The Experience of Eating
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In the developed world, eating is no longer solely a matter of sustenance and survival. Once simply a physiological necessity, the practice of eating is now closely associated to the pleasures of taste, smell, touch, as well as specific behaviors and emotions. It comes as no surprise, then, that eating is associated with many complex brain functions, some of which we are just beginning to understand. Hoping to elucidate the role of the brain in the experience of eating, members of the John B. Pierce Laboratory at the Yale School of Medicine are using the latest neuroimaging techniques to scan the brain for clues on what makes food taste good and why we eat it. Specifically, Dana M. Small, Ph.D. has been researching taste, odor, and responses to flavor stimuli with the help of graduate student Kristi Rudenga, Marga Veldhuizen, and several others.Recently, members of the Pierce lab have found interesting connections between how taste works, what tastes are appealing, and eating behaviors.
In particular, research in the Pierce Lab has shown that the sensory experience of eating is not exactly equivalent to the desire for food, and that certain types of so-called “nutritious” foods elicit differing activity in the brain. Through this research, Dana Small and the Pierce Lab hope to gain a better understanding of the causes of obesity and behavioral responses to calorie consumption.
Your Brain: An Overview
To better appreciate the nature of taste and smell, we must have a basic understanding of how the brain works. There are many parts of the brain that are directly related to the experience of eating: with three of the most important being the amygdala, the orbitofrontal cortex, and the caudate (shown in Figures 1a and 1b).
The amygdala is the region of the brain involved in the processing and memory of emotions, especially fear, in higher vertebrates. With regard to food, the amygdala is responsible for detecting the flavor intensity of a particular food and inducing the desire to eat more. The orbitofrontal cortex (Figure 1a) is an outer region of the brain located just above the eyes responsible for the cognitive process of decision-making. The orbitofrontal cortex detects how “pleasant” a food is – a sensation distinct from desire, according to Yale graduate student Kristi Rudenga. The caudate nucleus (Figure 1b) is an important part of learning and memory. Although its role with regards to food is not well defined, studies have shown that the caudate responds differently to food stimulus in healthy versus in obese people.
In general, there is no single area of the brain responsible for sensing and responding to food. Three different types of receptors on the tongue, including thermoreceptors, which detect temperature, mechanoreceptors, which determine touch or texture), and nociceptors, which are responsible for feeling pain, work together to detect all the qualities of food – taste, smell, temperature, texture, and spiciness – which the brain fuses together in the cortex. “Flavor” is the compounded effect of all of these sensory inputs. Many connected areas of the brain then distinguish the intensity of the flavor and determine degrees of hunger and desire.
Looking Inside the Brain
Discovering the nature of these receptors is no trivial matter. In order to detect brain activity in certain regions, Small and her lab use the latest neuroimaging technology, including functional magnetic resonance imaging (fMRI) and positron emission topography.
fMRI scanning is the primary technology used in behavioral experiments, as it allows for detection of brain activity in real time. It works by detecting increases in blood flow in certain areas of the brain due to neural activity. Activated neurons increase energy consumption and therefore require more oxygen delivered by the bloodstream. Hemoglobin is a blood protein that carries oxygen to various parts of the body and comes in oxygenated and deoxygenated forms. Therefore, when the blood flow to the brain changes, the concentrations of oxygenated and deoxygenated hemoglobin in the blood also change.
In particular, deoxygenated hemoglobin is a paramagnetic molecule, meaning it has a magnetic moment when an external magnetic field is applied. Detected by the fMRI scanner, this magnetic moment describes the characteristic motion of a group of nuclei when placed in a magnetic field. The detected realaxation of nuclei back to equilibrium after bombardment by radio waves indicates increased blood flow and therefore activity in certain regions of the brain. fMRI machines give results like those shown in Figure 2, where colored regions represent elevated brain activity.
The fMRI scanner is essentially a human-sized tunnel with a large magnet that creates a magnetic field used to detect paramagnetic molecules such as deoxygenated hemoglobin (Figure 3). Many experiments in the Pierce Lab use the fMRI technology to detect the parts of the brain that are activated during consumption of different types of food.
Difficulties with fMRI Technology and Food Studies
While fMRI technology is a beneficial tool to brain and food studies, several aspects increase the difficulty of conducting food and taste experiments. First, the technology requires the subject to lie completely still while inside the fMRI scanner. Chewing is out of the question, so experiments can only be done with liquids and not with solid foods. According to Rudenga, most of the experiments performed assess differences in brain activity “when you are drinking something like a fruit drink or a chocolate milkshake as compared to drinking water.”
The timing of the brain’s response to food stimulus creates an additional difficulty in using fMRI technology because brain activity increases just seconds after tasting food. Therefore food cannot be consumed or chewed before entering the scanner. Instead, food must be administered drop by drop (therefore requiring no chewing or swallowing) while the individual is inside the scanner.
Finally, due to the strong magnetic fields, only the subject is allowed inside the scanning room while the fMRI is on. Therefore, all liquid foods must be able to be administered through a long tube from another room and dropped into the subject’s mouth. Although this has limited the types of foods and drinks the Pierce Lab can use in their experiment, they have been able to register clear differences between sugary drinks and water.
fMRI technology was the primary method used to determine the interconnectivity of the brain areas that distinguish the components of “flavor”. To show this interconnectivity, subjects in Small’s lab were fed liquids containing capsaicin, a compound that causes a burning sensation when it comes in contact with any tissue.
Capsaicin – the active component of chili peppers and spicy foods – activates the thermoreceptors and nocireceptors that detect temperature and pain. Although these sensors are distinct from the actual “taste” receptors, studies have shown that the same part of the cortex (see Figure 1) responds to this compound and basic tastes, such as sweetness.
More interestingly, according to Rudenga, researchers have determined that there are “different connections based on the physiological significance of the taste.” Sugars and salts taste good because they provide humans with calories and electrolytes needed for bodily functions. Bitter flavors, however, usually indicate the presence of toxins and are therefore often perceived as unpleasant.
Rudenga points out that even though we can override a natural aversion to bitterness behaviorally (humans often enjoy bitter foods, such as coffee), the brain signals remain the same: “good” foods activate one series of connections, while “potentially harmful” foods activate another part of the brain.
Although sugary foods have good “taste” and “flavor”, taste alone doesn’t determine whether a person chooses to continue eating or not. Recent research has determined that, while the orbitofrontal cortex (Figure 4) is involved with detection of how pleasant food tastes, a different part of the brain – the amygdala – controls wanting and desire. This fact may play an important role in understanding obesity, as obesity is often caused by desire for food that may or may not have a pleasant taste.
The Brain and Obesity
According to the American Heart Association, over 74 million Americans are obese (defined as having a BMI of 30.0 or higher). Therefore, understanding obesity on a biochemical level is crucially important in improving the health of the nation.
Researchers have identified a specific gene, the TaqIA gene, which is seemingly related to obesity. Although the mechanism is not yet entirely understood, patients with a particular allele of the TaqIA gene are more likely to become obese than those without it. fMRI studies have shown that the brain response in the caudate (Figure 2) is different in individuals with and without this allele, suggesting that obesity may be partially influenced by genetic predisposition.
Further studies have shown that eating habits are closely related to certain behavioral and personality traits. For example, studies suggest that individuals classified as obese are often more impulsive. By studying brain responses to food over time, researchers like Small hope to one day make it possible for genetic and brain activity screening to indicate the likelihood of future obesity.
The brain is a fascinating enigma humans are just beginning to understand. As neuroscience research on food continues, scientists are finding more links among personality, circumstance and eating. For example, Rudenga mentioned a recent study concerning the impact of stress on the way the brain responds to food and commented on “how immediate situations can impact your response to food.” Studies suggest that stress impacts the brain’s response to food stimuli by increasing the activity of the amygdala’s response to food and creates a response almost equivalent to that of an obese person.
More research is being done on the relationship between brain activity and the practice of eating, and the Pierce lab is at the forefront of this exciting endeavor. Food is a central part of any society, so understanding food’s connection to emotions and behavior will prove relevant and influential. Evidently, there is still much to be learned about the brain and the wonder-fully delicious experience of eating.
About the Author
SHIRLEE WOHL is a sophomore in Calhoun College majoring in Molecular Biophysics and Biochemistry. She currently works in the Anastas Lab in the Department of Green Chemistry and Engineering.
The author would like to thank Kristi Rudenga for her help in understanding the basic relationship between taste sensors, food, and the brain.
K. Rudenga, B. Green, D. Nachtigal, D. M. Small “Evidence for an integrated oral sensory module in the human anterior ventral insula.” Chemical Senses (2010) in press.
J. Felsted, F. Chouinard-Decorte, X Ren, D. M. Small “Genetically determined brain response to a primary food reward.” Journal of Neuroscience (2010) 30(7): 2428-2432.
E. Stice, S. Spoor, C. Bohon, D. M. Small “Relation between obesity and blunted striatal response to food is moderated by Taq1A1 DRD2 gene” Science (2008) 322: 449-452