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The structural organization of animal eyes provides the first step toward visual perception. There is substantial diversity among animal eyes, yet they tend to fall into a few distinct categories such as single chamber and compound eyes. Underlying nearly all light perception are visual pigments based on a family of proteins called opsins. These molecules are tightly linked to retinal, a chromophore related to vitamin A, which undergoes a conformational change in response to light exposure. This activates a photoreceptive cell (either by hyperpolarization, as generally is the case for vertebrates, or by depolarization, which is typical for most invertebrates) and, ultimately, after many computational steps, results in the animal's perception of light. Differences in eye design influence factors such as the direction from which ...

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