Can We See With Our Skin?
What is “seeing” anyway?
Can We See With Our Skin?
No, of course not, we see with our eyes.
OK, but what do we mean when we say we see with our eyes?
What is “seeing”?
Light hits our retina, flicks some molecules in a nerve, triggering a process for that nerve to send signals to various places in our brain, and (in an unknown process that has whole philosophical and neurological fields built around figuring it out) the brain represents those signals as the qualia of our conscious visual experience.
That’s what allows us to see things.
So we have light, some nerves, a signal to the brain, some neurological computational processes and, voilà: sight.
But what happens when I put my hand into the path of direct sunlight?
The light from the sun hits my skin, my skin absorbs that light and heats up, nerves in my skin detect the heat and send a signal to various parts of my brain, and some neurological computational processes occur and, voilà: I can feel the warmth of the sun on my skin.
How is that different from “seeing”?
Seeing The Light
Those nerves in your skin aren’t light-sensitive nerves.
OK, they might not be sensitive to light, and definitely not sensitive to light in the visible spectrum (380–700nm), but the heat from the sun is just infrared light, and so those nerves are indirectly sensitive to infrared light (750nm — 1000µm), which is fundamentally the same as visible light.
Besides, the light in the visible spectrum is visible not because of some special property of that band of wavelengths, but because of a special property of a few different molecules and proteins packed into our retinas.
And the light is “visible” because the cells those molecules are packed into send signals to an area of the brain — your visual cortex — which has evolved to produce the experience we call “vision”.
Whatever I feel when I put my arm into the sun I can feel because light is heating up my skin, triggering nerve cells that sends off a signal to my brain.
And, although the process in the nerve might not be the same, once that nerve in my skin (or the one in my retina) has done its job, then it’s just passing along an electrical signal down a string of neurons to my brain (same as what the electrical signal does from my retina).
A Look At The Visual Cortex
But the signals from your skin don’t go to your visual cortex, hence it’s not “seeing”. “Seeing” is what happens in the visual cortex.
OK, maybe, but what is the visual cortex?
That might also seem obvious: it’s the part of the brain that processes visual information.
But that definition is somewhat circular, and it also doesn’t present the full picture (no pun intended).
While it’s true that the visual cortex processes signals from the eyes (specifically the retina), which is what we label as “visual” information, it also processes information from other parts of the body in special circumstances, like the ears in blind people using echo-location.
We know this because when people use echo location their visual cortex lights up.
So it makes more sense to think of the visual cortex as the part of the brain that creates our visual experience from the information that gets sent to it — it analyses signals into the qualia we experience as “visual” — and that it does so regardless of where that information comes from.
But then, what does that mean?
What is the visual cortex doing? What is it processing? What information is represented in the experience that it creates?
Seeing What’s Out There
Well, one way to think of it is to say that the visual cortex is the part of the brain that builds a 3D model of the world.
Or, at least (because the 3D-ness is a result of having more than one input), it produces the experience of the relative location of things in our world.
That would explain our normal experience of visual information, as well as the fact that echo-location involves the same systems.
Yes, visual data also contains information about things other than location — shape, colour, etc — but you can take many of those characteristics out of the data and still be able to get through life without too much difficulty — you can, for example, still understand what’s happening in a black-and-white movie.
But take away the location information from the visual field and you’ll suffer a significant detriment in your capacity to navigate the world.
It’s also hard to imagine what visual information would be without the location information anyway— perhaps because that’s all visual information fundamentally is.
Seeing Specialists
Again, we can think of the visual cortex as the part of the brain that creates our visual experience regardless of where the information comes from.
But then, why does the information from the eyes clearly dominate our experience?
Well, the information that comes specifically from our eyes has extra importance because they’re specialised at detecting light. (Not to add to the growing list of obvious statements).
If an organism has any hope of getting through existence, then it needs to tap into and take advantage the information available in its environment.
And for that purpose light is a spectacular source: it’s extremely stable, practically instantaneous and reasonably error-free.
And so developing sensors that are sensitive to light is good idea. That’s why eyes evolved (likely from light-sensitive areas of the evolutionary precursor to skin). Though, something was bound to take on the job.
Imagine a tiny early version of the brain trying to figure out where things are in the world (this is in no way meant as an accurate representation of how biological evolution works), and it’s attached to a few different cells all sensitive to different things, sending different information in.
And imagine one of those cells is sensitive to light. Then, because light carries a dense amount of information about the outside world, slight improvements in the way the light-sensitive cells work (and how the brain processes the information), will translate into massive improvements in how the brain attached to those cells is able to navigate the world.
There’s therefore pressure to keep improving light-sensitive cells at the cost of improving the other types of cells that are sending other types of information in.
Eventually (with some other improvements) you get to a patch of high-density (and hence high resolution) sensitivity to light.
If there’s so much information available in the light, and you’ve already improved the light-sensors (your eyes) so much, then it makes sense to expend most of your resources in the outside-world-model-making part of the brain on processing the information from those sensors, perhaps even at the cost of the information from the other sensors.
And hence other signals, like those from the ears or skin or nose, get mostly drowned out in our location-determining (a.k.a, visual) experience.
But if we lose the eyes, then we — our visual cortices — would have to rely on other sources.
Skin Sees The Sun
So then that brings us back to the initial question:
Can we see with our skin?
If I stand in the shade on a sunny day, close my eyes and put my arm out into the sun and try to locate where in the sky the sun is, I will be able to do so with a reasonable amount of accuracy.
Sure, I might not be able to tell the difference between its position at 12:05 and 12:06; but 9:05 and 12:06, sure; 12:05 and 1:06, maybe.
Haven’t you rigged the game by adding “sunny day” into the equation?
Well, at the level of resolution of our skin, requiring a “sunny day” in order for our skin to “see” is the equivalent of there being at least some level of visible light present in order for our eyes to “see” — you can’t see anything in a pitch-black room.
Strange To See
Of course, I’m not trying to convince you that we can actually see with our skin — by any reasonable treatment of definitions and concepts we cannot.
But properly thinking about this question sheds light (again, no pun intended) on what is actually happening between our neurons and the world.
“Seeing” is just a reconstruction of that outside world, an interpretation of a deluge of information about the things around us.
The point of thinking about whether we can “see with our skin” (or any similar proposal) is to re-examine this all-too-obvious and taken-for-granted “visual” experience of our world.
The point is to recognise that “seeing with our skin” is no stranger an idea than “seeing with our eyes”.
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