week 5 (2) Valuation System and Relative Value
The Ventromedial Frontal Cortex (vmPFC) and Valuation
The ventromedial frontal cortex (vmPFC) is a critical component of the brain's valuation system, playing a key role in guiding choices, especially when comparing and integrating various types of value and reward.
Nomenclature Note: For the purposes of this course, the terms ventromedial frontal cortex (vmPFC) and orbital frontal cortex (OFC) are used interchangeably. While technically the OFC is a specific part of the broader medial frontal cortex, both terms in this context refer to the anterior (frontal) regions of the medial frontal cortex that are involved in value-based decision-making.
Function: The vmPFC is considered the primary candidate for the neural substrate that facilitates the comparison of multiple, often highly dissimilar, types of rewards. It enables individuals to make choices between disparate options, such as evaluating the subjective worth of a painting versus a political leader, or a car versus a social interaction. This region integrates information about potential outcomes, personal preferences, and the current context to form a unified value signal.
Complexity: The valuation system is highly complex and dynamic, involving intricate feedback loops. The vmPFC not only receives inputs from other brain regions but also sends information back to areas associated with reward processing, such as dopamine-producing neurons in the midbrain. These feedback loops are essential for modulating reward sensitivity, learning from outcomes, and refining future valuations.
Comparing Dissimilar Rewards: Money vs. Juice Experiment
Challenge: A significant challenge in neuroscience is understanding how the brain manages to compare subjective values of fundamentally different reward types. For example, how does an individual weigh the subjective benefit of a monetary gain (an abstract, delayed reward) against a direct, sensory pleasure like a taste of juice (an immediate, visceral reward)? People constantly make choices that require evaluating such diverse options and assigning them a common 'currency' or subjective value.
Experimental Paradigm: To investigate this, researchers often design experiments where participants must choose between monetary incentives and sensory rewards, such as different quantities or concentrations of a flavorful juice. Typically, participants are presented with a series of trials where they are offered a choice between, for instance, a small amount of money (e.g., 0.50) or a certain volume of juice (e.g., 10 ext{ mL}$$). The amounts and types of rewards are systematically varied.
Neural Correlates: During these choices, fMRI (functional magnetic resonance imaging) is used to monitor brain activity. Studies consistently show that the vmPFC, along with other interconnected regions, becomes active when participants evaluate and choose between these dissimilar rewards. The level of activity in the vmPFC often correlates with the subjective value assigned to the chosen option, regardless of whether it's money or juice. This suggests that the vmPFC serves as a 'common currency' area, translating different reward types into a unified value signal that allows for direct comparison and decision-making.