Frequency of gamma oscillations routes flow of information in the hippocampus

Information flow in the hippocampus In neuronal networks, the gamma frequency oscillation is thought to be important for several higher-end cognitive processes, such as attention and memory. Coordinated firing by widely distributed cells is required to produce an oscillation, but this network activity can be quite variable. It is not clear why gamma oscillations should differ so greatly across time and space. New evidence reveals that information trafficking within the hippocampus and parts of temporal cortex is dependent upon the speed of gamma. Faster oscillations link entorhinal cortex to CA1, providing information on the animal's location, while slower oscillations link CA1 to CA3 for information storage. Thus, one possible function underlying a variable gamma is to properly route information within a circuit. Gamma oscillations in the brain are thought to 'bind' spatially distributed cells, a function that is probably important in perception, attentional selection and memory. However, it is unclear why the frequency of gamma oscillations varies substantially across space and time. Here, the study of the frequency of gamma oscillations in the CA1 area of the hippocampus suggests that variations in gamma frequency may be important for routeing information in the brain. Gamma oscillations are thought to transiently link distributed cell assemblies that are processing related information 1 , 2 , a function that is probably important for network processes such as perception 1 , 2 , 3 , attentional selection 4 and memory 5 , 6 . This ‘binding’ mechanism requires that spatially distributed cells fire together with millisecond range precision 7 , 8 ; however, it is not clear how such coordinated timing is achieved given that the frequency of gamma oscillations varies substantially across space and time, from ∼25 to almost 150 Hz 1 , 9 , 10 , 11 , 12 , 13 . Here we show that gamma oscillations in the CA1 area of the hippocampus split into distinct fast and slow frequency components that differentially couple CA1 to inputs from the medial entorhinal cortex, an area that provides information about the animal’s current position 14 , 15 , 16 , 17 , and CA3, a hippocampal subfield essential for storage of such information 14 , 18 , 19 . Fast gamma oscillations in CA1 were synchronized with fast gamma in medial entorhinal cortex, and slow gamma oscillations in CA1 were coherent with slow gamma in CA3. Significant proportions of cells in medial entorhinal cortex and