Different theta frameworks coexist in the rat hippocampus and are coordinated during memory-guided and novelty tasks

Hippocampal firing is organized in theta sequences controlled by internal memory processes and by external sensory cues, but how these computations are coordinated is not fully understood. Although theta activity is commonly studied as a unique coherent oscillation, it is the result of complex interactions between different rhythm generators. Here, by separating hippocampal theta activity in three different current generators, we found epochs with variable theta frequency and phase coupling, suggesting flexible interactions between theta generators. We found that epochs of highly synchronized theta rhythmicity preferentially occurred during behavioral tasks requiring coordination between internal memory representations and incoming sensory information. In addition, we found that gamma oscillations were associated with specific theta generators and the strength of theta-gamma coupling predicted the synchronization between theta generators. We propose a mechanism for segregating or integrating hippocampal computations based on the flexible coordination of different theta frameworks to accommodate the cognitive needs.

Gravitational Waves in Axion Dark Matter

Axion dark matter is interesting as it allows a natural coupling to the gravitational Chern–Simons term. In the presence of an axion background, the gravitational Chern–Simons term produces parity violating effects in the gravitational sector, in particular on the propagation of gravitational waves. Previously, it has been shown that the coherent oscillation of the axion field leads to a parametric amplification of gravitational waves with a specific frequency. In this paper, we focus on the parity violating effects of the Chern–Simons coupling and show the occurrence of gravitational birefringence. We also find deviation from the speed of light of the velocity of the gravitational waves. We give constraints on the axion-Chern–Simons coupling constant and the abundance of axion dark matter from the observation of GW170817 and GRB170817A.

Exploring the string axiverse and parity violation in gravity with gravitational waves

We show that the parametric resonance of gravitational waves (GWs) occurs due to the axion coherent oscillation and the circular polarization of GWs is induced by the Chern–Simons coupling. However, we have never observed these signatures in the data of GWs. Using this fact, we give stringent constraints on the Chern–Simons coupling constant [Formula: see text] and the abundance of the light string axion. In particular, we improved the current bound [Formula: see text] by many orders of magnitude.

Electromagnetic waves propagating in the string axiverse

It is widely believed that axions are ubiquitous in string theory and could be dark matter. The peculiar features of axion dark matter are coherent oscillations and a coupling to the electromagnetic field through the Chern–Simons term. In this letter, we study the consequences of these two features of axions with mass in the range $10^{-13}\,{\rm eV}$ to $10^{3}\,{\rm eV}$. First, we study the parametric resonance of electromagnetic waves induced by the coherent oscillation of the axion. Since the resonance frequency is determined by the mass of the axion dark matter, if we detect this signal, we can get information on the mass of the axion dark matter. Second, we study the velocity of light in the background of the axion dark matter. In the presence of the Chern–Simons term, the dispersion relation is modified and the speed of light will oscillate in time. It turns out that the change in the speed of light would be difficult to observe. We argue that future radio wave observations of the resonance can give rise to a stronger constraint on the coupling constant and/or the density of the axion dark matter.