How does the brain generate and store sequences? We hypothesize that the mechanisms for representing a path through an environment are similar to those used to represent sequences in memory (Fig. 4a). Like the position-dependent sequential firing of neurons along a linear path, sequences of arbitrary items in episodic memory tasks are essentially unidimensional. Therefore, the same encoding mechanisms can be used for temporal integration of positions and episodic items54. The dominantly unidirectional linking of items in episodic memory, analogous to linking of place cells55, can explain two important principles of free recall: asymmetry, or the fact that forward associations are stronger than backward associations56, and temporal contiguity, or the fact that recollection of an item is facilitated by the presentation or spontaneous recall of another item that occurred close in time to the item just recalled56. There are also key differences between mechanisms of navigation in the physical world and mental recall. A fundamental difference is that while the navigation system can rely on environmental or body-derived cues, as it does in invertebrates, ‘simulated’ or ‘internalized’ travel12,14,57 requires internally generated cell assembly sequences (Fig. 4).
