A Computational Study On Theta-Nested Gamma Rhythm In Hodgkin-Huxley Neuronal Network

Multiple brain regions can generate rhythm in different frequency bands when animals, including humans, are undergoing cognitive tasks, such as learning, memory tasks and so on. These rhythms can occur simultaneously and work respectively, they can also interact with each other and much more complex brain activities can take place when nested rhythms generate as the amplitude of the high frequency rhythm is modulated by the phase of the low one. Resent research have indicated that these nested rhythms play an important role in memory coding and recalling.According to our computational simulation, we find that the HH neuronal net-work can generate both theta(4-8Hz) and gamma (30-100Hz)rhythm. And the theta-nested gamma rhythm occurs when the amplitude of high frequency gamma rhythm is modulated by the phase of low frequency theta rhythm. However, this modulating needs the synaptic connection from inhibitory fast neurons to excitatory neurons and the connection from inhibitory slow neurons to excitatory neurons, accompanied by the appropriate applied current. It is found that nested rhythm can't generate if any one of these three terms is not available, namely the applied current Iapp, the synaptic conductances gGAse and gGAfe which can't be zero. We find that the power of theta rhythm increases when the conductance gGAse increases, it leads to the increasing of the magnitude of phase-amplitude cross-frequency coupling(CFC) between theta and gamma rhythm, and the phase synchronization of theta rhythm among excited neurons increases in this process. However, a completely opposite result takes place when the conductance gGAfe increases.An important function of brain is to take charge of learning, and the current me-diated by NMDA receptor can facilitate learning. However, if we only increase the conductance gNMee, the learning activity can't be aroused. Only when the conduc-tances gGAse and gNMes, as well as gNMee increase simultaneously, will the learning activity occur. In other words, the cooperation between NMDA receptor and GABAA receptor is the key for generating the learning effect. The power of theta rhythm in-crease during learning, and the magnitude of the phase-amplitude CFC also increases, but the phase synchronization of theta rhythm among excited neurons increases.