How long is a sequence? Neuronal representation of travel paths does not consist of long uninterrupted neuronal chains but are often broken up into repeating chunks by prominent landmarks, state changes or reinforcers. Furthermore, the firing characteristics of grid cells and place cells are context dependent. In corridors or on elevated tracks, the firing fields of entorhinal cortex grid cells in one direction are generally independent of the positions in the other direction61. Similarly, the omnidirectional place fields of hippocampal neurons62 become unidirectional in linear environments, such that different sets of neurons are active at each location in the two directions of travel63. The independent representations of travel paths in opposite directions (Fig. 2) may reflect mechanisms similar to those underlying remapping between different environments, such as independent resetting among modules of the entorhinal cortex grid map. Chunking is even more prominent in complex environments. When different segments of a maze are geometrically similar, such as for the corridors of a hairpin labyrinth, hippocampal and entorhinal cortex neurons fire the same sequences in each corridor, although distinct sets of neurons
