Theta Phase Modulates Multiple Layer-Specific Oscillations in the CA1 Region

2012-10

amplitude modulation

cross-frequency coupling

gamma

hippocampal oscillations

local field potential

It was recently proposed that fast gamma oscillations (60--150 Hz) convey spatial information from the medial entorhinal cortex (EC) to the CA1 region of the hippocampus. However, here we describe 2 functionally distinct oscillations within this frequency range, both coupled to the theta rhythm during active exploration and rapid eye movement sleep: an oscillation with peak activity at ~80 Hz and a faster oscillation centered at ~140 Hz. The 2 oscillations are differentially modulated by the phase of theta depending on the CA1 layer; theta-80 Hz coupling is strongest at stratum lacunosum-- moleculare, while theta-140 Hz coupling is strongest at stratum oriens--alveus. This laminar profile suggests that the ~80 Hz oscillation originates from EC inputs to deeper CA1 layers, while the ~140 Hz oscillation reflects CA1 activity in superficial layers. We further show that the ~140 Hz oscillation differs from sharp wave--associated ripple oscillations in several key characteristics. Our results demonstrate the existence of novel theta--associated high-frequency oscillations and suggest a redefinition of fast gamma oscillations.