Every Smell Has Its Place

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.

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What is she smelling? The recording reveals the answer. A volunteer demonstrates the experimental apparatus for detecting olfactory neuron activity

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That device in the photo is quite Kubrick! :)

I was undergoing radiation therapy for a brain mass a few weeks ago and whenever the field was active I could smell something very unpleasant, like a cross between iron and chlorine. Mind you, the field itself has no smell. The machine has no smell. However it was stimulating my olfactory nerves (as well as my optic nerves, which caused me to see a soft violet light).

Would've been nice if they could have hit the "pleasant smell" area instead!

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

I thought Terry Gillian's Brazil.

Sorry to everyone who posted comments over the last few days. We had a four-day holiday/weekend here and we did not open our email even once.

What can be considered a good smell? Some people like the smell of car exhaust and paint, but these aren't necessarily good for you. How did that factor into the research? How does Smoking effect these receptors.

I found it interesting how the chemical structure of an odor molecule can relate to where the odor would be placed on the pleasantness scale. Even though Sobelâs theory goes against the predictable claim that the smell receptors are randomly placed around the membrane, he might be onto something. He discovered a connection between the patterns of the nerve reactions to different scents and the pleasantness of the scents. Although Iâm not sure how this was tested. What was his process for the experiment that took place?

They used smells that rated at one end of the scale or the other in previous research on smell pleasantness. So lemon is nice, cow manure nasty. They do admit that there can be cultural and personal aspects to smell (I, for instance, worked with cows for many years, and don't find their manure smell bad). But underlying them is a basic scale we are all born with.

That apparatus in the photo releases odors and has electrodes that can pinpoint where the receptor nerves have the strongest reaction

What can be considered a good smell? Some people like the smell of car exhaust and paint, but these aren't necessarily good for you. How did that factor into the research? How does Smoking effect these receptors.

Clearly there are learned preferences and dislikes. But underlying that, we also have a built-in scale. This showed up quite clearly in the research, despite personal and cultural differences.