From The Olfactory Lives of Primates by Robert M. Sapolsky (Virginia Quarterly Review, not online):
For sensory systems, how many steps does it take to get from the eye, ear, or path of skin to that emotional limbic system. Roughly ten steps. Take vision. First there's a layer of neurons in the cortex that breaks the visual scene into dots, then a next layer turning the dots into lines, then collections of lines, on and on. Finally, an Ice Age later, by a neuron's temporal standard, visual information trickles to the limbic system, and you activate an emotional response appropriate to seeing the face of someone intent on, say, seducing you or ethnically cleansing you. All of the sensory systems, that is, except olfaction. How many steps from smelling something to the limbic system? Just one.
See also: NY Times article on smell.
