Atul Gawande has a piece in the New Yorker about a woman who went from a normal, kids ‘n’ apple-pie life, through a series of disastrous life choices and wound up with an itching sensation in her scalp that she just couldn’t shake. This wasn’t just any itching sensation, I’m talking the mother of all itches that caused her to eventually scratch all the way through her skin, skull, and start gouging out chunks of her brain. Lucky for her (it’s all relative, right?) she was rushed to hospital, and spent the next few years wearing a football helmet and giant padded boxing gloves to bed every night in an attempt to stop the scratching. The dude in the bed next to her managed to fight off the gloves in his sleep and puncture a hole in his neck to get at the itching there, which killed him.

These feel-good anecdotes are all well and good, but they aren’t the interesting part of the article. This blew my mind:

The images in our mind are extraordinarily rich. We can tell if something is liquid or solid, heavy or light, dead or alive. But the information we work from is poor-a distorted, two-dimensional transmission with entire spots missing. So the mind fills in most of the picture. You can get a sense of this from brain-anatomy studies. If visual sensations were primarily received rather than constructed by the brain, you’d expect that most of the fibres going to the brain’s primary visual cortex would come from the retina. Instead, scientists have found that only twenty per cent do; eighty per cent come downward from regions of the brain governing functions like memory. Richard Gregory, a prominent British neuropsychologist, estimates that visual perception is more than ninety per cent memory and less than ten per cent sensory nerve signals.

I had a weird sensation last week while riding through a park I pass through every day. Looking at the bare tree branches, I wondered when the last time I’d actually looked at those trees was. It felt like I’d never seen them before, and maybe I hadn’t. Maybe my brain just created a mockup of what it should look like and every time I rode past this illusion kicked in, and I never actually looked at the trees. It’s probably pretty common, and raises all kinds of questions for me. Do everyone’s brains create the same kinds of illusions? Or does the same scene look completely different to each person? Are hallucinations that rare, or are we all walking around with what are in effect hallucinations? Does everyone have the same proportion of visual perception to sensory signals?

I’ll never look at anything the same again.

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