Theta phase coding in a network model of the entorhinal cortex layer II with entorhinal-hippocampal loop connections |
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Authors: | Jun Igarashi Hatsuo Hayashi Katsumi Tateno |
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Institution: | (1) Department of Brain Science and Engineering, Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino Wakamatsu-ku, Kitakyushu 808-0196, Japan |
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Abstract: | We investigated successive firing of the stellate cells within a theta cycle, which replicates the phase coding of place information,
using a network model of the entorhinal cortex layer II with loop connections. Layer II of the entorhinal cortex (ECII) sends
signals to the hippocampus, and the hippocampus sends signals back to layer V of the entorhinal cortex (ECV). In addition
to this major pathway, projection from ECV to ECII also exists. It is, therefore, inferred that reverberation activity readily
appears if projections from ECV to ECII are potentiated. The frequency of the reverberation would be in a gamma range because
it takes signals 20–30 ms to go around the entorhinal-hippocampal loop circuits. On the other hand, it has been suggested
that ECII is a theta rhythm generator. If the reverberation activity appears in the entorhinal-hippocampal loop circuits,
gamma oscillation would be superimposed on a theta rhythm in ECII like a gamma-theta oscillation. This is a reminiscence of
the theta phase coding of place information. In this paper, first, a network model of ECII will be developed in order to reproduce
a theta rhythm. Secondly, we will show that loop connections from one stellate cell to the other one are selectively potentiated
by afferent signals to ECII. Frequencies of those afferent signals are different, and transmission delay of the loop connections
is 20 ms. As a result, stellate cells fire successively within one cycle of the theta rhythm. This resembles gamma-theta oscillation
underlying the phase coding. Our model also replicates the phase precession of stellate cell firing within a cycle of subthreshold
oscillation (theta rhythm). |
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Keywords: | Network model Entorhinal cortex layer II Loop connections Stellate cell Theta rhythm Phase coding Spike-timing dependent plasticity |
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