James Howard, a PhD student in the Northwestern University Interdepartmental Neuroscience Program (NUIN) recently published an article in Neuron which found that distinct regions of the human brain encode an odor’s molecules not only altogether in a mixture, but also as individual parts. Howard studies in the laboratory of senior author Jay Gottfried, MD, PhD, associate professor in the Ken and Ruth Davee Department of Neurology.
What was the goal of this research?
Most natural scents contain dozens of molecular components, but we perceive them together as one unified object—say, peanut butter. We wanted to deconstruct one of these natural odors to determine what molecules make up that smell, and then to determine how the brain breaks down this information, from the molecules individually and the whole odor mixture.
We 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.
What is the key finding?
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.
Tell us about the research.
We used a common food odor—peanut butter—to test the idea. Using functional magnetic resonance imaging (fMRI) to measure brain activity, we 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.
We 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. 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, we looked for differences in fMRI activity in certain areas of the brain after lunch.
We then 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.
This suggests that another region, the orbitofrontal cortex, integrates all of the mixture and component information.
How can this finding be applied to future research?
These 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. Smells of food odors are really powerful to help you start or finish eating.