The above examples demonstrate that theta cycles do not simply compress first-order sequences of past and upcoming positions of travel82 but can chunk large parts of the environment, without replaying all actually visited locations. Such nonsequential, higher-order connections are possible because place fields have long ‘tails’ that provide opportunities for spikes of neuron pairs with distant place fields to fire together, at least occasionally, in the same theta cycle. The large place fields of neurons in more ventral parts of the hippocampus and the large grid size in the more ventral parts of the medial entorhinal cortex may be especially important in creating higher-order links and may provide the flexibility needed for efficient navigation64,91. These same neuronal mechanisms can enhance the flexibility of episodic memory as well. In the same way that an infinite number of paths can connect the origin and end point of a journey, a recalled story can be told in many ways, connecting the beginning and end through innumerable variations. The neuronal mechanisms that allow the selection of the optimal path in navigation in the physical
