"Our analysis shows that the human capacity for discriminating smells is much larger than anyone anticipated," said Howard Hughes Medical Institute (HHMI) investigator Leslie Vosshall, who studies olfaction at the Rockefeller University.
Vosshall and her colleagues published their findings March 21, 2014, in the journal Science. "I hope our paper will overturn this terrible reputation that humans have for not being good smellers," she says.
The 10,000 odours smelling capacity was estimated back in the 1920s without any data and it has bothered scientists like Vosshall. For one thing, it didn't make sense that humans should sense far fewer smells than colours. In the human eye, Vosshall explains, three light receptors work together to see up to 10 million colours. In contrast, the typical person's nose has 400 olfactory receptors.
But no one had tested humans' olfactory capacity.
Vosshall and Andreas Keller, a senior scientist in her lab at Rockefeller University, knew they couldn't test people's reactions to 10,000 or more odours, but they knew they could come up with a better estimate. They devised a strategy to present their research subjects with complex mixtures of different odours, and then ask whether their subjects could tell them apart.
They used 128 different odorant molecules to concoct their mixtures. The scientists presented their volunteers with three vials of scents at a time: two matched, and one different. Volunteers were asked to identify the one scent that was different from the others. Each volunteer made 264 such comparisons.
Afterwards, the scientists tallied how often their 26 subjects were able to correctly identify the odd-man-out. From there, they extrapolated how many different scents the average person would be able to discriminate if they were presented with all the possible mixtures that could be made from their 128 odorants. In this way, they estimated that the average person can discriminate between at least one trillion different doors.
"I think we were all surprised at how ridiculously high even the most conservative lower estimate is," Vosshall says. "But in fact, there are many more than 128 odorants, and so the actual number will be much, much bigger."
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