Loss of Hippocampal Theta Rhythm Results in Spatial Memory Deficit in the Rat

TLDR

The task required rats to locate a single goal cup among eight equidistant cups on a circular track, a spatial deficit similar to that seen in hippocampally lesioned patient H.M. Rats were trained to run to the goal cup using distal room cues, then electrolytic lesions of the medial septal nucleus were applied to eliminate hippocampal theta rhythm in some animals. Rats lacking theta rhythm failed to perform the spatial task but could still recognize the goal cup when encountered, and they were able to learn the task when deprived of theta rhythm before training.

Abstract

Rats learned, using distal room cues, to run to a goal on an elevated, circular track starting from any position on the track. The goal was one of eight equidistant, recessed cups set around the track, the goal cup being distinguished from the others solely by its position in the room. After learning, electrolytic lesions were made in the medial septal nucleus eliminating hippocampal theta rhythm in some animals but not in others. Rats without theta rhythm were no longer able to perform the spatial task, whereas rats with undisturbed theta rhythm retrained normal performance. Although rats without theta rhythm could not find their way directly to the goal, they recognized its location when they came upon it by chance. This type of spatial deficit appears similar to that shown by hippocampally lesioned patient H.M. Subsequent tests demonstrated that rats deprived of theta rhythm before training could nevertheless learn the task.