Four weeks into the NBB Paris program, I now know my friends in our apartment very well. In our apartment (maybe this applies to the others too), neither neuroscience nor soccer is the most discussed topic. This conversation is by far the most frequent. “What are we getting for dinner?” “I don’t know!” All of us are from different parts of the United States and even different parts of the world, which makes our restaurant selection processes a bit tricky. Nick Maamari, an Emory NBB senior from Dubai, thinks that cheese is the most disgusting food in the world. On the other hand, Daniel Son, a Korean-American student from Portland, Oregon, enjoys eating cheese so much that he bought a slice of gouda cheese from Monoprix on the first day we arrived in Paris.
France is a country known for its cheeses. In 1962, President Charles de Gaulle said, “how can you govern a country which has two hundred and forty-six varieties of cheese?” (Mignon, 1962) Being a neuroscience student, I ask, “what happened in Nick’s brain when he ate these French cheeses?”
Disgust has been identified as a basic emotion since Charles Darwin (Rozin and Fallon, 1987). Like other emotions, disgust has a characteristic facial expression (like the one shown in Nick’s photo), an appropriate action (Nick would definitely leave a restaurant if all available food has cheese), a distinctive physiological manifestation (nausea), and a characteristic feeling state (revulsion) (Rozin and Fallon, 1987). Research has found two brain structures that are considered as neural sites for processing disgust: insular cortex and basal ganglia (Sprengelmeyer, 2007). First, let’s start with some neuroanatomy.
The insula cortex (or insula) lies deep within the lateral sulcus (as shown above) and it sits on an island (hence the name insular) covered with frontal, parietal and temporal opercula (“the lid”). It is interconnected with many cortical regions and subcortical structures, placing it at an ideal position to integrate homeostatic information with information about the physical and external environment (Sprengelmeyer, 2007).
Basal ganglia is a group of subcortical nuclei responsible primarily for motor control and other roles such as executive functions, reward and emotions (Lanciego et al.). Two major neurodegenerative disorders, Huntington’s disease, and Parkinson’s disease are caused by the hyperactivation or hypoactivation of this structure, respectively (Cepeda et al., 2014). Previous research has identified cases where patients with Huntington’s disease were unable to recognize disgust (Sprengelmeyer et al., 1998).
Numerous research has shown the role of insula and basal ganglia in mediating disgust (Sprengelmeyer, 2007). However, most previous studies have focused on the recognition of facial expressions of disgust. The reason for the lack of research on food aversion is mainly due to a great variation between how each individual perceive what food is disgusting and also due to ethical issues associated with invoking uncomfortable feelings in experiments (Royet et al., 2016). A group of French researchers narrowed down their experimental food to … guess what, cheese (Royet et al., 2016). Cheese is loved by people like Daniel and hated by people like Nick, therefore, making it a great model to study the cerebral processes of food disgust.
In the first part of the study, the authors conducted a survey of the French population (this may be a biased sample and results do not apply to the rest of the world), to evaluate individual preferences for 75 foods and estimate the proportion of individuals who are disgusted by cheese. The authors have found a higher percentage (11.5%) of people disgusted by cheese than by other types of food. Now they have a study sample of individuals expressing a deep disgust for cheese.
The second part of the study involved Functional Magnetic Resonance imaging (fMRI), which is a tool to show activations of brain regions. The participants were asked to begin their experiments in a hunger state (these poor people did not even get a full breakfast) in order to make sure that the results were not biased by different metabolic rates after a meal. The researchers first presented participants in an MRI scanner with both the image and the smell of six different types of cheese and six other control foods. The participants were asked to judge whether the smell and sight of food are pleasant or not and whether they have a strong desire to eat the food.
After analyzing their data, the researchers found that global pallidus and substantia nigra (shown above) of the basal ganglia are more activated in people who dislike cheese. The authors also found that another structure of the basal ganglia, the ventral pallidum was inactivated in individuals disgusted by cheese. These structures are involved in what’s called the “reward pathway” of the brain, which regulates our perception of pleasure and facilitates the reinforcement of a particular behavior (Berridge and Kringelbach, 2015). Taken together, the authors proposed that perhaps a modified version of the pathway for encoding reward was involved when we were presented with food that aroused strong feelings of dislike. Interestingly, the authors did not observe any differences in activation of insula in people who like or dislike cheese.
One thing to keep in mind as you read fMRI studies is that “correlation does not imply causation”. A structure active for a task does not mean it is critical for the task. Also, conclusions made in papers generally involve heavy statistics and morphing all brains of the participants, who of course, have different brain shapes and sizes (Logothetis, 2008). Therefore, research results should be interpreted cautiously.
This study fills in some gaps in the research of disgust, specifically for food. It helps us understand the role of different parts of the basal ganglia in processing disgust. The null finding of insula also supports that insula has more complicated functions than simply processing disgust. A foundation of knowledge on this topic can be applied to a wide variety of eating disorders that affects many people in our lives. I would like to end with my favorite celebrity chef, Gordon Ramsay, who must have his brain structures associated with disgust constantly activated when judging his students’ dishes!
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