Since eyes can contain an extraordinary number of photoreceptors and they can’t all connect directly to the optic nerve, receptors are connected by a smaller number of ganglion cells, which pool visual input and transfer the consensus along to the optic nerve and onward to the brain.
The “perception” part of vision doesn’t begin until these neural impulses, coded as cascades of electrical signals, travel from the retina to the brain. This three-part sensitivity results in millions of combinations that we perceive as colors, not unlike the way in which computer monitors may display images based only on the axes of red, green, and blue. Microraptor gui, a tiny four-winged dromaeosaur dinosaur, is now known to have been iridescent black in life.
Cone cells, for example, transduce photons into neural impulses via three classes of photopigments-called photopsins-each of which absorb a different range of light wavelengths. These receptors translate light energy into neural impulses by the differential bleaching of photopigments. These receptors are specialized neurons classically divided into rods and cones, named for their shape and adapted to handle the value and color, respectively, of our visual environment. Each retina has at least one fovea, a densely packed concentration of photoreceptors that’s responsible for sharp central vision. Bird eyes, like ours, function by admitting light through the transparent cornea, lens, and vitreous body to stimulate photoreceptors on the retina at the back of the eye. The Vertebrate EyeĪll vertebrate eyes descend from a common ancestor, and most of them share a number of basic qualities. But this specialization must be based around a common model. This also applies to flight in the animal kingdom, where the unforgiving demands of flight have led to an astonishing diversity in form, function, and style.įalcons and other birds of prey represent the upper end of what seems possible for animal vision, and their eyes are highly specialized for a unique predatory lifestyle on the wing. In order to fly a jet, a strong and responsive visual system is absolutely paramount, because a single mistake in judging distance, depth, or timing could spell instant death. But dragonflies and F-16s notwithstanding, nothing has conquered the skies quite like the bird.įor us-clever, indeed, but weak, fragile, and relatively blind-it should come as no surprise that the vision standards for becoming a US Air Force pilot are remarkably stringent: no color blindness, no abnormal refraction or accommodation issues, and corrected 20/20 vision. A suite of highly specific characteristics is necessary for the defiance of gravity, from lightweight, lift-generating surfaces to precise sensing devices. Powered flight has arisen independently on our planet a total of five times: in insects, pterosaurs, bats, birds, and, of course, human beings.
Falcons and other birds of prey have highly specialized eyes for a predatory lifestyle on the wing.
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