Episodic memory refers to the collection of past personal experiences that occurred at a particular time and place, a highly effective process for future planning. Dr. Tulving coined this process: mental time travel (Tulving 1983).
How do place cells fit into episodic memory retrieval?
Place cells respond to a particular location in the physical world. The real-time activity of place cells represents where a rat is in space. When rats pause, place cells replay allowing the rat to mentally map out the route it needs to take. After spatial navigation in an environment, hippocampal place cells replay their activity, which can be useful for future planning.
Dr. Takahashi at Doshisha University is studying how exactly the reactivation of hippocampal place cells contributes to future planning when rats are navigating a spatial environment.
To do this he has rats perform a variety of tasks inside of a figure 8 maze.
Adapted from Takahashi 2015
In one maze, rats need to assess which one of two lights is lit and run towards it. In the other task, rats need to remember the direction they went previously and go in the opposite direction.
As rats perform the mazes, he simultaneously monitors hippocampal place cell activity using implanted tetrodes to examine what place cells are doing at each decision-making point.
Hippocampal Place Cell Replay for Memory Retrieval
So far, Dr. Takahashi has discovered that place cells replay information about both the path the rat needs to take as well as the task it needs to carry out. That is, hippocampal place cell firing conveys both the “where” information and “what” information during retrieval. Previously, hippocampal place cell replay was thought to only contribute spatial information during memory retrieval. However, these results suggest that hippocampal place cell activity can simultaneously represent both spatial and nonspatial information!
In fact, it was this research that was the inspiration for our newest product, The Free Maze. Dr. Takahashi teamed up with O’Hara to design the first modular maze for mice and rats that researchers can design and redesign themselves!