Take a Hike on the Dark Side

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How often are you in the dark? I mean truly, no glow of your phone, in the dark? Could you navigate around your house, a place you probably spend a fair amount of time, for even one evening, without any light?

I’ve been spending a lot of my road trip in national parks. Recently, I was able to go on a night hike to record bat activity with a park ranger in Yosemite. Learning about bats and their incredible ability to hunt and navigate through the dark made me jealous that most humans don’t share that “superpower”. But what if we do?

Therein lies the power of rhodopsin. Before we go into that, let’s go over the basics of eyesight.

Your retina has two types of photoreceptors: rods and cones. Your eyes have more rods, which are light-sensitive and lie in the periphery. Cones are concentrated in the central spot called the macula and provide color sensitivity. Inside the macula, you have the fovea centralis which contains only cones. Since it has no blood vessels in the way, it’s easier for light to get through. When we want to get a better look at something, we move our head or our eyes until the object falls into our fovea, which has no rods. This is the reason why astronomers suggest you look slightly off to the side of a dim star. If you look directly at it, it seems to “disappear”.

eyeretfovearc

When the power goes out in your house suddenly and you’re left in the dark, you might feel that you can’t see anything because your eyes’ supply of the protein rhodopsin goes down. Your irises need to expand to let more light hit your retinas. The light is then turned into nerve impulses by breaking rhodopsin molecules down, which takes time. The process for your eyes to become dark adapted takes about 40 minutes. Then, if someone shines a flashlight your way, the process of replenishing rhodopsin has to start all over again.

Our second night in Yosemite, we went on a night hike in total darkness. When you walk in the dark, you can feel your body start to try and compensate for the temporary loss of a sense. Your heartbeat rises, your footsteps become uncertain, your hands reach out in front of you, you cock your head towards every background noise and your sense of smell becomes more sensitive.  After a while, you start to get your bearings and the violence of being thrust into darkness starts to seem more manageable.

A few days later, when we were at the Oregon Sand Dunes, I decided to try walking with my eyes closed. (I chose the soft, forgiving sand over walking in a canyon or forest for obvious reasons.) It was unsettling and I kept having to take a peek every few seconds, but I’m glad I tried it. I’ll definitely keep the practice going, if for no better reason that to have a greater sense of empathy for those who thrive without sight.

For more resources on vision and even an example of humans using echolocation like bats (!) visit the following links:

http://www.intropsych.com/ch04_senses/fovea.html

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html

https://faculty.washington.edu/chudler/eyetr.html

http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html

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