In addition to reporting the instantaneous position of the animal or the explicit identity of objects and events, neurons in the hippocampus can also predict where the animal is coming from or where it is going45,58. When environmental or body-derived signals are kept constant—for example, during running in a wheel in a memory task—perpetually changing neuronal assembly sequences are present in the hippocampus59 (Fig. 4b). The unique patterns of the cell assembly sequences can reliably predict an animal’s correct or even erroneous choice in a maze many seconds before the actual motor turn. It has been hypothesized that such self-organized cell assembly sequences or neural ‘trajectories’ underlie the numerous episodes recorded in one’s lifetime59. Because many more sequences can be generated than the numbers of the member neurons, the syntactical rules underlying self-evolving neuronal trajectories can enormously expand the memory capacity of the hippocampal system60. In short, assembly sequences in the hippocampus can be generated by two different but possibly interacting mechanisms, driven, respectively, by external inputs (environmental or body-derived cues) and internal self-organization (Fig. 4a). Self-organized cell assembly sequences disengaged from the environmental or body-derived inputs, in turn, may support mental travel.
