Dept. of Brain and Cognitive Sciences & Center for Learning and Memory, Massachusetts Institute of Technology
Neural activity in the primate prefrontal cortex during associative learning
The prefrontal (PF) cortex has been implicated in the ability of primates to form and rearrange arbitrary associations rapidly. This ability was studied in two monkeys, using a task that required them to learn to make specific saccades in response to particular non-spatial (foveal) cues and then repeatedly learn to reverse these responses. We found that the activity of individual PF neurons represented both the cues and the associated responses, perhaps providing a neural substrate for their association. Furthermore, during learning, neural activity conveyed the direction of the animals' impending responses progressively earlier within each successive trial. The final level of activity just before the response, however, was unaffected by learning. The progressively earlier appearance of directional selectivity with learning might be related to dopaminergic gating by subcortical inputs and/or to plasticity mechanisms intrinsic to the prefrontal cortex. Furthermore, when the animals were trained to perform an additional, non-associative, task using the same visual stimuli (and interleaved block-wise with the associative task), many neurons reflected the task currently being performed in their firing rates or correlations, despite identical visual events. Finally, in the associative task, some cells displayed direction selectivity in response to the foveal visual cues, but not when the same directions were cued explicitly with a spatial cue. These results suggest a role for the PF cortex in learning arbitrary cued response associations, an ability critical for complex behavior.