[The brain mechanism of memory encoding and retrieval: a review on the fMRI studies].

Memory encoding and memory retrieval are two important processes of the memory. The main results of studies on the neural basis of the memory encoding and memory retrieval by functional magnetic resonance image (fMRI) technique were summarized in this review. The neural basis of memory encoding and retrieval phases varies with different materials, memory types, and age stages. It means that the neural networks of these memory activities are separate. The functional locations of the activated brain areas during memory encoding and during memory retrieval phases are overlapped with distinction. The activated brain areas of memory encoding mainly locate in the prefrontal lobe, the temporal lobe, the parietal lobe, the anterior hippocampus, the thalamus, and the basal ganglia (including the striatum and the marginal division of the striatum). The activated brain areas of memory retrieval mainly locate in the prefrontal lobe, the temporal lobe, the entorhinal cortex, the perirhinal cortex, the posterior hippocampus, the thalamus, and the basal ganglia. The anterior hippocampus is mainly activated during the encoding phase, whereas posterior hippocampus is mainly activated during the retrieval phase. The intensity of the activated cerebral cortex regions during the encoding phase is stronger than that during the retrieval phase, whereas the opposite activated pattern is found in the subcortical structures, mainly the basal ganglia and thalamus, during the two phases. It seems that the stimulation might activate certain cerebral cortex areas during the memory encoding phase, then the information is transported to the subcortical structures and comes back to the cerebral cortex to complete the memory retrieval phase. The encoding and retrieval phases of the memory are supposed to be accomplished by a neural circuit among the cerebral cortex, basal ganglia, thalamus and cerebral cortex, rather than the cerebral cortex only.