Lost in time: rats are unable to return to a start location that varies

Path integration provides guidance based on cues generated by a point of reference (usually start location) and subsequent self-movement. This well-established mechanism suggests start location may have special significance and might provide a useful window into memory in rats. In an earlier study rats did not learn to return to a start location in a four-arm radial water maze when the start location varied across trials. Here we examine return to start location in appetitive tasks. Initially, rats were released from one of three arms with the food located in the fourth arm. Once a rat found the food, a second arm was baited; either the start arm, for one group, or another fixed location, for a control group. Rats had difficulty finding the second food reward in the start arm, but not in another fixed location. Performance was similar when rats were trained with a three-arm maze. It was also observed that rats learned the initial fixed location more slowly if they were required to learn a variable second location. This suggested the nature of the journey affected the rate of problem solution. One explanation for the failure of rats to return to the start location is that the path integrator is reset upon reaching the first correct arm. In a final experiment, a foraging task was used where resetting of the path integrator should not occur. Again, rats failed to return to the start location when it varied across trials. These findings suggest rats did not time tag start locations, which indicates that there may be constraints on the occurrence of episodic-like memory in rats.