Different neural codes serve long and short-term memory functions in primate Hippocampus and Lateral Prefrontal Cortex during virtual navigation

The primate hippocampus (HPC) and lateral prefrontal cortex (LPFC) are two brain structures deemed essential to long- and short-term memory functions respectively. Here we hypothesize that although both structures may encode similar information about the environment, the neural codes mediating neuronal communication in HPC and LPFC have differentially evolved to serve their corresponding memory functions. We used a virtual reality task in which animals navigated through a maze using a joystick and selected one of two targets in the arms of the maze according to a learned context-color rule. We found that neurons and neuronal populations in both regions encode similar information about the different task periods. Moreover, using statistical analyses and linear classifiers, we demonstrated that many HPC neurons concentrate spikes temporally into bursts, whereas most LPFC neurons sparsely distribute spikes over time. When integrating spike rates over short intervals, HPC neuronal ensembles reached maximum decoded information with fewer neurons than LPFC ensembles. We propose that HPC principal cells have evolved intrinsic properties that enable burst firing and temporal summation of synaptic potentials that ultimately facilitates synaptic plasticity and long-term memory formation. On the other hand, LPFC pyramidal cells have intrinsic properties that allow sparsely distributing spikes over time enabling encoding of short-term memories via persistent firing without necessarily triggering rapid changes in the synapses. ### Competing Interest Statement The authors have declared no competing interest.