Long-term object location memory in rats: effects of sample phase and delay length in spontaneous place recognition test.

This study investigates the effects of sample phase and delay length on discrimination performance in the spontaneous place recognition (SPR) test in rats. Rats were allowed to explore an arena where two identical objects were presented for 5-20 min (sample phase). After a delay interval, rats were placed again in the same arena but one of the two objects was moved to a novel place (test phase). Results showed that when the sample phase was as long as 20 min, rats preferentially explored the moved object during the test phase even after a 6-24h delay was interposed. Further sequential and cumulative analyses of the test phase revealed that the preference for the object in a novel place was evident in the first and 2nd min of the test phase in rats with a longer sample phase duration. Correlation analysis showed that locomotor activity and object exploration in the sample phase were not decisive factors in spatial memory performance. The present results demonstrate the importance of the sample phase exposure time and the test phase length.