The olfactory membranes in your nose are densely packed with smell receptors. These receptors come in some 400 different subtypes; complex odors like that of rose petals can waft around 175 distinct kinds of odor molecules in the direction of your nose. In other words, the number of discrete odors we can perceive runs to the tens of thousands. No wonder scientists had thought that the whole smell arrangement was basically random.
But research by Prof. Noam Sobel and his team in the Weizmann Neurobiology Department is bringing our noses into line with our other sensory organs. The arrangement of the photoreceptors in our eyes, for instance, is set up to relay spatial information to the brain. And the membranes in our inner ears are graded by tone.
Sobel provided one piece of the puzzle several years ago when he revealed that odors can be placed on a scale that runs from sweetly pleasant to highly disagreeable. His research linked the apparently subjective – how volunteers rated a smell – to the objective – the chemical structure of an odor molecule. Once he had cracked the secrets of the scale, he could then predict how people smelling a scent for the first time would rate it.
Now he and his team have shown that, like the sensory receptors in our eyes and ears, smell receptors are also arranged in patterns that reflect a rational scale. In other words, they identified separate regions in the olfactory membrane; each responds most actively to one part of the pleasantness scale. While there are, of course, many more dimensions to smell than pleasantness, the scale seems to be an organizational framework that is hardwired into our brain. To Sobel, it makes sense that our basic sense of smell is both innate and based on pleasantness: A bad smell is an instinctive, universal signal to us that we need to avoid its source.