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| Artwork by Holly Fischer, CC BY 3.0, via Wikimedia Commons |
I
was at the opticians on Thursday for my two-yearly check-up. The
optician was showing me the image of my retina on the computer and
pointing out the salient features: the blind spot, the macula and fuzzy parts of the image due to floaters. The blind spot is the region of the retina where the optic nerve connects all the rods and cones
to the brain. Cones are structures in the retina, responsible for
sensing colour in the daytime, and rods are more sensitive for night
vision, but they can't discern colour.
The macula is the area of the retina that has the highest concentration of rods and cones. It's located towards the centre of our vision and what allows us to see greatest detail. When people develop a disorder called macular degeneration, the function of the macula is degraded, making it more difficult to read and perform other tasks. If you hold your thumb in front of your eye, blocking your central vision and try to focus on text in the distance, but not looking directly at it, you'll find it difficult to make out detail and read the text.
Floaters are pieces of debris, the remains of cells, floating about in the vitreous humour, a gel-like material that fills our eyeballs. If you try to look at them, they flick into the centre of vision, then slowly sink again.
The eye does the mechanics of converting an upside down image focused on the retina into an electrical signal and sending it to the brain, similar in principle to the charge coupled device (CCD) sensor in a digital camera. But that's only part of the story. Perception is something done by the brain, effectively "projecting" what we see onto the equivalent of a cinema screen. But what appears on that screen isn't exactly what's "out there" and not a one-to-one mapping like what would happen on a television or cinema screen. For starters, the image on the retina is upside down, so that has to be turned the right way up. We think we see a high resolution image of what's in front of use, but our eyes are really scanning a scene, building up a collage of images from the macula, so that makes a sort of composite image that our brain remembers. We can't see anything from the region of the retina where the optic nerve joins it, so the brain "fills in" that blank spot in the image we perceive. What's most amazing I think is how we don't see the blood vessels in our eyes. They're visible in the retinal image taken by an optician, but our brain does something like the post processing used in movies to remove wires that support flying actors. If you look at the ceiling just as you wake up and open your eyes, you can sometimes see the blood vessels as black lines, like the roots of a tree, for a split second before your brain kicks in and removes them. The optician hadn't experienced this phenomenon, but said she'd look out for it. Optical illusions are another quirk of perception where what we see isn't necessarily what's out there. Two illusions are worth checking out, including the checker shadow illusion and the simultaneous contrast illusion.
The macula is the area of the retina that has the highest concentration of rods and cones. It's located towards the centre of our vision and what allows us to see greatest detail. When people develop a disorder called macular degeneration, the function of the macula is degraded, making it more difficult to read and perform other tasks. If you hold your thumb in front of your eye, blocking your central vision and try to focus on text in the distance, but not looking directly at it, you'll find it difficult to make out detail and read the text.
Floaters are pieces of debris, the remains of cells, floating about in the vitreous humour, a gel-like material that fills our eyeballs. If you try to look at them, they flick into the centre of vision, then slowly sink again.
The eye does the mechanics of converting an upside down image focused on the retina into an electrical signal and sending it to the brain, similar in principle to the charge coupled device (CCD) sensor in a digital camera. But that's only part of the story. Perception is something done by the brain, effectively "projecting" what we see onto the equivalent of a cinema screen. But what appears on that screen isn't exactly what's "out there" and not a one-to-one mapping like what would happen on a television or cinema screen. For starters, the image on the retina is upside down, so that has to be turned the right way up. We think we see a high resolution image of what's in front of use, but our eyes are really scanning a scene, building up a collage of images from the macula, so that makes a sort of composite image that our brain remembers. We can't see anything from the region of the retina where the optic nerve joins it, so the brain "fills in" that blank spot in the image we perceive. What's most amazing I think is how we don't see the blood vessels in our eyes. They're visible in the retinal image taken by an optician, but our brain does something like the post processing used in movies to remove wires that support flying actors. If you look at the ceiling just as you wake up and open your eyes, you can sometimes see the blood vessels as black lines, like the roots of a tree, for a split second before your brain kicks in and removes them. The optician hadn't experienced this phenomenon, but said she'd look out for it. Optical illusions are another quirk of perception where what we see isn't necessarily what's out there. Two illusions are worth checking out, including the checker shadow illusion and the simultaneous contrast illusion.