We found that, counter to previous findings in the water maze, rats on dry-land mazes do not seem to reset their internal maps upon passage through the curtain surrounding the maze. My current research involves analysis of the relationship between place cell firing and the hippocampal theta rhythm, as part of our investigation into the nature of the inputs which control place cells and endow them with their spatial specificity. Collectively, a the population of place cells active in a given environment form a representation of all positions within the environment. However, as an animal travels through the field for a given cell, the firing of the cell tends to occur at successively earlier phases of the theta cycle. The phase precession of spike bursts as the animal passes through the field suggests that place cell firing may be produced by dual oscillatory input - one input probably being the theta rhythm itself, and the other being an oscillator which changes it's phase relationship with theta as the animal traverses the place field.
Velocity is plotted blue.
As the animal is moving left to right, this cell's field is clearly on the left. The image on the right represents data collected over multiple passes through the field. The top panel is the theta phase at which the cell fires plotted against position. The bottom panel illustrates the overall firing rate of the cell as a function of position. Control by an internal sense of direction carried across environments.