Noam Sobel, a neurobiologist who is the primary investigator for the Olfaction Research Group of Israel's Weizmann Institute of Science, and his team of graduate students, exposed the 56 participants in their study to the smells of different compounds, ranging from one compound up to a mixture of 43 different compounds. Each participant was given pairs of smells, each with the same number of compounds (but with no common compounds between them), and were asked to make comparisons between them. The results of the study showed that the more compounds present in the smell, the harder it was to tell them apart.
Using these results as a basis, they designed a new study where they created four specific mixtures of 40 different compounds in equal parts. They made up a name, 'Laurax', split up the 12 participants into four groups of three each, and told each group that a different compound was 'Laurax'. They spent three days in the lab while the participants were familiarized with the smell of their particular 'Laurax', and then they were given four new smells and four labels, one of which said 'Laurax'. The results of this second study showed that the more compounds that were in a particular smell made it more likely that it was labeled 'Laurax' by the participants.
Just to test for any 'catch-all' bias in the test, they performed it again, but allowed for participants to use the label 'other' if needed. The results of this second test showed similar results as the first - people used Laurax more often when labeling smells with more compounds in them.
According to Sobel and his team, these results mean that we treat particular smells as 'units', such as "coffee", "rose", "pine" and "bacon". That may seem like a bit of a 'duh' statement, since that is how we typically relate to smells. However this research confirms that our brains don't separate and analyze distinct parts of a smell and then put them back together to form a label, otherwise we wouldn't be able to smell two mixtures of completely differing compounds and end up labeling as the same.
The key to 'olfactory white', though, is the same as for white light and white noise — the compounds that comprise it must be from the entire olfactory range and in equal proportions.
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Now, you might be asking, "what does 'olfactory white' smell like?" That might be a difficult question to answer, though. Try accurately describing the smell of a rose to someone who has never smelled one before. It's possible, but it's not easy.
As you might expect, the researchers response is: "The best way to appreciate the qualities of olfactory white is to smell it."