceive any color. Where you see red or green， they see shades of gray.

看蓝色和黄色，狗和人类都靠眼部视网膜内的神经元。这些神经元感知到视锥细胞（同样位于视网膜内）中的黄光，就会变得兴奋，而蓝光对视锥细胞的刺激则会抑制神经元活动。于是，狗的大脑将神经兴奋、神经抑制分别解读为感知到黄光和蓝光。然而，对狗和色盲人群来说，无论红光还是绿光，对神经元都没有刺激。既然没有解读这两种颜色的信号，狗大脑就不去感知红色或绿色。我们看见了红色和绿色，狗看见的则是深浅不一的灰色。

“A human would be missing the sensations of red and green，” Neitz said. “But whether or not the dog’s sensations are missing red and green， or if their brains assign colors differently， is unclear.”

“人可能会对红绿颜色丧失感知，”奈茨说，“但犬类是否同样无法感知红绿，其大脑是否在色彩认知上有差异，尚不明确。”

Furthermore， like people with colorblindness， dogs may use other cues to distinguish the color we call “red” from the color we call “green.”

又或许，狗也像色盲人群那样，要借助别的线索，才把我们所说的“红色”“绿色”区分开。

“A lot of the time， there are good cues to help them figure it out; for example， red objects tend to be darker than green objects，” Neitz said. “So， if it’s a dark apple， a red-green colorblind person would know that it’s probably a red one， and if it’s a lighter apple， it may be a Granny Smith.”

“很多时候，有很恰当的线索可供色盲人士识别颜色。比方说，红色的物体一般比绿色的暗一些。”奈茨说，“那么，假如看到的是一个暗灰色的苹果，红绿色盲的人就明白，那八成是个红苹果；若是浅灰色，没准就是澳洲青苹。”

There is some evidence that dogs may be able to see colors humans cannot. A 2014 study published in the journal Proceedings of the Royal Society B found that the lenses in the eyes of a dog transmit significant amounts of ultraviolet light， whereas these wavelengths are blocked by human lenses. This suggests that dogs might see more blue light than we do.

有证据表明，狗或许能看见人看不见的颜色。《英国皇家学会学报B》2014年发表的一项研究证实，狗眼部的晶状体可让大量紫外线透过，而这些波长则被人眼晶状体所阻挡。这就说明狗看到的蓝光或许比我们更多。

How sharp is dog vision？

狗的视力如何？

In addition to missing some of the hues perceived by human eyes， dog vision lacks some of the sharpness of human vision. In a 2017 study， published in the journal PLOS One1 and conducted at Linköping University in Sweden， researchers designed a canine visual acuity test similar to the tests ophthalmologists give to people. Instead of having to discern letters of decreasing size， the dogs were rewarded with treats for correctly identifying images containing vertical or horizontal lines with ever-decreasing amounts of space between them.

除了没有人类肉眼对某些色彩的感知力，狗的视力也缺乏人类视力的一些敏锐度。2017年，《科学公共图书馆·综合》刊发了一项在瑞典林雪平大学做的实验，实验人员设计了一项犬类视敏测试，与人类眼科测试类似。但狗要识别的不是字号递减的字母，而是间距递减的垂直线条或水平线条构成的图像，辨别准确即可获得奖励。

The researchers discovered that dogs—or at least the whippets2， pugs and the single Shetland sheepdog that participated in the experiments—were very nearsighted. The results of the experiment suggest that dogs， in well-lit conditions， have roughly 20/50 vision. This means that they have to be 20 feet （6 meters） away from something to see it as well as a human who is 50 feet （15 m） away from that same object.