Color my world

When you think about it, color perception is really strange.

Just about all of us have wondered whether we all see colors the same way -- if, for example, what you see as blue is the same as what I see as yellow, but we both identify them using the same word because there's no way to know we're not seeing them the same way.  I've always thought that unlikely.  After all, with few exceptions (other than genetic or structural abnormalities, about which q. v.) our eyes and brain are all built on the same basic plan.  I guess it's possible that we each see the world's colors differently, but the most parsimonious explanation is that because the underlying structures are the same, we're all pretty much perceiving identical color palettes.

Of course, there's no way to know for certain, and I ran into two things just in the last couple of days that leave me wondering.

The first is a curious conversation I had with my friend, the awesome writer Andrew Butters, whose books -- especially the staggeringly good Known Order Girls -- should be on everyone's TBR list.  It started out with an amusing discussion of words that sound like they should mean something else.  One of Andrew's was ambulatory, which to him sounds like "someone who is so incapacitated they need an ambulance."  I personally believe that pulchritude should mean "something that makes you want to puke," and not what it actually does, which is "beauty."  And then Andrew mentioned that he always thought the color words vermilion and chartreuse were wrong, and in fact backwards -- that vermilion should mean a light green and chartreuse a bright orangey-red.

This struck me as really weird, because those two words have never given me that sense.  This may be because I've known them both since I was little.  I knew vermilion because I grew up a mile away from Vermilion Bayou, so named because the red mud of southern Louisiana stains the water reddish brown.  Chartreuse I knew because my grandma's employer, Father John Kemps, was an eccentric, bookish, cigar-smoking Dutch expat who was very fond of a post-meal tipple and loved chartreuse, the pale green herbal French liqueur from which the color got its name.

So I asked Andrew where his misapprehension came from.  He said he wasn't sure, but that perhaps the vermilion one came from the French vert (green); Andrew, like most Canadians, is English-French bilingual.  But where his thinking chartreuse should mean "red" came from, he had no idea.

What baffled me further, though, was when he pointed out that he's not alone in this.  There's a whole page on Reddit about thinking that vermilion and chartreuse are backwards, and an astonishing number of people chimed in to say, "Yeah, me too!"  So why those particular words, and not another pair?  Why not citron and azure, or something?

The second is that I'm finally getting around to reading Oliver Sacks's book An Anthropologist on Mars, which has to do with the intersection between neurological disorders and creativity.  The very first chapter is about a painter who was in a car accident that resulted in brain damage causing cerebral achromotopsia -- complete colorblindness due not to abnormalities in the cones of the retina, but because of damage to a region of the brain called the V4 prestriate cortex.  Afterward, he saw the world in shades of gray -- but with some distinct oddities, because pure white surfaces looked "dirty" or "smudged" to him, red looked black, and blue looked a pale gray.

This brought up an interesting discussion about how we see color in the first place, and that color perception (even within a single, normally-sighted individual) isn't absolute, but comparative; we assess the color value of a region by comparison to the entire visual field.  If the whole "what color is this dress?" thing that was going around a few years ago didn't convince you of that, try this one out:

Every one of these spheres is exactly the same color; they were, in fact, cut-and-pasted from a single image.  The only thing that differs is the color of the foreground stripes that cross each one.  But since your eyes judge color based on context, it's impossible to see them as the same even once you cognitively know what's going on.

Don't believe it?  If you go to the link provided, the article author (the wonderful Phil Plait) created an animation that cycles between the image with and without the stripes.  It's mind-blowing.

All of this circles around to the weird topic of synesthesia, which is a still-unexplained sensory phenomenon where people have a sort of cross-wiring between two senses.  Russian composer Alexander Scriabin was a synesthete, who experienced sensations of colors when he heard chords; C# minor, for example, looked a bright emerald green.  (If you want to find out more, the amazing book by Richard Cytowic, The Man Who Tasted Shapes, is still considered one of the seminal works on this odd disorder.)

I wonder if what Andrew (and the others with the vermilion-chartreuse switch) are experiencing is a form of synesthesia.  A former student of mine is a synesthete for whom printed letters (and whole words) evoke sensations of colors, so his word choices while writing took into account whether the colors were harmonious, not just that the words made semantic sense.  (I hasten to add that he was and is one of the most brilliant people I've ever known, so his synesthesia didn't cause his writing to lack any clarity to non-synesthetes like myself -- although it has been known to slow him down as he struggled to find words that satisfied both meaning and appearance.)  So perhaps the "vermilion = light green" thing comes from the fact that for Andrew and the others on the Reddit page, the word looks green irrespective of its association with an actual (different) color.

What I find odd still, though, is that so many people have those two particular color words backwards.  Synesthesia is remarkably individual; while one of its hallmarks is a complete consistency within a particular person (Scriabin always saw C# minor as green, for example), it varies greatly from person to person.  The fact that vermilion and chartreuse are reversed for so many people is just plain peculiar.

So there's still a lot we don't know about how exactly we perceive color, and maybe my "parsimonious" explanation that (other than those with colorblindness, synesthesia, and other visual disorders) we're all seeing colors more or less the same way fails to capture the complexity of the real world.  Wouldn't be the first time I've thought things were simpler than they turned out to be.  Maybe it's just my perception because I'm a non-synesthete with intact color vision.

But until we're somehow able to see things through someone else's eyes and brain, that's a limitation I can't escape except for in my imagination.

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Pink, pink, gold

When I was in Ecuador in 2019, I was blown away by its natural beauty.  The cloud forests of the mid-altitude Andes are, far and away, the most beautiful place I've ever been, and I've been lucky enough to see a lot of beautiful places.  Combine that with the lovely climate and the friendliness of the people, and it puts the highlands of Ecuador on the very short list of places I'd happily move to permanently.

What brought me there were the birds.  It's a tiny country, but is home to 1,656 species of birds -- about one-sixth of the ten-thousand-odd species found worldwide.  Most strikingly, it has 132 different species of hummingbirds.  Where I live, in upstate New York, we have only one -- the Ruby-throated Hummingbird (Archilochus colubris) -- but there, they have an incredible diversity within that one group.  Because each species is dependent on particular flowers for their food source, some of them have extremely restricted ranges, often narrow bands of terrain at exactly the right climate and altitude to support the growth of that specific plant.  You go a few hundred meters up or downhill, and you've moved out of the range where that species lives -- and into the range of an entirely different one.

The most striking thing about the hummingbirds is their iridescence.  My favorite one, and in the top five coolest birds I've ever seen, is the Violet-tailed Sylph (Aglaiocercus coelestis):

[Image licensed under the Creative Commons Andy Morffew from Itchen Abbas, Hampshire, UK, Violet-tailed Sylph (33882323008), CC BY 2.0]

What's most fascinating about birds like this one is that the feathers' stunning colors aren't only due to pigments.  A pigment is a chemical that appears colored to our eyes because its molecular structure allows it to absorb some frequencies of light and reflect others; the chlorophyll in plants, for example, looks green because it preferentially absorbs light in the red and blue-violet regions of the spectrum, and reflects the green light back to our eyes.  Hummingbirds have some true pigments, but a lot of their most striking colors are produced by interference -- on close analysis, you find that the fibers of the feathers are actually transparent, but when light strikes them they act a bit like a prism, breaking up white light into its constituent colors.  Because of the spacing of the fibers, some of those wavelengths interfere destructively (the wavelengths cancel each other out) and some interfere constructively (they superpose and are reinforced).  The spacing of the fibers determines what color the feathers appear to be.  This is why if you look at the electric blue/purple tail of the Violet-tailed Sylph from the side, it looks jet black -- your eyes are at the wrong angle to see the refracted and reflected light.  Look at it face-on, and suddenly the iridescent colors shine out.

So the overall color of the bird comes from an interplay between whatever true pigments it has in its feathers, and the kind of interference you get from the spacing of the transparent fibers.  This is why when you recombine these features through hybridization, you can get interesting and unexpected results -- as some scientists from Chicago's Field Museum found out recently.

Working in Peru's Cordillera Azul National Park, on the eastern slopes of the Andes, ornithologist John Bates discovered what he'd thought was a new species in the genus Heliodoxa, one with a glittering gold throat.  He was in for a shock, though, when the team found out through genetic analysis that it was a hybrid of two different Heliodoxa species -- H. branickii and H. gularis -- both of which have bright pink throats.

"It's a little like cooking: if you mix salt and water, you kind of know what you're gonna get, but mixing two complex recipes together might give more unpredictable results," said Chad Eliason, who co-authored the study.  "This hybrid is a mix of two complex recipes for a feather from its two parent species...  There's more than one way to make magenta with iridescence.  The parent species each have their own way of making magenta, which is, I think, why you can have this nonlinear or surprising outcome when you mix together those two recipes for producing a feather color."

The gold-throated bird apparently isn't a one-off, as more in-depth study found that it didn't have an even split of genes from H. branickii and H. gularis.  It seems like one of its ancestors was a true half-and-half hybrid, but that hybrid bird then "back-crossed" to H. branickii at least once, leaving it with more H. branickii genes.  All of which once again calls into question our standard model of species being little cubbyholes with impermeable walls.  The textbook definition of species -- "a morphologically-distinct population which can interbreed and produce fertile offspring" -- is unquestionably the most flimsy definition in all of biology, and admits of hundreds of exceptions (either morphologically-identical individuals which cannot interbreed, or morphologically-distinct ones that hybridize easily, like the Heliodoxa hummingbirds just discovered in Peru).

In any case, the discovery of this hybrid is fascinating.  You have to wonder how many more of them there are out there.  The fact that its discovery ties together the physics of light, genetics, and evolution is kind of amazing.  Just further emphasizes that if you're interested in science, you will never, ever be bored.

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Color my world

Our ability to perceive color is, when you think about it, a peculiar thing.

First, there's the rather hackneyed question of whether we all see color the same way (exclusive, of course, of people who are colorblind).  The way the question is usually phrased goes something like, "How could we tell if when you look at something red, what you see what I call green, but you still call it red because that's what you've learned?"  In other words, if I were to take a look through your eyes and brain, would the colors of objects be the same as what I see?

The answer is: we can't know for sure.  Thus far there's no way for one person to perceive the world through another person's sensory organs and brain.  But the great likelihood is that we all see colors pretty similarly.  All of our visual receptors are put together the same way, as are the visual cortices in our brains.  To assume that even with this structural and functional similarity, each person is still perceiving colors differently, runs counter to Ockham's Razor -- so without any evidence, it seems like a pretty untenable position.

More interesting is the comparison between how we see the world and how other animals do.  Once again, we run up against the issue that we can't see through another's eyes, but at least here we're on more solid ground because we can see that different animals have differently structured eyes.  Dogs, for example, have retinas with a much higher density of rods (the structures that operate in dim light, but only see in shades of gray) and a much lower density of cones (the structures that operate in bright light, and are able to differentiate by wavelength -- i.e., see colors).  Dogs aren't completely colorblind -- their two types of cones peak in sensitivity in the blue and yellow regions of the spectrum -- but they're relatively insensitive to colors outside that.  (This explains why my dog, Guinness, routinely loses his bright orange tennis ball in the bright green lawn -- to me, it stands out from fifty meters away, but he'll walk right past it without seeing it.) 

Then, there are bees and butterflies, which have eyes sensitive not only in the ordinary visible light spectrum but in the infrared and ultraviolet regions, respectively.  There are flowers that look white to our eyes, but to a butterfly they're covered with streaks and spots -- ultraviolet-reflecting markings that advertising nectar to those who can see it. 

A flower of the plant Potentilla reptans, photographed in ultraviolet light.  To the human eye, the flower looks solid yellow -- this is what it might look like to a butterfly.  [Image licensed under the Creative Commons Wiedehopf20, Flower in UV light Potentilla reptans, CC BY-SA 4.0]

But the winner of the wildly complex vision contest is the mantis shrimp, which has sixteen different color receptors (contrasted with our paltry three), rendering them sensitive to gradations of color we aren't, as well as detecting ultraviolet and infrared light, and discerning the polarization angle of polarized light.  How the world looks to them is a matter of conjecture -- but it certainly must be a far brighter and more varied place than what we see.

The reason all this colorful stuff comes up is because of a paper that appeared last week in Proceedings of the National Academy of Sciences, called "What We Talk About When We Talk About Colors," by Colin Twomey, Gareth Roberts, David Brainard, and Joshua Plotkin of the University of Pennsylvania.  The researchers looked at how words describing different colors vary from language to language.  "The color-word problem is a classical one," Plotkin said, in an interview with Science Daily.  "How do you map the infinitude of colors to a discrete number of words?"

And, more central to this research: does everyone do it the same way?  If you showed me a series of gradations from pure blue to pure green, at what point to I switch from saying "this is blue" to saying "this is green" -- or do I call the intermediate shades by a third, discrete name?

What the researchers found was that across 130 different languages, humans tend to group and name colors the same way.  Further, if you give people tiles with varying shades of red and asked them to pick out "the reddest red," the results show remarkable consistency.

Another interesting result of the research was that the sensitivity of our eyes to color variation isn't the same from color to color; we are much better at picking out subtle variations in red, orange, and yellow than we are at seeing differences in (for example) different shades of brown.  The researchers believe this is due to a difference in what they call communicative need; since reds, oranges, and yellows are the colors of ripe fruit, we've evolved eyes that are most sensitive to variations in those colors.  "Fruits are a way for a plant to spread its seeds, hitching a ride with the animals that eat them," Twomey said.  "Fruit-producing plants likely evolved to stand out to these animals.  The relationship with the colors of ripe fruit tells us that communicative needs are likely related to the colors that stand out to us the most...  No one really cares about brownish greens, and pastels aren't super well represented in communicative needs."

So it seems like the great likelihood is that we all see the world pretty much the same way.  Well, all humans, at least.  What the world looks like to a dog, with their better dim-light vision and better motion detection, but far poorer color discrimination, can only be guessed at; and what colors a mantis shrimp sees is beyond the ability of most of us to imagine.

Study lead author Colin Twomey wonders whether the same techniques could be used to study other facets of sensory perception.  "This is something that could be carried to other systems where there is a need to divide up some cognitive space," he said, "whether it's sound, weight, temperature, or something else."

One I wonder about is the sense of taste.  We know that taste differs a great deal between different individuals, not only because everyone likes (and dislikes) particular flavors, and those preferences differ greatly; but there are some people called "supertasters" who are sensitive to minor variations in flavor that the rest of us don't even notice.  (I am most definitely not a supertaster; the joke in my family is that I have two taste buds, "thumbs up" and "thumbs down.")  The daughter of a friend of mine, for example, has amazingly sensitive taste buds, to the point that she can discern whether the coffee was brewed with filtered water or ordinary tap water.

Me, as long as it's brewed with water and not turpentine, I'm fine with it.

But that's all potential future research.  For now, we have a better idea of how each of us colors our world.  And despite our individual differences, the answer appears to be that what you're seeing and what I'm seeing look very much alike.  

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As someone who is both a scientist and a musician, I've been fascinated for many years with how our brains make sense of sounds.

Neuroscientist David Eagleman makes the point that our ears (and other sense organs) are like peripherals, with the brain as the central processing unit; all our brain has access to are the changes in voltage distribution in the neurons that plug into it, and those changes happen because of stimulating some sensory organ.  If that voltage change is blocked, or amplified, or goes to the wrong place, then that is what we experience.  In a very real way, your brain creates your world.

This week's Skeptophilia book-of-the-week looks specifically at how we generate a sonic landscape, from vibrations passing through the sound collecting devices in the ear that stimulate the hair cells in the cochlea, which then produce electrical impulses that are sent to the brain.  From that, we make sense of our acoustic world -- whether it's a symphony orchestra, a distant thunderstorm, a cat meowing, an explosion, or an airplane flying overhead.

In Of Sound Mind: How Our Brain Constructs a Meaningful Sonic World, neuroscientist Nina Kraus considers how this system works, how it produces the soundscape we live in... and what happens when it malfunctions.  This is a must-read for anyone who is a musician or who has a fascination with how our own bodies work -- or both.  Put it on your to-read list; you won't be disappointed.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]