Dissociation of LFP Power and Tuning in the Frontal Cortex during Memory

Working memory, the ability to maintain and manipulate information in the brain, is critical for cognition. During the memory period of spatial memory tasks, neurons in the prefrontal cortex code for memorized locations via persistent, spatially tuned increases in activity. Local field potentials (LFPs) are understood to reflect summed synaptic activity of local neuron populations and may offer a window into network-level processing. We recorded LFPs from areas 8A and 9/46 while two male cynomolgus macaques (Macaca fascicularis) performed a long duration (5.1–15.6 s) memory-guided saccade task. Greater than ~16 Hz, LFP power was contralaterally tuned throughout the memory period. Yet power for both contralateral and ipsilateral targets fell gradually after the first second of the memory period, dropping below baseline after a few seconds. Our results dissociate absolute LFP power from mnemonic tuning and are consistent with modeling work that suggests that decreasing synchronization within a network may improve the stability of memory coding.

SIGNIFICANCE STATEMENT The frontal cortex is an important site for working memory. There, individual neurons reflect memorized information with selective increases in activity, but how collections of neurons work together to achieve memory is not well understood. In this work, we examined rhythmic electrical activity surrounding these neurons, which may reflect the operation of recurrent circuitry that could underlie memory. This rhythmic activity was spatially tuned with respect to memorized locations as long as memory was tested (~7.5 s). Surprisingly, however, the overall magnitude of rhythmic activity decreased steadily over this period, dropping below baseline levels after a few seconds. These findings suggest that collections of neurons may actively desynchronize to promote stability in memory circuitry.