Hippocampal lesions produce a temporally graded retrograde amnesia on a dry version of the Morris swimming task

Using a dry version of the Morris hidden-goal swimming task, we reexplored the retrograde and anterograde effects of hippocampal lesions on spatial performance in rats. Rats were trained to find food buried under sawdust in a 183-cm-diameter gray cylinder; the location of the buried food was constant for each rat. Either 1 week or 14 weeks after training the rats were subjected to massive hippocampal + dentate lesions by multiple injections of neurotoxic agents. Rats with a 1-week training-lesion interval could not reacquire the ability to get to the hidden food, although they performed normally when the goal location was marked with a dowel. In contrast, rats with a 14-week training-lesion interval relearned finding the buried food at a nearly normal rate. The two groups together suggest that memory for the buried food task is susceptible to graded retrograde amnesia in a fashion similar to explicit memory in humans. Additional testing of the 14-week training-lesion group revealed, however, that the consolidated memory was not fully formed; these rats were incapable of learning to go to a new location in the same cylinder, although sham lesioned rats readily transferred. To explain this pattern of results, we propose that (1) rats with an intact hippocampus have available a neural map of the environment that permits them to learn to go from anywhere to anywhere else, and (2) hippocampally lesioned rats have no such map, but are able to show some navigational skills by using a vector field representation of the environment that permits them to get from anywhere to a single location. In a separate experiment, we provide support for the notion that the rodent hippocampus has a specialized role in spatial memory by showing that hippocampally lesioned rats performed normally in a visual object recognition task whose motor and motivational components were the same as in the buried food task.