Humans can't tell apart 1 trillion smells: scientist

A researcher is calling into question recent findings that the human nose is capable of distinguishing at least 1 trillion odours, saying that the data used in last year's study does not support this claim.
According to Rick Gerkin, an assistant research professor with Arizona State University School of Life Sciences, this is important because the findings are already making way to neuroscience textbooks, misinforming upcoming investigators and cutting off potentially productive lines of research that do not adhere to those findings.
The last year's study was authored by researchers from The Rockefeller University and Howard Hughes Medical Institute and published in the journal Science.

"We disagree with several aspects of the 2014 study," said Gerkin, who co-authored the new paper with Jason Castro, a professor with Bates College in US.
"First, the assertion that humans can discriminate between at least 1 trillion odours is based on a fragile mathematical framework - one that's capable of creating nearly any result with small variations in the data or the experiment design. So the result in question could be tens of orders of magnitude - a factor of one with dozens of zeros after it - larger or smaller than first reported," said Gerkin.
"We also point out that the conclusion in the 2014 paper relies heavily on untested assumptions about smell perception," added Castro.

"And the equation used actually shows that the number of distinguishable smells is fewer than 1 trillion, not more, making the original claim inaccurate, and in fact the exact opposite of what the calculation actually shows," Castro said.

Gerkin and Castro show in their study that if the experiment had used approximately 100 additional subjects, the same analysis would have shown that the human nose could discriminate all possible odours - clearly at odds with the data in the 2014 paper.
Moreover, had the researchers used a slightly more conservative statistical analysis, it would have shown that humans can distinguish only 5,000 odours - about the same number generally believed to be true prior to 2014, they said.

According to Gerkin, the number of smells humans can discriminate is the kind of fact that could flow naturally from a deeper understanding of olfaction - something we currently lack, but that must be established to better understand olfactory health and diseases of smell.
"Scientists can easily compute the number of discriminable colours because they know the organisation of colour perception," said Gerkin.
"For example, think about the colour wheel we learned in elementary school or the red-green-blue colour values that make it possible to display colour on television. For smells, there is no accepted 'smell wheel' just yet," Gerkin added.

The new study challenging the findings was published in the journal eLife.