To see or not to see: What is perception?

country lane with autumn colours, woodland either side

“Country lane” by Jongleur100 on Wikimedia

A stoke damaged my ability to see things on my right hand side, in both eyes. I have homonymous hemianopsia, homonymous meaning the same in each eye and hemianopia meaning half-blind.

A 19th century sufferer joked that the name of his condition was too long for him to read, and I can see why. If I fix my gaze steadily on the first ‘h’ then here’s what can read

homonymush blur er

I don’t see nothing beyond, I just don’t see anything.

It’s really hard to explain the sensation, because most people naturally imagine something like a shadow, or whiteout, or lack of sharp vision (“Please let us know if you would like the large print version”). But that doesn’t capture the subjective experience at all. Philosophers speak of the quality of “redness”, as an example of qualia – “redness” is that unique quality of subjective experience that we get when we see blood, wine or roses, and so on. Red things.

I experience a qualia something like “glassiness right in front of me” when I actively don’t see something. Let’s call it grifom.

Just as it’s impossible to explain red to someone who’s never experienced it, conveying grifom is a challenge but I’ll do my hardest. Unfortunately there’s no way of passing out my inner perceptions in digital form but I’ve faked it a bit with the help of my phone.

On my kitchen wall I stuck a row of index markers and took a photo. If you were to stare fixedly at the green marker on the left this is what you might see. All the markers would be visible, if you have normal sight, although the ones off to the right would be rather indistinct, because they are in peripheral vision. But you’d see them.

six index markers, green at left then rest red, equally spaced on a wall

Not me. This is what I see see with the naked eye. There’s a blank green wall ahead, with just two index markers, the first and last. The first because that’s the one I am staring at and the last because I’ve kept a crescent-shaped bit of vision at very far right. In between, nothing. Blind spot. Nada. But notice there is a wall there still! I can see it.

only the far left and far right marker are now visible

Next step of the experiment> I hold my hand up in front of me. Your view (and mine through the camera!)

a hand obscures some of the markers

My view with the naked eye again. Notice the absent hand.

only the far left and far right marker are now visible

How can this be? Am I seeing through my own glass hand? No, some part of my brain is filling in for me. I can’t actually see behind my hand, or see anything where my hand is, but a bit of my brain is photoshopping it in – “That missing bit’s most likely green wall like the rest” and so that’s what I experience.

This ability to fill in such a large gap is truly astonishing. Measurement shows I have a blind spot that covers about one third of my vision. But I still see the wall.

What is more my brain can fill in far more complex pictures than a plain green wall. If I drove down the country lane you see at the top of this post I would be unaware of vision loss. The whole scene would be spread before me. Here it is again.

country lane with autumn colours, woodland either side

But I’ve got a great big blind spot in reality. So a big chunk of what I imagined I was seeing would be a lie, something my brain had stitched together from partial information to appear convincing and blend in with the surroundings.

Beautiful – but deadly dangerous. I might simply not see a person, or an animal, or a vehicle, that was coming towards me. Which is why I can’t drive anymore.

About partialinsight

One evening I had a stroke. Half my sight vanished overnight. Adapting made me grasp how amazing the visual system and brain are. It also taught me to understand disability completely differently and I'm grateful for the lesson.
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