How animals see in the dark?
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On a moonless night the light level can be 100 million times lower than in bright daylight. And if we are almost blind and completely helpless in the dark, cats quite successfully hunt down prey, and butterflies quickly fluttering between flowers on our balconies. While we sleep, millions of other animals rely on their visual system to survive. The same can be said about animals that live in the eternal darkness of the deep sea. Moreover, the vast majority of animals in the world mainly active in dim light. How they manage to use such powerful visual characteristics, especially insects, with their tiny eyes and brains less than a grain of rice? What optical and neural strategies they have developed to see well in dim light?
To answer these questions, let's turn our attention to the night insects. Despite their miniature visual system, night insects see well in dim light. In recent years, we found that nocturnal insects can avoid and fixed obstacles during the flight, distinguish colors, to detect weak movements, to learn visual landmarks and use them for homing. They can even navigate using a weak pattern of polarization of the star by the Moon, and to navigate using the constellation of stars in the sky.
In many cases, this visual performance seems completely violate what is physically possible. For example, the Central American nocturnal bee Megalopta genalis absorb five photons with their tiny eyes when light levels are at extremely low level — imperceptible visual signal. Nevertheless, in the deep of the night she can navigate through dense and tangled tropical forest during foraging and safely return to its nest, invisible hollowed-out stick, dangling in the undergrowth.
To see how all this is possible, scientists began to study hawk moths. These beautiful insects — Hummingbird invertebrate in the world — presents an elegant, fast-flying butterflies that are constantly searching for flowers with nectar. Once the flower was found, the moth hovers in front of him, sucking the nectar through the proboscis, ratovitski tube.
Night European Deilephila elpenor hawk moth is a beautiful creature, hiding in feathered pink and green scales that collect nectar in the deep of the night. A few years ago, scientists have discovered that this butterfly can discern colors in the night, the first night animal, that is known.
Recently, this butterfly has revealed another of its secrets: neural tricks that she uses to see well in very dim light. These stunts, of course, use and other night insects, such as Megalopta. After studying the physiology of neural circuits in the visual centers of the brain, scientists have discovered that Deilephila can see in the dim light, effectively folding the photons collected in different points of space and time.
It's a bit like increasing the shutter speed on the camera in low light. If you allow the shutter to stay open longer, more light will reach the image sensor and will get brighter picture. The disadvantage is that anything moving fast like driving a car, I don't get permission, so the insect will not see it.
theNeural summation
To combine the photons in space, the individual pixels of the image sensor you can pool, creating smaller, but large number "superpixel". Again, the disadvantage of this strategy is that even with the high brightness of the image will be blurry and devoid of clear detail. But for night animal that tries to live in the dark, able to see the bright, but lacking in detail and slow world will be better than not seeing anything at all (which is the only alternative).
Psychologists have shown that the neural summation of photons in time and space is extremely useful for night deilephila. With all the intensity of night light, from twilight to the stars, the summation significantly improves the ability of deilephila to see well in dim light. In fact, due to these neural mechanisms, deilephila can see at 100 times dimmer light than otherwise. The advantages of a sum so great that other night insects too, very likely rely on him to see well in the night.
The World observed the night insects, may not be as sharp or well resolved as the one that they see day-active relatives. But the summation ensures that it will be bright enough to catch prey, fly back to the nest and to avoid obstacles. Without this ability they would be as blind as we all are.
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