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Peanut Butter Odor Perception

By Ashley Ahn | October 28

When humans detect a scent, tens to hundreds of different odor molecules rush into the nose to bind to olfactory receptors. The Gottfried Lab is using the familiar peanut butter odor to determine whether all or only a fraction of these odorants contribute to the perceived peanut butter smell.  

After removing specific molecules from the peanut butter odor to test variabilities in participants’ odor perception, The Gottfried Lab has identified a specific set of molecules that are essential to the peanut butter smell. Visiting scholar Koichiro Iwai recently began the second phase of the study at the beginning of October which examines changes in brain activity in response to different sets of peanut butter odorants.

“The main purpose of my research is to understand the pattern separation and competition mechanism in the odor perception,” Iwai said.

Pattern completion is the ability of neurons in the cerebral cortex to “fill in” missing features of a familiar input odor, while pattern separation allows for perceptual difference in odors. 

In order to remove specific odorants from the whole peanut butter odor, the lab used gas chromatography-mass spectrometry — a common technique for separating chemical compounds based on their affinity to the liquid phase in the tube, or column, and their different molecular weights — with lighter molecules exiting the column faster than heavier molecules. Research specialist Kyle Mabry said the team also employed the Dean’s switch which redirected the flow of the GCMS in order to collect the desired odor molecules in a vial.

The team divided each chromatogram run, which took 20 minutes, into four chronological time zones during which the different molecules exited the column.

Gas chromatography-mass spectrometer

After eight to 10 different trials to hone-in on the different areas of the chromatogram, Iwai said the four final sets of peanut butter odors presented to participants were the whole odor, whole – zone 1 odor, whole – zone 3 odor, and whole – (zone 3 + zone 4) odor. 

The lab used an olfactometer to present each of the odors three times to 12 participants for the behavioral testing portion of the study, which concluded in February, Mabry said. 

“We had the different odors attached to the olfactometer, and we have a behavioral script in MATLAB that asks them different questions regarding the odor like intensity, similarity to peanut butter, and which odor they could smell,” Mabry said. 

The lab found that participants reported high similarity to peanut butter odor for the whole odor and whole – zone 1 odor, which is consistent with pattern completion, and low similarity to peanut butter for the whole – zone 3 odor and whole – (zone 3 + zone 4) odor, which is consistent with pattern separation.

Iwai said Zone 3 appeared to be particularly significant to the peanut butter odor perception. While three to four participants could not differentiate between the whole peanut butter odor and whole – zone 3 odor, the majority of participants reported a difference in smell perception.

“Some participants could not identify the differences between whole odor – zone 3 and whole odor, but most participants could, so I needed more participants to understand if it was statistically significantly different or not” Iwai said.

Some of the compounds found in Zone 3 include 2,5-Dimethyl-pyrazine, 2,6-Dimethyl-pyrazine, ethyl pyrazine, and acetic acid, he added.

After wrapping up the behavioral testing portion of the study in February, Iwai had planned to start the second phase of the study on March 12 or March 13. Days before the commencement of the fMRI scanning, the COVID-19 pandemic hit the United States and forced Penn to shut down campus. 

Iwai said in one sense, he considers himself lucky that the coronavirus outbreak happened right in between the behavioral testing and fMRI scanning stages of the study but is still disappointed about the delay in his research. 

To prepare for the fMRI scanning portion of the study, Iwai returned to the lab in June to create the odor samples he will present to subjects. He plans to present whole odor, whole – zone 1 odor, whole – zone 3 odor, and whole – (zone 3 + zone 4) odor to 30 subjects during the fMRI scans, requiring him to make a total of 120 odor samples.

Because each 20-minute run of the GCMS only produces a faint odor, Iwai said it takes approximately 24 hours to create one vial of an odor with a strong enough intensity to present to a participant. After four months preparing the peanut butter odor samples, Iwai has just started conducting fMRI scans on participants and will continue to do so for the next three months.

“Our guess is the pattern of activation in the cerebral cortex will be changed if the odor percept is changed, but if we remove some odorants and the perception does not change, the brain activity pattern will not change,” he said.


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