THE EFFECT OF MEDIAL SEPTAL AND MAMMILLARY BODY LESIONS ON THE SERIALPOSITION CURVE IN RATS

The effect of medial septum and mammillary body radio-frequency lesion s on memory for list items was examined in rats. Subjects were exposed , one arm at a time, to a list of 7 arms presented in a 12-arm radial maze and were then required to return to a list arm in the subsequent test phase. Before surgery, rats in both groups displayed a serial pos ition curve-that is, superior recognition for arms presented at the be ginning of the list (primacy effect) and the end of the list (recency effect) but not the middle of the list. Medial septal lesions had two major effects on performance: accuracy at all serial positions was red uced, but while the primacy effect disappeared, a recency effect was r etained. Following lesions of the mammillary bodies, the primacy and t he recency effects disappeared, but there was no significant reduction in overall accuracy. The disruptive effect of both lesions on memory performance was robust despite extended training after surgery and was unaffected by increasing the exposure time to list arms during presen tation. The memory impairment observed in rats following a medial sept al lesion mimics the disruption in memory for list items that has been observed in patients with mild Alzheimer's disease and with Korsakoff 's syndrome, two diseases that involve neuropathological changes in th e medial septum.