Neuronal oscillations and memory retrieval

Brain oscillations, reflecting rhythmic excitation and inhibition of neuronal populations, play essential role in coordination of neuronal activity. Study of rhythmic activity in hippocampus revealed its importance for memory processing. However, memory retrieval, the final stage of memory cascade, remains an elusive phenomenon. Analysis of hippocampal network activity during spatial memory retrieval pointed to significance of theta oscillations, the EEG pattern in hippocampus (5–12 Hz) that dominates during periods of active (“online”) processing of information. Recurrent excitation in CA3 subregion of hippocampus keeps the memory network in a stable attractor state. Periodic inhibition during theta oscillations disrupts attractor dynamics and enables memory recall by transition to corresponding network state. The main source of input to hippocampus is entorhinal cortex, whose medial division is dedicated to spatial position representation, while the lateral part is associated with coding for local sensory stimuli. The communication between entorhinal cortex and hippocampus is coordinated by gamma rhythms (25–100 Hz). Which inputs originating in the entorhinal cortex initiate retrieval of memory for spatial context remains unclear. Further studies with simultaneous recording of activity from various hippocampal sub-regions and associated structures could reveal more about dynamics of memory retrieval and their coordination by neuronal oscillations.