Something mysterious and remarkable resides in the sense of smell, most famously captured by the novelist Marcel Proust in The Remembrance of Things Past. 

In the years following its original publication, scientists were examining the mechanisms that gave rise to scent profiles such as lemon and rose. It was already known that visual and auditory perceptual qualities were not complete psychological constructions but tied to real and quantifiable properties of electromagnetic radiation. Early theories suggested, much like hearing and vision, that an odorant molecule bears an electromagnetic signature that interacts uniquely with sensory receptors in the nose. Like objects resonating at a specific, characteristic frequency, complementarity between the molecule and receptor progressively grew into a distinct perceptual experience (e.g. “rose” or “lemon”). More recent work has since challenged this quantum mechanical “resonance”-based explanation, however, this and other early theories treated olfactory coding as a tractable, scientific problem, which was a key development in shaping current day thinking. 

It now seems clear that factors such as molecular shape, functional groups, and additional physicochemical properties contribute to the initial interaction between an odorant molecule  and receptors in the nose. Given an acceptable fit between the key (the molecule) and lock (the receptor), the sensory neuron in the nose expressing the receptor passes an activity threshold, leading to a signaling cascade that ultimately terminates in the olfactory or piriform cortex where the final identity (rose, lemon, etc.) is assigned. Yet many nuances remain unclear such as how very subtle shape differences can result in wildly different perceptual experiences (Explanation Box 1)