Blind Spot In Eye Rods And Cones

Vision Crash Course AP 18

Take a good long look at this we're gonna mess with your brain. This is the first stage of an optical illusion. Many illusions use patterns of light or perspectiveto exploit the disconnect that exists between sensation and perception between what youreyes see and what your brain understands. But not all illusions work that way. Someproduce ghost effects, or afterimages, that take advantage of glitches in the physiologyof human vision. Like this flag. I'm not trying to make a political statementhere. And I'm not going ask you to swear

allegiance to the Republic of Hank or anything.I mean, if I was gonna start my own country, my flag would be way cooler than that notthat I've thought about that a lot. And now, look at this white screen. If you looked at that flag for at least 30seconds without moving your eyes, you'll see something, even though this screen isblank an afterimage of the flag. But instead of being turquoise, and black, and yellow,it's red, white, and blue. OK so that's pretty cool, but I'm nothere just to entertain you. This kind of illusion is actually a great way to explain your verycomplex sense of vision.

And I do mean complex… nearly 70 percentof all the sensory receptors in your whole body are in the eyes! Not only that, but in order for you to see,perceive, and recognize something whether it's a flag or a handsome guy in glassesand a sport coat sitting behind a desk nearly half of your entire cerebral cortex has toget involved. Vision is considered the dominant sense ofhumans and while we can get along without it and it can be tricked, what you are aboutto learn is NOT an illusion. When we talked about your sense of hearing,we began with the mechanics of sound. So before

we get to how your eyeballs work, it makessense to talk about what they're actually seeing light bouncing off of stuff. Light is electromagnetic radiation travelingin waves. Remember how the pitch and loudness of a sound isdetermined by the frequency and amplitude of its waveé Well, it's kind of similar with light, exceptthat the frequency of a light wave determines its hue, while the amplitude relates to itsbrightness. We register short waves at high frequenciesas bluish colors, while long, low frequencies look reddish to us.

Meanwhile, that red might appear dull andmuted if the wave is moving at a lower amplitude, but super bright if the wave has greater amplitudeand thus higher intensity. But the visible light we're able to seeis only a tiny chunk of the full electromagnetic spectrum, which ranges from short gamma andX rays all the way to long radio waves. Just as the ear's mechanoreceptors or thetongue's chemoreceptors convert sounds and chemicals into action potentials, so too doyour eyes' photoreceptors convert light energy into nerve impulses that the braincan understand. To figure out how all this works, let'sstart with understanding some eye anatomy.

Some of the first things you'll notice aroundyour average pair of eyes are all the outer accessories like the eyebrows that help keepthe sweat away if you forgot your headband at raquetball, and the supersensitive eyelashesthat trigger reflexive blinking, like if you're on a sandy beach in a windstorm. These features, along with the eyelids andtearproducing lacrimal apparatus are there to help protect your fragile eyeballs. The eyeball itself is irregularly spherical,with an adult diameter of about 2.5 centimeters. It's essentially hollow full of fluidsthat help it keep its shape and you can

2Minute Neuroscience The Retina

Welcome to 2 minute neuroscience, where Isimplistically explain neuroscience topics in 2 minutes or less. In this installmentI will discuss the retina. The retina contains the neural component ofthe eye. When light reaches the back of the eye, it enters the cellular layers of theretina. The cells of the retina that detect and respondto light, known as photoreceptors, are located at the very back of the retina. There aretwo types of photoreceptors: rods and cones. Rods allow us to see in dim light, but don'tallow for the perception of color. Cones, on the other hand, allow us to perceive colorunder normal lighting conditions. Throughout

most of the retina, rods outnumber cones.In one area called the fovea, however, there are no rods but many cones. The fovea representsthe area of our retina that provides our highest acuity vision, and thus is at the center ofour gaze. When light hits photoreceptors, it interactswith a molecule called photopigment, which begins a chain of events that serves to propagatethe visual signal. The signal is transmitted to cells called bipolar cells, which connectphotoreceptors to ganglion cells. Bipolar cells pass the signal on to ganglion cells,which leave the eye in a large cluster at an area called the optic disc. The optic discdoesn't contain any photoreceptors, and so

represents an area of the retina that can'tprocess visual information, creating a natural blind spot. However, we normally don't noticeour blind spot. The brain uses information from surrounding photoreceptors and the othereye to fill in the gaps in images that are processed by the retina. After leaving theretina, the ganglion cell fibers are called the optic nerve. The optic nerve carries visualinformation toward the brain to be processed. There are two other cell types in the retinathat should be mentioned: horizontal and amacrine cells. Horizontal cells receive input frommultiple photoreceptor cells. They integrate signaling from different populations of photoreceptorcells, make adjustments to the signals that

will be sent to bipolar cells, and regulateactivity in photoreceptor cells themselves. Amacrine cells receive signals from bipolarcells and are involved in the regulation and integration of activity in bipolar and ganglioncells.

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