Theta modulation in the medial and the lateral entorhinal cortex

37 38 Hippocampal neurons show a strong modulation by theta frequency oscillations. 39 This modulation is thought to be important for temporal encoding and decoding of 40 information in the hippocampal system, as well as for temporal ordering of neuronal 41 activities on time scales at which physiological mechanisms of synaptic plasticity 42 operate. The medial entorhinal cortex (MEC), one of the two major cortical inputs to the 43 hippocampus, is known to show theta modulation. Here, we show that the local field 44 potentials (LFPs) in the other major cortical input to the hippocampus, the lateral 45 entorhinal cortex (LEC), show weaker theta oscillations than the MEC. Neurons in LEC 46 also show weaker theta modulation than neurons in MEC. These findings suggest that 47 LEC inputs are integrated into hippocampal representations in a qualitatively different 48 manner than the MEC inputs. Furthermore, MEC grid cells increase the scale of their 49 periodic spatial firing patterns along the dorso-ventral axis, corresponding to the 50 increasing size of place fields along the septo-temporal axis of the hippocampus. We 51 show here a corresponding gradient in the tendency of MEC neural firing to skip 52 alternate theta cycles. We propose a simple model based on interference of delta 53 oscillations with theta oscillations to explain this behavior.