Most natural scents contain dozens of molecular components, but we perceive them together as one unified object – say, peanut butter. In a new study published in Neuron, Northwestern Medicine scientists discovered that distinct regions of the human brain encode an odor’s molecules not only altogether in a mixture, but also as individual parts.
“These findings provide the first evidence that the human brain can engage object-level and component-level mechanisms to process a natural food odor mixture, implying that both modes work simultaneously to guide odor-related behavior,” said first author and NUIN PhD student James Howard.
Howard and senior author Jay Gottfried, MD, PhD, associate professor in the Ken and Ruth Davee Department of Neurology, were inspired by previous studies that suggest specific odor molecules within a mixture drive behavior in animals – for instance, particular molecules within a flower’s scent mixture prompt flight and feeding in moths.
“We wanted to deconstruct a natural odor to determine if and how the brain breaks down the molecules individually and as a whole,” Howard said.
The scientists used a common food odor – peanut butter – to test the idea. Using functional magnetic resonance imaging (fMRI) to measure brain activity, they had study participants sniff 14 individual molecular components of the food odor, the whole food odor, and a control, banana. Participants rated each one for pleasantness and intensity.
They did this both before and after a lunch of peanut butter on crackers to examine the sensory-specific satiety effect, which holds that food odor pleasantness tends to decrease after you’ve consumed the food to satiety. Indeed, participants rated the peanut butter odor, as well as a handful of its molecular components, as less pleasant after eating lots of peanut butter. Likewise, the scientists looked for differences in fMRI activity in certain areas of the brain after lunch.
They analyzed the fMRI activity in seven brain regions associated with odor and reward value processing. Whole mixture processing involved the posterior piriform cortex, the central smell-processing center, a finding that replicated previous studies.
“Our really novel finding was that the amygdala, a part of the brain that’s typically involved in processing emotions or salience of stimuli, is processing individual components of the mixture – not just what they are but how good they smell,” Howard said.
The scientists suggest that another region, the orbitofrontal cortex, integrates all of the mixture and component information.
These new findings could apply to future research on controlling appetite.
“If there are certain molecules that seem to be more relevant for processing the value of a food odor, adding or removing these components could affect feeding behavior,” Howard said. “Smells of food odors are really powerful to help you start or finish eating.”
This work was supported by NIH grants 5F31DC013500, R01DC010014 and 3R01DC010014-01S1 and Brain Research Foundation grants BRF SG2010-07 and SG2011-06.