Your Sense of Sight
The following is excerpted from A Primer on Vision.
If the world were the way it appears on your retina, the universe would jerk every time you took a step or moved your head. It would disappear every time you blinked. And since the retina receives a flat picture, we’d all be two-dimensional, living in a world of meaningless fragments.
If you think about it, you’ll notice that in fact you seldom see an entire object. Half the rocking chair will sit behind the doorframe. Chunks of tree are hidden by picket fence. Yet your mind makes these fragments whole, without giving it a thought. We infer continuity from blink to blink, jerk to jerk. We understand three dimensions so deeply that even when we view an object from an unfamiliar angle–imagine a rocking chair seen through a glass floor–we still know what it is. At a glance, literally in milliseconds, we routinely make sense of what we see.
No computer, however powerful, can perform such marvels. Cameras can easily produce an image of a scene, no problem, but within the image computers cannot even reliably pick out an edge, to decide where one object stops and the next begins.
I studied all about rods and cones and corneas in 8th grade. Could you jump ahead to how the system interprets the disjointed jumble we see?
The neat stuff starts in the retina, which is technically part of the brain, only displaced toward the light. However, we can keep the eyeball minimalist.
So: Light energy bounces off each person’s personal universe (whatever’s out there); enters the eye; and is focused on the retina as an isomorphic (iso = same, morphic = shape) picture of what you see. The image is shaped by some 5 million rods and 100 million cones. These serve much like pixels on a TV screen, because each responds only to photons of a given wavelength and/or brightness. Those that are stimulated go ready, set, fire!–and the picture appears. We see.
The picture is upside-down on the retina, but your brain doesn’t care. To know where things are, it only needs consistency. “People always ask about that,” says Michael McCloskey, a professor of cognitive science. “They’re kind of assuming that there’s some kind of homunculus inside, somebody looking at the image who needs to have it right-side up.”
It works better to think of the retinal image as 100 million bits of raw data–nothing the brain can use till it’s been deconstructed and recoded into dozens and dozens and dozens of categories. These the cortex will finally reassemble some 4 to 5 milliseconds later. (‘Be sure to step around that chair.’)
Deconstruction: The retina itself starts the sorting, as many millions of signals pass through its two layers of neurons to converge in only one million ganglion cells. Random neural firings (noise) don’t make the cut, because the ganglions pass along only strong signals. Nor do stimuli reporting more of the same. Rather, the ganglions single out indications of movement, edges, and color contrast, as coded by the retinal neurons.
From the retina, the picture goes to the brain per se?
Not yet. First comes a structure in the thalamus that is much like a layer-cake: six copies of the isomorphic image, one stacked precisely on top of the other. This is the lateral geniculate nucleus, or LGN, and you can think of it as an organizer.
Each layer gets signals from both eyes, the better to see three dimensions (and therefore to know which stimuli belong together). But only two layers get clues about motion, destined for what neurologists call the ‘Where’ pathway of the brain. The other four layers seize on stimuli to do with ‘What.’
The layers talk with one another, and the LGN talks with regions elsewhere in the brain. For example, it gets feedback from memory, presumably to help it process signals. Indeed, researchers have recently learned that traffic is heavier between the LGN and the cortex than between the LGN and eyeball.
From the LGN, streams for What and Where go separately to the striate cortex, where the isomorphic image will be analyzed for the orientation of edges. Specialized receptors fire if stimulated by a line that is vertical, horizontal, at 122º, 127º, or whatever. Most signals for Where then go to the parietal lobe (in the crown of the head), and those for What to the temporal lobe (near the temples). Now the isomorphic picture transmutates into enigmatic blips (to researchers). But for the see-er, meaning emerges.
From A Primer on Vision
Sight is associated with the solar plexus (3rd) chakra, which governs self-definition, ego identity, thoughts and power systems (like will power and your constitution). Our effectiveness and spontaneity also flow via the third chakra, and many people today rely quite heavily on their sense of sight to receive data. No wonder advertising is big business!
Humans see as much color as three receptor types for color make possible, as do many fish and birds.
Dogs and cats have only two color receptors. Pigeons have 340º surround vision, although 3D capacities are only right in front of their beaks.
Eyes are the busiest muscles in our bodies, with movements estimated at 100,000 per day.
Super great content made for curious birds without a science background.
A Primer on Vision – Johns Hopkins Magazine
Do You Wear Glasses? (Here’s Why) – Neuroscience for Kids
Eye/Vision Review – Neuroscience for Kids
Vision: In Short – Johns Hopkins Magazine
Sight – BBC Science & Nature: Human Body & Mind
Balance – BBC Science & Nature: Human Body & Mind
Chakras Chart: shows corresponding sense, area of consciousness, color vibration, musical vibration, gland, nerve and system of the body and element – The Brofman Foundation for the Advancement of Healing