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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