Hippocampus-independent phase precession in entorhinal grid cells

Spatial awareness: Phase precession in grid cells In the brain, both rate and temporal codes are critical for information storage. Theta phase precession is a change in action potential timing in the hippocampus where place cells fire at progressively earlier phases of the theta rhythm as the animal moves across the firing field of the neuron. Hafting et al . explore the circuitry of theta phase precession and show that phase precession is expressed independently of the hippocampus in spatially modulated grid cells in parts of the entorhinal cortex. In the brain, both rate and temporal codes are critical for information storage. Theta phase precession is a change in action potential timing in the hippocampus where place cells fire at progressively earlier phases of the theta rhythm as the animal moves across the firing field of the neuron. This paper explores the circuitry of theta phase precession and shows that phase precession is expressed independently of the hippocampus in spatially modulated grid cells in parts of the entorhinal cortex. Theta-phase precession in hippocampal place cells 1 is one of the best-studied experimental models of temporal coding in the brain. Theta-phase precession is a change in spike timing in which the place cell fires at progressively earlier phases of the extracellular theta rhythm as the animal crosses the spatially restricted firing field of the neuron 1 , 2 , 3 , 4 , 5 . Within individual theta cycles, this phase advance results in a compressed replication of the firing sequence of consecutively activated place cells along the animal’s trajectory 2 , 6 , 7 , 8 , at a timescale short enough to enable spike-time-dependent plasticity between neurons in different parts of the sequence. The neuronal circuitry required for phase precession has not yet been established. The fact that phase precession can be seen in hippocampal output stuctures such as the prefrontal cortex 9 suggests either that efferent structures inherit the precession from the hippocampus or that it is generated locally in those structures. Here we show that phase precession is expressed independently of the hippocampus in spatially modulated grid cells 10 , 11 in layer II of medial entorhinal cortex, one synapse upstream of the hippocampus. Phase precession is apparent in nearly all principal cells in layer II but only sparsely in layer III. The precession in layer II is not blocked by inactivation of the hippocampus, suggesting that the phase advance is