The hippocampus and spatial memory: findings with a novel modification of the water maze.

For many tasks and species, remote memory (but not recent memory) is spared after damage to the hippocampus. An exception to this pattern of findings has been that both recent and remote memory are impaired after hippocampal lesions when rats are trained in the conventional water maze task. We explored the effect of introducing a navigational beacon for rats to use during testing. Four identical beacons were hung directly over each of the water maze quadrants, equidistant from each other (multiple-beacon maze). One of the beacons was always directly over the hidden platform. By using distal spatial cues, rats could select the correct beacon and use that beacon as a guide to the hidden platform. Probe tests indicated that rats did use the beacons to guide performance throughout training. Two months after the completion of training, rats were given hippocampal or sham lesions. Controls performed well, but the lesion group performed at chance on the retention probe trials. Furthermore, the rats with lesions not only searched indiscriminately in all four quadrants, they also did not use the beacons. These results indicate that impaired performance in the water maze after hippocampal damage reflects more than a loss of spatial information.