Neuronal selectivity for stimulus information determines prefrontal LFP gamma power regardless of task execution

Abstract Local field potential (LFP) power in gamma frequency has been thought to be modulated by cognitive variables during task execution. We sought to understand how the properties of neurons and LFPs from same sites representing information before and after training in cognitive tasks. We therefore trained monkeys to perform working memory tasks and analyzed the neurons and LFPs in different prefrontal subdivisions. In agreement with previous studies, we found that sites containing neurons selective for stimuli held in memory exhibit increased gamma power during and after the presentation of stimuli compared to the baseline period. Unexpectedly, we also found that the elevation of gamma power above the baseline was diminished after training, compared to power observed prior to training in the same monkeys; despite an overall in firing rate observed after training. Learning to perform the task did alter some properties of LFP gamma power, it decreased at the time that subjects needed to make a judgment about whether two stimuli were same or not, and differential gamma power was observed for matching and nonmatching stimuli. Our results indicate that gamma power elevation in the prefrontal cortex represents automatic stimulus processing, not necessarily tied to a cognitive task being executed.