You’re lying in bed at night, looking at the ceiling. Out of the corner of your eye, you see something – a shadow or a dim light. You move your gaze to investigate, and it’s gone. Maybe it’s nothing, you think. But when you look up again, it’s in your peripheral vision again. You move your head to look, and it’s gone again.
Don’t panic, you’re not imagining it – this vanishing effect is a result of the way our eyes are structured.
Our eyes have two types of light-sensing cells: rods and cones. These are called photoreceptors because they respond to light. (“Photo” comes from the Greek word “phos,” which means light. So the prefix “photo” means light. Think photons, photosynthesis, photography.)
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Both rods and cones line the inside of the retina at the back of the eye. But these light sensors are not distributed evenly, and they respond to light in different ways. This difference between rods and cones is why objects appear to disappear in the dark.
Cone cells help us see color and fine detail. They are concentrated in the central area of the retina, called the fovea. When you look directly at an object, the image is captured on the fovea. Cone cells help us see objects in vivid color and detail. However, cones are not very sensitive to light. That’s why they only work well in sunny or bright places, not in dim light.
This is where the rod comes in handy.
Rods make up the majority of the photoreceptors in our eyes. They are located on the edge of the retina, outside the fovea. very They are sensitive to light – perhaps even single photons – that enable them to see in a dark bedroom or outside at night, and they are also very good at detecting motion, especially in their peripheral vision.
Peripheral vision
If someone walks into the room while you’re reading this, you might notice them out of the corner of your eye. That’s peripheral vision at work. Peripheral vision helps you see what’s going on around you without having to turn your head. Images of objects in your peripheral vision fall on parts of the retina that are covered primarily by rod cells.
These rods also work well in low light, allowing you to better distinguish objects in the periphery of your field of vision. Try it for yourself: in the dark, look just to the side of an object. Then look straight at the object and it seems to disappear. Without the rods in your fovea, you wouldn’t be able to focus on objects in the dark.
Remember, only cones can perceive color, not rods. So why don’t objects appear gray in your peripheral vision? Our brains compensate for color. Here’s how to see it for yourself:
Have your friend sit in a chair. Stand next to your friend, about 1 meter (3 feet) away from one of their ears. Ask your friend to stare straight ahead at a point. Pick up a colored marker, but make sure your friend can’t see the color. Hold the marker just in front of your friend, at eye level (make sure your friend doesn’t turn their head – they need to keep staring ahead). When you do this, your friend should just be able to see you in their peripheral vision. Ask your friend if they can see the marker. If they can’t, move the marker slightly forward until your friend first notices it in their peripheral vision.
Next, ask them to identify the color of the marker. They probably can’t – the marker is detected by the color-insensitive rod cells in the periphery of the retina.
Again, move forward in an arc toward your friend. Can your friend name the color? Repeat this process until your friend can see the color. This is when the color-sensitive cones in the middle of the retina first find the marker.
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This structure of the retina – with cones in the center and rods on the edges – makes a lot of sense from an evolutionary standpoint. Motion can convey important information, including danger. Rods are good at detecting motion, even at the edge of your vision. Instinctively, we turn toward moving objects, asking the cones to help us see the object in detail.
However, this system only really works well in bright light.
What if the danger comes at night? Many biologists believe this is one of the reasons why humans evolved to be active during the day and sleep when it was dark: our eyes are not adapted to life in the dark.
So if you see something out of the corner of your eye in the dark, you look at it and it’s gone. But you’re not seeing something that’s not there. It’s just that not enough light is reaching the cones. It could also be time to close your eyes and get some sleep.
Source: www.snexplores.org
