Crozet Annals of Medicine: Blindsight

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If the doors of perception were cleansed every thing would appear to man as it is, Infinite. For man has closed himself up, till he sees all things thro’ narrow chinks of his cavern.  –Wm Blake

“Wow.” The word escaped my lips unplanned. “That was really cool.”

“You saw it, didn’t you?” said my ophthalmologist with a chuckle. She was examining my eye with a prism that bends light around a corner so she could see the hidden recesses of my inner eye.

“Just for a split second, but very cool.”

What I saw oh so briefly was sublimely beautiful; it was the lacy web of the arteries and veins of my retina.

Vision is a complicated miracle that we mostly take for granted. Light waves reflecting off objects enter our eyes in straight lines that are then bent (refracted) by our corneas, the outermost layer of our pupils. The bent rays then pass through the anterior chamber of our eye, the part in front of our pupil, then through the adjustable diameter pupil, then though the adjustable focus lens that bends them some more, then through the posterior chamber of our eye, finally ending up on our retina where the light sensors are.

Through some evolutionary quirk the blood supply to the retina actually passes in front of the retina, not behind it as you might expect. This means that the arteries and veins constantly shadow the light falling onto our retinas. In a literal sense, the blood supply to the eye is always visible yet we never see it. The brain filters the image, extrapolating information to fill in the shadows so that we see a complete and uninterrupted image of the world and cannot perceive what is actually there, the world seen through a spidery web.

When my ophthalmologist bent the light rays into my eyes, the shadows of my blood vessels were cast on unfamiliar portions of my retina that weren’t desensitized to them and thus briefly I could see them. Almost instantly my brain kicked in and stopped perceiving them.

I wonder how much else my brain filters for me.

Vision gets even more complicated once the light gets to the back of the eye. The light receptors of the retina create tiny electrical pulses that travel through the optic nerve, then traverse the entire brain to finally get sorted out in the very back of our brains, the occipital lobe. Without a working occipital lobe you cannot “see,” even if your eye and optic nerve are working just fine. Strokes to this part of the brain can cause what we call cortical blindness.

And this is where it starts to get weird.

When the occipital lobe is damaged by stroke or trauma or infection, patients can no longer see anything in the corresponding visual field. Each occipital lobe controls the visual field of the opposite side of the eye. For example the right occipital lobe perceives sight from the left side of each eye. Damage to both occipital lobes causes complete blindness. Unfortunate.

And yet a small group of these completely blind patients exhibit something called blindsight. If a ball is tossed to them, many times they can catch it. When asked how they did it, they cannot say. They still cannot see it even though it is now in their hand. When asked to guess about the presence of objects or movement, they guess correctly far more often than chance alone would account for. Researchers have to phrase it as a guess in order for the patients to participate because they simply cannot see anything consciously.

The most prevalent theory explaining this phenomenon of blindsight is that small branches of the optic tract wander off into other diffuse parts of the brain involved with initiating and maintaining eye movements when new moving visual clues present suddenly. These small parts of the brain can “see” without perceiving. They are unable to interpret images the way the visual cortex in the occipital lobe can, but they do receive the input and act on it.

Whatever the origin of the phenomenon, it does provide intriguing clues that we can respond to visual input from our environment without ever being consciously aware of seeing the input. We all may have some degree of blindsight. Our brains filter out some images and can act on some images that we may not ever see.  The doors of perception indeed.

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