A special #NeuroThursday for the winter #solstice: how we see darkness and light on the scale of hours, days, and seasons!
Most people know the basics of visual perception, the rod and cone cells that let us see color and contrast. But I'm here today to talk about something more obscure: how we detect days and seasons.
(But I could discuss some fun tricks of the visual "basics" in the new year, if that sounds fun!)
Humans, like all animals (and plants, and fungi, and some bacteria), have circadian rhythms: internal biological rhythms that work on a ~24-hour cycle.
Circadian rhythms make your physical, mental, and biological performance rise and fall throughout the day. This is what it means to be a "morning person" or a "night owl" (or a "coffee person").
The body keeps time via biochemical oscillators - processes with feedback loops that take 24 hours to get back to the original state. But you're not here for biochemistry, and neither am I.
Here's the trick: if you have a clock, you need a way to reset it. The time and pattern of day/night cycles can't be inborn. If they were everything would asplode if you were born in a different time zone from your parents.
Clock setting doesn't just happen at birth, of course. That's what jet lag is: a super annoying process, but after a few days you sync back up to the (local) sun.
In theory, our standard visual receptors (the rods & cones in the back of our eyeballs) are probably capable of this. But for whatever reason, they don't do this job.
(I imagine this is because the mammalian system for clock-setting evolved earlier than "normal" precision vision, but that's just a guess.)
Clock-setting occurs through the third kind of light-sensitive cell (photoreceptor) in the eye: "ipRGCs." An abbreviation for the poetically-named "intrinsically photoreceptive retinal ganglion cells."
That name is a mouthful, but we can make sense of it. "Intrinsically photoreceptive" - they themselves are sentitive to light. "Retinal" - the back of your eye, same place as the rods & cones. But "Ganglion cells," well…
Retinal ganglion cells are little visual preprocessors in the back of the eye. They draw together input from many rods and/or cones, and transmit information on to the brain. (Image is overcomplicated, but shows a RGC doing color-contrast work.)
But a tiny fraction (~1%) of retinal ganglion cells have their own light-sensitive pigments. These are our new friends the ipRGCs.
These folks don't form precise images like rods and cones do. Instead, their response to light is a slow slow arc, over the course of minutes or hours.
They provide the anchor to reset our circadian clocks. They also control our body's other responses to ambient light, like making your pupils grow and shrink.
These cells only respond to blue light, which is why standard screens (with their blue-toned glow) are a bad idea before bed. health.harvard.edu/staying-health…
So on this #NeuroThursday, be glad your eyes contain these secret cells devoted to telling light from dark, bright from dim, winter from spring. And #happysolstice to all!
(Apologies to those of you who've previously used the Storify version of these threads, but that website is shutting down, so I can no longer provide.)
If this #NeuroThursday brought light into your day, share it around, or check out all the fiction & nonfiction I've published this year! benjaminckinney.com/awards-eligibi…

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More from @BenCKinney

Aug 13, 2018
Handedness comes in two groups, "right handed" and "not right handed." Most people use their right hands for almost all precision movement, but the other group is a broad spectrum from weakly-right to strongly-left. baen.com/handedness
The way we describe and define handedness creates the effect @CStuartHardwick rightly notices. Culture defines how we talk about it - but the behavior is mostly genetic. The % of righties has remained constant across continents and milennia.
Hand dominance is a more squirrelly thing than most people realize. For example, righties are better at *some* things with their left hand... and *some* of these asymmetries flip in lefties. Take a few minutes on #LeftHandersDay to learn more!
Read 4 tweets
Aug 8, 2018
Quick heads up on the #BlackSpecFic report: the story counts for @escapepodcast @Pseudopod_org and @PodCastle_org are incomplete, and revisions will be forthcoming.
But you should read and learn from the #BlackSpecFic report anyways! The missing data is due to idiosyncrasies of the @EAPodcasts model, and has no impact on any other magazine's numbers.
Long story short, we treat reprints very differently from other magazines. For @escapepodcast specifically, they were ~45% of our 2017 stories, and our editorial process has one unified pipeline for originals + reprints together.
Read 4 tweets
Jul 23, 2018
Regretting organizing my two Worldcon panels this year. It means I'm not free to throw up my hands in frustration and give up on programming. The last 24hrs have been the last worst icing on a bad cake that's long been baking.
I mean, my panels will be awesome. But if you're skipping programming because you don't trust the con, you've made a sensible choice.
There are always more people who want to be on programming than can fit. There's no way to make everyone happy. I get that. But this weekend's screwups come in the context of a long chain of trust-erosion.
Read 4 tweets
Jul 13, 2018
So glad this one came out! "After Midnight at the Zap Stop" by @ouranosaurus is an awesome story - full of late-night grease, and the luckless & the worthy. But also because it's a #neuroscience teaching opportunity. Might even be a #NeuroThursday!
One offhand line explains a technology as "stimulating a particular set of mirror neurons." Which works as a story element just fine. It sounds plausible and authoritative! But as a neuroscientist, I have strong opinions about #mirrorneurons. I don't think they're real.
To be clear, mine is a controversial opinion. Many neuroscientists would disagree. But it's a hill I'm willing to fight on, especially given how often "mirror neurons" crop up in popular science.
Read 14 tweets
Jul 8, 2018
This Lindsey Sterling + Evanescence concert has been going for 3 minutes and it is already amazing.
Update: she is simultaneously dancing, playing violin, and kicking skeletons.
P.S. She too is a skeleton. Hard to be sure at this distance but I believe she has glittery bones.
Read 14 tweets
Jul 4, 2018
This phenomenon - when you look away from a moving thing, and you briefly see illusory motion in the other direction - is the "Motion Aftereffect," and it comes from some very basic brain maneuvers. Who wants to join me on going full #NeuroThursday here? en.wikipedia.org/wiki/Motion_af…
Most neurons in the brain (and elsewhere) do this thing called "adaptation," where they accept whatever's going on as the new normal. For example, if you sit down with your laptop on your lap, you'll soon stop noticing the weight.
This can arise from the crudest single-cell level: some ion channels in the cell membrane have negative feedback loops that self-dampen.
Read 14 tweets

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