跟读练习: Dog vision, explained - 通过YouTube学习英语口语
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How do they know what colors dogs can see?
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How do they know what colors dogs can see?
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There's this myth that won't go away,
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that dogs only see in black and white,
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but that's just not true.
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There's a bunch of apps and filters now that claim to show dog color vision.
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But where does our understanding of this canine rainbow come from?
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And do these filters really capture how dogs see the world?
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Before we get to dogs,
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how do we know what colors humans can or can't see?
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I'm sure you've seen these before, right?
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Yeah.
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Do you have normal color vision?
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I have, I think, normal color vision.
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I think that says 45.
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So what just happened there?
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Light from this image entered Cleo's eye,
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where it hit cells in her retina called cones that in turn sent electrical signals to her brain,
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which created her perception of color.
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This rainbow is just how our brain translates different wavelengths of light from one small section of the electromagnetic spectrum.
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Clio, like most people, has three types of cone cells,
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each one sensitive to a different part of the spectrum,
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relatively short wavelengths of light, medium wavelengths, and long.
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Their sensitivity peaks at specific points,
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but it drops off smoothly in both directions.
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So, when what we call orange light comes in and hits an S-cone,
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its wavelength lands outside this sensitivity curve.
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But if it hits an M-cone, a signal is produced.
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And with the L-cone, an even stronger signal.
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These signals are combined and interpreted by the brain as orange.
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When so-called green light comes in,
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the cones send a different set of signals.
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And so people with normal vision can make out the number 45 in this pattern.
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But what What if you were missing this M-cone,
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like about 0.5% of the population?
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Both orange and green light would stimulate the L-cone about the same amount.
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And without the M-cone, the signals would be too similar.
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The brain wouldn't be able to distinguish between them,
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and the number 45 would disappear into the background.
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For those who can't read numbers,
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like toddlers, there are versions of this test that just have the subject trace a loopy line.
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But how do you design a vision test for a dog?
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If I were trying to do this with Thor,
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I would concoct a test that involves getting food at the end,
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if and only if he can demonstrate that he can see two different colors.
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That's pretty much what Jay Knights did back in the late 1980s.
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I said, we have to solve this question once or for all.
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I went home at lunchtime and got my dog.
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A toy poodle appropriately named Retina.
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Brought him into the laboratory seven days a week for a year.
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Lab setup was a little chamber.
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There were three discs illuminated from behind.
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The dog was trained to boop whatever disc looked different with her nose.
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If she chose right, she heard a click and got a little treat.
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If she chose wrong, no treat and a buzzer sound.
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Once Retina learned that Jay wanted her to identify the different circle,
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it was time for the real experiments.
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One of the first experiments that we did,
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we had three different white lights,
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and then we added a little bit of the particular color.
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At first we'd add a little bit and the dog can't see it.
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But add more, and the dog can pick it out.
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And if it takes a very small amount of any particular wavelength,
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that means that they must be very sensitive.
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For each color, they plotted out this sensitivity.
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And we just marched across the whole entire rainbow.
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This sensitivity curve strongly suggests that dogs have just two cones,
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centered here and here.
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And at this low point in the middle, something interesting happened.
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Retina couldn't tell the difference between this color and white light.
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There's some wavelength in the middle part of the spectrum that's kind of where the two cones overlap.
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The light equally stimulates the two cones.
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When all three of our cones are equally stimulated,
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humans see shades of gray.
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And the same would be true for dogs with their two cones.
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For them, this is the point of that equal stimulation.
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Another set of experiments mimicked those colorblindness tests we talked about before.
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And as you'd expect from someone with just two cones,
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retina struggled to tell the difference between oranges and greens.
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Jay confirmed his results with two other dogs and published them in 1989.
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Now my dog is much more famous in the realm of science than I am.
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Follow-up studies gathered similar data,
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and if you put that together with what we know about human vision,
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you can start to paint a picture of the rainbow that dogs see.
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Blue on one end, fading into gray in the middle,
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and then yellow on the other end.
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It's probably not that different from what a human missing their M-cone would see.
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It makes me wonder how many times I have interacted with Thor in a way
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that kind of expected him to be able to see the full spectrum of color.
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Can I just put a filter on an image and be like,
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oh, this is what a dog is seeing,
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or are there complications to that?
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Oh, I think it's way more complicated than that.
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But that's a good first pass.
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Is it anywhere near the subjective experience?
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I think not.
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I mean, there's a lot more complications just in the retina to begin with.
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In addition to cones, mammals have rod cells that help them pick up light and motion.
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And dogs have way more of these kinds of cells than humans.
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That makes them more sensitive to movement and helps them see in low light.
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They also have a layer called the tapetum lucidum that reflects light back through the retina.
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That's what makes dogs' eyes shine at night.
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Then there's a difference in the arrangement of cells in the retina.
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We have lots of cells across all of the back of the eyeball,
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but an especially dense group of cells in the center,
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which enables us to see the thing that's right in front of us.
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But some breeds of dogs are different.
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They have a density of cells that sort of extends...
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Oh, I just made a hard...
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They have a density of cells that I love.
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And
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that horizontal band of cells might make it easier for them
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to take in visual information across a broader swath of the horizon.
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The position of their eyes often gives them a wider field of vision,
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but less overlap, which means weaker depth perception.
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Dogs are also mostly nearsighted.
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Distant things are fuzzier.
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And additionally, dogs' visual experience of the world is going to be completely tied up in the olfactory experience of the world.
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This is the aspect of dog vision that I don't think we humans can ever fully understand.
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So, this is just an image of the olfactory peduncles in the human on the left and the dog on the right.
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These are called peduncles?
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These are olfactory peduncles, yeah.
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That's a great word, wow.
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The olfactory system in the human takes up approximately 0.03% of the brain.
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Dogs do have a hugely larger olfactory system.
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They are detecting things that we are completely unable to detect.
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There's a bunch of studies that show dogs can smell cancer,
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malaria, COVID-19, explosives, diseased avocado trees,
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whale poop on the ocean,
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even the emotional state of their owners.
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You know, I do sometimes think
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when I put perfume on what my dog is thinking because it must be quite overwhelming for them.
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Here's another way to look at the brain.
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These are tractograms made with a special MRI technique.
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These lines trace the paths of white matter,
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the fibers that speed signals between different regions of the brain.
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You can virtually dissect these tractograms, highlighting different roots.
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For example, here are the paths extending from human olfactory peduncles to other regions.
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But here is the dog brain.
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Whoa.
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There's information freeways running from the nose to other parts of the brain.
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And these weren't found before just because no one really looked?
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Yeah, pretty much.
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Can you guess what this is circling?
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It's going here.
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Yeah, what is there?
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Do you know?
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I have no idea what's here.
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It's a little counterintuitive because in both humans and dogs,
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the place that we process vision is actually the back of our head,
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this occipital lobe back here.
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Oh, so what you're saying is dogs are processing smell like we are processing sight, sort of?
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Well, it's interesting because it's not unique to the dog that they might use a different sense to generate vision.
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I mean, we know that bats are a classic example.
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They can navigate through their auditory pathways.
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It's very possible that dogs can identify structures in the room from scent as well as from vision.
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Seeing and smelling are interwoven in an important way,
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and that is something completely unlike what we have in our brain.
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When I think about my sense of the world,
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my mental map of where I am in space,
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where other things are, like how those things relate to each other and move,
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like all of that is visual.
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We literally use the word visualize like that's what I'm thinking.
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But for a dog like now I'm imagining Thor's sense of the world
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and his place in it is like a spatial awareness that depends on smell.
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That's fascinating.
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Wow I don't even know I can't grok this fully.
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Yeah I mean I think it's impossible to grok.
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It's ungrokkable.
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Have you come across this term Umwelt?
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No. It's a term that comes from this German biologist whose name I can't remember right now,
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but it's something I couldn't pronounce even if I could remember it.
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Jakob Johann von Uxgut.
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And Umwelt, it refers to the different realities experienced by different organisms,
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the set of things that are meaningful to them,
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some of which we can't even sense.
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You come to see that we're just seeing like
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but one of many worlds which are existing in parallel and other animals are seeing in another slice.
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Star Rainbow might be more varied than that of a dog, but...
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Their olfactory world is so explosively colorful that it wouldn't occur to them for a second that there's something that's being missed.
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This is.. it really does help me understand my little best friend better.
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I feel like I feel connected to his world and sort of an appreciation that his world is different and also beautiful.
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I don't see Rowdy in his box today,
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but you're also a cat guy.
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Can cats do the booping test?
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I find it hard to believe.
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That might be my stereotypes about cats,
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but have people investigated this?
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And what do we know about cat color vision?
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Well, hey, everybody, it's Adam from the future.
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It's the next morning.
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I've got Rowdy back with me.
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And I've been up all night reading cat studies.
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