After a Meal, Foods Aren't the Same in the Orbitofrontal Cortex
Material below summarizes the article Identity-Specific Reward Representations in Orbitofrontal Cortex Are Modulated by Selective Devaluation, published on March 8, 2017, in JNeurosci and authored by James D. Howard and Thorsten Kahnt.
Our decisions are influenced by the value of the outcome we expect to obtain. For example, a person who prefers pizza over pasta will naturally choose pizza if both are on a menu.
Prior work suggests that these expected values are encoded in the activity of the orbitofrontal cortex (OFC). This region lies along the ventral surface of the frontal lobe of the human brain, just above the eyes.
However, the value of future outcomes often changes rapidly, and these fluctuations must be taken into account to make rational decisions. Thus, if the aforementioned pizza enthusiast recently became sick after eating pizza, she will likely choose pasta the next time.
This example highlights two important aspects of reward-based decisions. First, value can be updated “on the fly” without the need for trial-and-error learning. One does not have to repeatedly get sick after eating pizza to lower the value for pizza. Second, value can be updated independently for the reward that has changed without changing the value for other rewards. Humans and other animals with damage to the OFC have trouble changing their decisions based on changes in value, such as in the above example. However, the way in which brain activity in OFC changes to reflect these value changes for specific rewards remains poorly understood.
To address this question, we used functional magnetic resonance imaging (fMRI) to investigate changes in neural activity in response to food odor rewards before and after one specific odor was devalued.
Hungry participants first rated the appeal of several odors and two different but equally appetizing odors were selected. These odors were then used in a subsequent choice task, which was performed in the fMRI scanner. On each trial of the task, subjects chose whether to sniff a high or low concentration of one of the appealing odors. They then consumed a meal outside of the scanner consisting of food associated with one of these odors, thus devaluing that odor. Finally, subjects performed the choice task in the scanner again. We used pattern-based analysis of the brain imaging data to compare neural encoding of the expected food odors before and after the meal.
Before the meal, subjects chose to sniff the higher concentration of both odors. After the meal, subjects chose the lower concentration of the devalued odor, but continued to choose the high concentration of the non-devalued odor. Pattern analysis indicated that the identity of the expected odor was encoded in medial and lateral aspects of the posterior orbitofrontal cortex (pOFC). While encoding of both expected odors changed in the medial pOFC after the meal, only the representation of the devalued reward changed in the lateral pOFC.
Additional analyses indicated that functional coupling between the lateral pOFC and ventromedial prefrontal cortex (vmPFC), an area that encodes identity-independent reward value, decreased selectively for the devalued odor after the meal. The larger this decrease for a given participant, the more this person’s preference for the high concentration of the devalued odor decreased.
These results corroborate previous work showing that the specific identity of expected rewards is encoded in the OFC, while general reward value is encoded in vmPFC. These findings extend this idea by demonstrating how neural activity in these brain areas changes to support the redirection of reward-based decision making. How representations of expected outcomes change at the neuronal ensemble level remains unknown and should be investigated in future studies.
Identity-Specific Reward Representations in Orbitofrontal Cortex Are Modulated by Selective Devaluation. James D. Howard and Thorsten Kahnt. JNeurosci Mar 2017,37 (10) 2627-2638; DOI: 10.1523/JNEUROSCI.3473-16.2017