Odourants

the definition of odourants has been less successful than the mathematical descriptions of sound and light
numerous attempts have been made define odour qualities --- essentially on a scale from pleasant to unpleasant--- but, until recently, development of a scale related to chemical structure was unsuccessful, and the inability to define the stimulus hindered the study of olfactory physiology

Incremental changes in chemical structure produce detectably different odours.
Johnson and Leon (2007)

olfactory coding must account for the ability to distinguish among chemicals that have subtle differences along several dimensions

our understanding of olfactory transduction was significantly advanced with the discovery of olfactory receptor genes, which allowed the receptor code to be reverse engineered
- i.e., instead of definining the stimulus, then studying transduction (as was done in the auditory and visual systems), in the olfactory system, the major breakthrough was discovering the molecular structure of the receptors, and then piecing together what stimulates each type of receptor
there are approximately 330 olfactory receptor genes, which code for ~1000 receptor proteins in the main olfactory epithelium
the definition of the receptors reinivigorated field to take another look stimuli - a recent analysis of responses to 345 odourants was able to define at least 6 different dimensions that lead to different patterns of activity in neurons in the olfactory bulb

2013 Sensory Physiology