Bilateral Orbital Prefrontal Cortex Lesions in Rhesus Monkeys Disrupt Choices Guided by Both Reward Value and Reward Contingency

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Abstract
The orbital prefrontal cortex (PFo) operates as part of a network involved in reward-based learning and goal-directed behavior. To test whether the PFo is necessary for guiding behavior based on the value of expected reward outcomes, we compared four rhesus monkeys with two-stage bilateral PFo removals and six unoperated controls for their responses to reinforcer devaluation, a task that assesses the monkeys' abilities to alter choices of objects when the value of the underlying food has changed. For comparison, the same monkeys were tested on a standard test of flexible stimulus-reward learning, namely object reversal learning. Relative to controls, monkeys with bilateral PFo removals showed a significant attenuation of reinforcer devaluation effects on each of two separate assessments, one performed shortly after surgery and the other ∼19 months after surgery; the operated monkeys were also impaired on object reversal learning. The same monkeys, however, were unimpaired in acquisition of object discrimination learning problems and responded like controls when allowed to choose foods alone, either on a food preference test among six different foods or after selective satiation. Thus, satiety mechanisms and the ability to assign value to familiar foods appear to be intact in monkeys with PFo lesions. The pattern of results suggests that the PFo is critical for response selection based on predicted reward outcomes, regardless of whether the value of the outcome is predicted by affective signals (reinforcer devaluation) or by visual signals conveying reward contingency (object reversal learning).