... by a species of shrimp.
You've probably heard the term refresh rate used in regards to computer monitors, but it also applies to our eyes. The photoreceptors in your retina have to reset after firing, and during that time -- the refractory period -- the receptor cell is insensitive to further stimuli. I recall finding out about this in my animal physiology class at the University of Washington thirty years ago, and finding out that human photoreceptors reset in about 1/60th of a second. This is why the flicker in a fluorescent light is barely detectable to the human eye; it's driven by the oscillations of alternating current at a frequency of sixty hertz; and the fact that we have millions of photoreceptors, all out of phase with each other, smooths out the signal and makes it look like one continuous, evenly-bright light.
To a fly, however, which has a refresh rate double ours -- about 120 times per second -- a fluorescent light would look like a strobe, brightening and dimming every sixtieth of a second.
Must be really freakin' annoying. Yet another reason I'm glad I'm not a fly.
But even they are not the fastest. A paper in Biology Letters this week describes research into the visual systems of a species of snapping shrimp (Alpheus heterochaelis), which already is badass enough -- it snaps its claws together with such force that it creates a shock wave in the water, stunning its prey. And this little marine crustacean has a refresh rate of 160 times per second.
So what looks like a blur of motion to other animals is visible as clear, discrete images moving across its field of vision.
Not only that, they have one of the widest ranges of sensitivity to light level known, functioning well with only 1 lux of incident light (the light intensity of late twilight) all the way up to 100,000 lux (direct, intense sunlight).
[Image licensed under the Creative Commons Rickard Zerpe, Snapping shrimp (Synalpheus sp.) (23806570264), CC BY-SA 2.0]
No wonder the folks in the Caribbean have nicknamed the native species of mantis shrimp "the thumb-splitter."
But wild as that is, it's not why I brought up mantis shrimp. They have the most sensitive color vision of any animal known. Humans are (mostly) trichromats, having three functioning types of color receptor in our eyes. Dogs are dichromats -- they have only two, which is why their color acuity is worse than ours. A few lucky humans, and a great many bird species, are tetrachromats, having four kinds of color receptors.
Mantis shrimp have sixteen. They can not only see in the ultraviolet region of the spectrum -- a range of light completely invisible to the human eye -- they can detect polarization angle, and even have sensors for detecting circular polarization, something that is thought to be unique in the animal kingdom.
Why they need this many different kinds of light receptors is unknown, although it probably has to do with predator-prey interactions -- finding lunch and avoiding being made into lunch. With so many different strategies used by shallow tropical marine species to confound the eye -- shimmering scales, transparency, cryptic coloration, countershading -- having eyes that beat everyone else in sensitivity and range would be a pretty neat adaptation.
So that's yet another excursion into the weird world of sensory perception. It never fails to fascinate me to think about what a different kind of animal's experience of the world must be like. As philosopher Thomas Nagel pointed out, the only way to know what it's like to be a bat is to be a bat; all of our ideas of echolocation and flight and being nocturnal only gives us the answer to what it's like for a human to think about being a bat.
But even so, and all pondering about the mind/body problem aside, I can't help but wonder what the world looks like to a shrimp.
I know I sometimes wax rhapsodic about books that really are the province only of true science geeks like myself, and fling around phrases like "a must-read" perhaps a little more liberally than I should. But this week's Skeptophilia book recommendation of the week is really a must-read.
No, I mean it this time.
Kathryn Schulz's book Being Wrong: Adventures in the Margin of Error is something that everyone should read, because it points out the remarkable frailty of the human mind. As wonderful as it is, we all (as Schulz puts it) "walk around in a comfortable little bubble of feeling like we're absolutely right about everything." We accept that we're fallible, in a theoretical sense; yeah, we all make mistakes, blah blah blah. But right now, right here, try to think of one think you might conceivably be wrong about.
Not as easy as it sounds.
She shocks the reader pretty much from the first chapter. "What does it feel like to be wrong?" she asks. Most of us would answer that it can be humiliating, horrifying, frightening, funny, revelatory, infuriating. But she points out that these are actually answers to a different question: "what does it feel like to find out you're wrong?"
Actually, she tells us, being wrong doesn't feel like anything. It feels exactly like being right.
Reading Schulz's book makes the reader profoundly aware of our own fallibility -- but it is far from a pessimistic book. Error, Schulz says, is the window to discovery and the source of creativity. It is only when we deny our capacity for error that the trouble starts -- when someone in power decides that (s)he is infallible.
Then we have big, big problems.
So right now, get this book. I promise I won't say the same thing next week about some arcane tome describing the feeding habits of sea slugs. You need to read Being Wrong.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]