Neuronal activity in dorsomedial frontal cortex and prefrontal cortex reflecting irrelevant stimulus dimensions.

Previous studies of the dorsomedial frontal cortex (DMF) and the prefrontal cortex (PF) have shown that, when monkeys respond to nonspatial features of a discriminative stimulus (e.g., color) and the stimulus appears at a place unrelated to the movement target, neurons nevertheless encode stimulus location. This observation could support the idea that these neurons always encode stimulus location, regardless of its relevance to an instrumentally conditioned behavior. Past studies, however, leave open the possibility that activity observed during one operant task might reflect the contingencies of a different task, performed at different times. To test these alternatives, we examined the activity of DMF and PF neurons in two rhesus monkeys conditioned to perform an operant eye-movement task in which only the color and shape of visual stimuli served as salient discriminative features. Each of eight stimuli was associated with a response to a different eye-movement target. The location of these stimuli varied from trial to trial but was of no behavioral relevance, and the monkeys did not perform any operant task in which stimulus location controlled behavior. A substantial minority of neurons in both DMF and PF nevertheless encoded stimulus location, which indicates that this property does not depend on its relevance in an instrumentally conditioned behavior.