Patterns of retrograde amnesia for recent and remote incidental spatial learning in rats.

A non-navigational test of incidental spatial learning was used to determine whether hippocampal damage causes temporally-graded retrograde amnesia (TGRA) for allocentric-spatial information. Rats were exposed to two identical objects in a circular open field for 7 min on seven consecutive days. In the 1-3 days after the last day of familiarization, rats received neurotoxic lesions of the hippocampal formation (HPC) or sham lesions. Another two groups received the same lesions 3 weeks after familiarization. The rats were then placed back in the open field with one object displaced, and the time spent in each of the quadrants as well as time spent exploring the objects was recorded. Rats that received HPC lesions 3 weeks but not 1-3 days after familiarization showed evidence of preserved remote spatial memory; however, their remote memory was expressed through different behavior than control rats. Rats with HPC lesions spent more time with the displaced object than with the object that remained in the same place, whereas control rats spent more time in the quadrant where the displaced object used to be. These results suggest that remote spatial memories may be preserved with a sufficiently long familiarization-to-surgery interval before HPC lesions, but that the nature of these memories may differ in quantity and/or quality from those of intact rats.