Sensation

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In psychology, sensation is the first stage in the chain of biochemical and neurologic events that begins with the impinging of a stimulus upon a sensory organ, and leads to perception, the sort of mental state that is reflected in statements like "I see a uniformly blue wall."

A sensation that might lead to that statement could include the excitation of cone cells in the retina, spatially varying in the proportion of "blue" and "green" cone excitation due to portions of the wall receiving different proportions of yellowish artificial and bluish sky-light; it is common for these variations to be compensated for, within the brain, so that the non-uniform sensation yields a perception of uniform color.