Oscillatory neuronal synchronization between prefrontal and extrastriate visual cortex during visual memory

Visual memory involves the timely interaction of distributed neural ensembles across the brain. However, how distant neural sites cooperate during visual memory is still an open question. In this study we investigate the neural interaction between two cortical areas, the extra striate visual area V4 and the lateral prefrontal cortex based on simultaneous recordings of local field potentials (LFP) and single unit activity (SUA) in V4 and PF while two rhesus monkeys performed a delayed matching to sample task. Specifically, we analyzed LFPs from 131 sites recorded in V4 (86/45 for monkeys 1/2) and 117 sites in PF (74/43 for monkeys 1/2) from 20 and 11 recording sessions in 2 monkeys. This results in a total amount of 507 pairs of simultaneously recorded channels. When we analyzed the phase locking (PL) of the LFP, we observed enhanced PL during the memory period of the task between both areas that predominantly occurred in the theta frequency range (3-8Hz). Here, 135/332 and 33/175 pairs for monkey 1 and 2 showed significantly elevated phase locking during the delay period of the task (Z-Test for significant proportion p<0.001). To test whether the increase in theta phase locking at the mesoscopic level of LFPs was accompanied by an increased phase locking between spiking activity and theta oscillations across both areas, we examined whether spiking systematically varied as a function of theta phase. We found that spike-phase locking significantly increased during delay compared to baseline for both V4 units locked to prefrontal theta (Z=6.5/5.7, p<0.01, M1/2, N=458/202 unit-channel pairs) and vice versa (Z=5.5/3.7, p