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Abstract

Sensory coding represents a basic principle of all phyla in nature: species attempt to perceive their naturalsurroundings and to make sense of them. Ultimately, sensory coding is the only way to allow a species tomake the kinds of crucial decisions that lead to a behavioral response. In this manner, animals are ableto detect numerous parameters, ranging from temperature and humidity to light and sound to volatileor non-volatile chemicals. Most of these environmental cues represent a clearly defined stimulus arraythat can be described along a single physical parameter, such as wavelength or frequency; odorants, incontrast, cannot. The odor space encompasses an enormous and nearly infinite number of diverse stimulithat cannot be classified according to their positions along a single dimension. Hence, the olfactory systemhas to encode and translate the vast odor array into an accurate neural map in the brain. In this review,we will outline the relevant steps of the olfactory code and describe its progress along the olfactorypathway, i.e., from the peripheral olfactory organs to the first olfactory center in the brain and then tothe higher processing areas where the odor perception takes place, enabling an organism to make odor-guided decisions. We will focus mainly on studies from the vinegar fly Drosophila melanogaster, but wewill also indicate similarities to and differences from the olfactory system of other invertebrate speciesas well as of the vertebrate world.