Time cells in the mouse dentate gyrus: An apparent instance of traveling waves in the brain

The role of standing brain rhythms in cognition has been well investigated, but traveling waves have only recently attracted interest for their potential role in memory and attention. A manifestation of traveling waves in the brain may be "time cells". In animal experiments, a task-relevant event triggers a reliable sequence of neural firing. Each "time cell" fires during a circumscribed period of a delay interval; different events can trigger different sequences. As the time interval since the event increases, the proportion of active time cells decreases. We present evidence of time cells in the mouse dentate gyrus whose activity spans a 20 s delay interval. We describe how information encoded about the time interval since the event is characteristic of a propagating wave traveling in a medium with graduated index of refraction. This framework allows us to characterise how the brain is continually aware of its orientation with respect to events in the world unfolding in time.