Why We’re Good at Identifying Smells, But Horrible at Describing Them

The simple smell of a childhood staple, peanut butter, is surprisingly complex: roasted peanuts emit nearly 200 airborne molecules, and peanut oil almost 100. But the real surprise of peanut butter’s complexity is what happens when we inhale its aroma. We don’t consciously detect peanut butter’s individual components—we simply smell peanut butter.

Deconstructing a scent is notoriously difficult for humans. Though we’re talented at identifying odors, when we attempt to break them down and name their component parts, we run into problems. The point at which that stumbling block in the sensory experience occurs has remained a mystery.

peanut-butter
Peanut butter is a surprisingly complex smell.

Which is why James Howard and Jay Gottfried at Northwestern University decided to investigate whether the human brain has access to the elements of peanut butter odor even if we consciously do not. Initially, Howard had planned to synthetically recreate peanut butter odor, but he found that “re-creating a smell like that is a whole industry. It was really naïve of me to even try,” he said. Instead, Howard and Gottfried settled on 14 representative components. They recently published their results in Neuron.

To test the participants’ smelling abilities, they puffed whiffs of whole peanut butter, the representative 14 molecules, and control banana odors through plastic tubes attached to the upper lips of 11 participants lying in an MRI machine, who were instructed when to sniff. The participants then scored the pleasantness of odors while lying immobile using a trackball mouse at their sides.

Everyone came to the lab having fasted, and Howard conducted one set of functional MRI scans while the participants were hungry. He then led them into a private room attached to the scanning facility, handed them a jar of all-natural peanut butter and encouraged them to eat until they were full. “Everyone ate a surprising—almost disturbing—amount of peanut butter,” Howard said, although he added that some were disappointed by the brand. “They fasted for a few hours, came really hungry, and then the payoff was this drippy, organic, peanut butter.”

After the peanut butter binge, Howard and Gottfried noticed a change in participants’ neural activity in the region of the brain tasked with identifying odors and associating them with similar smells, the posterior piriform cortex. This is the earliest stop that olfactory information makes in the brain after the olfactory bulb, and it suggests that the decrease in a smell’s value may start at the level of perception.

The participants also reported that they found the whole peanut butter odor significantly less pleasant after their peanut butter lunch. The second set of scans backed up their claims, revealing less activity in a brain region associated with reward signaling and less synchrony between the reward circuitry and the amygdala, a region of the brain that registers emotion—which might account for why we find food smells so much more pleasant when food is set on the table than after we eat too much of it.

Four of the 14 individual odors of peanut butter evoked similar reductions in activity in the brain’s reward circuitry and a decrease in synchrony with activity in the amygdala after the peanut butter meal, which correlated with the participants’ reports that these four odors were less pleasant after eating the peanut butter.

Even if the brain can access peanut butter’s reward value from some of its individual odor molecules, it can’t identify peanut butter from its component parts: none of the individual components induced the same pre- or post-peanut butter binge activity in the piriform cortex. “The brain seems to have access to some of these individual components in this example, whereas we, perceptually, don’t,” said Don Wilson, a professor at New York University and investigator at the Emotional Brain Institute who was not involved in the study.

Howard speculates that the unique ability of the brain’s reward circuitry to infer the whole from the parts allows people to quickly assess the value of a complex smell mixture without having to first consciously identify it.

Wilson agreed, saying, “Consciously we’re able to say this is a particular odor and that’s all I smell. But the brain often knows more than we do.”