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.  

**************************************

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!]

Color my world

Perception is such a mystery.

Neuroscientist David Eagleman, in his brilliant TED talk "Can We Create New Senses For Humans?", touches on this with a concept he calls the umwelt -- the slice of the objective reality we're aware of.  That differs from animal to animal -- as he points out, for dogs, the umwelt is all about smell; it's sound-related for echolocating bats; it consists of electrical field fluctuations for the black ghost knife-fish; and so on.  Eagleman says:

What this means is that our experience of reality is constrained by our biology.  And that goes against the common-sense notion that our eyes and our ears and our fingertips are just picking up the objective reality that's out there.  Instead, our brains are sampling just a little bit of the world...  Now, presumably, every animal assumes that its umwelt is the entire objective reality out there, because why would you ever stop to imagine that there's something beyond what we can sense?  Instead, what we all do is that we accept reality as it's presented to us. 

What never ceases to amaze me is that even the parts of the human umwelt most of us are pretty good at picking up on are still made largely of faulty and incomplete information.  Our brains have evolved to fill in the gaps in what we see and hear -- so your perception of the world is built of what you're actually sensing of the real world, and what your brain assumes is there and fills in for you.  (That it sometimes does this incorrectly is the basis of a lot of optical illusions.)

If you need further evidence that you're seeing some bits of reality but otherwise just kind of making shit up, consider a paper published this week in Proceedings of the National Academy of Sciences, by Michael Cohen (of Amherst College) and Thomas Botch and Caroline Robertson (of Dartmouth University).  In "The Limits of Color Awareness During Active, Real-World Vision," Cohen, Botch, and Robertson tested something that's been known for years -- that the acuity of our color vision in the periphery of our visual field is fairly poor -- and challenged the prevailing explanation, which is that cones (our color-sensitive retinal cells) are dense in the fovea (center of the retina) and sparse in the edges.

[Image is in the Public Domain]

For one thing, "sparse" is comparative, and not even especially accurate.  In a normal retina, the periphery still has four thousand cones per square millimeter.  Plus, even the statement that peripheral color vision is bad turns out to be a misstatement; we can detect the color of a small, brightly-colored object almost as well in the periphery as we can in the dead center of the visual field.

However, Cohen, Botch, and Robertson did an experiment that turns the whole question upside down.  They gave test subjects head-mounted visual displays that were equipped with devices for tracking eye movements.  They then showed the test subjects images of outdoor scenes, and without alerting them, began to decrease the color saturation in the edges of the image.  The test subjects failed to notice the fact that the image was gradually turning to black-and-white from the edges inward until the colored bit spanned an angle of only 37.5 degrees, something that "does not correspond to known limitations imposed by retinal or neuroanatomy."

It appears that what's going on is that the edges of our visual field are reasonably good at recognizing color, but our brain simply ignores the input.  Motion, on the other hand, is quickly detected even in the peripheral vision; makes some sense evolutionarily, where seeing the lion coming up from behind you is way more critical than determining what color his fur is.

It was a fairly shocking result even for the researchers.  "We were amazed by how oblivious participants were when color was removed from up to 95 percent of their visual world," said study senior author Caroline Robertson, in an interview with EurekAlert.  "Our results show that our intuitive sense of a rich, colorful visual world is largely incorrect.  Our brain is likely filling in much of our perceptual experience."

How and why the brain does this, however, is still a mystery.  The authors write:

If color perception in the real world is indeed as sparse as our findings suggest, the final question to consider is how this can be.  Why does it intuitively feel like we see so much color when our data suggest we see so little?  While we cannot offer a definitive answer, several possibilities can be explored in future research.  One possibility is that as observers spend time in an environment, their brains are able to eventually “fill-in” the color of many items in the periphery.  Of course, providing direct evidence for this explanation is challenging since it is extremely difficult to differentiate between scenarios where a subject knows the color of an object (i.e., “I know the tree behind me is green even though I currently cannot see the color green”) from instances where the subject is experiencing the color of that object online (i.e., “I can see the color green at this very moment”).

So our umwelt is apparently an even smaller slice of reality than we'd thought.  A little humbling, and something to think about next time you're in an argument with someone and you are tempted to say, "I know it happened that way, I saw it with my own eyes."

**********************************

This week's Skeptophilia book-of-the-week is for people who are fascinated with the latest research on our universe, but are a little daunted by the technical aspects: Space at the Speed of Light: The History of 14 Billion Years for People Short on Time by Oxford University astrophysicist Becky Smethurst.

A whirlwind tour of the most recent discoveries from the depths of space -- and I do mean recent, because it was only released a couple of weeks ago -- Smethurst's book is a delightful voyage into the workings of some of the strangest objects we know of -- quasars, black holes, neutron stars, pulsars, blazars, gamma-ray bursters, and many others.  Presented in a way that's scientifically accurate but still accessible to the layperson, it will give you an understanding of what we know about the events of the last 13.8 billion years, and the ultimate fate of the universe in the next few billions.  If you have a fascination for what's up there in the night sky, this book is for you!

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