No abstract

A recent study has shown that damage to the interpeduncular nucleus (IPN) of the midbrain abolishes a previously formed brightness discrimination habit in rats (Thompson & Massopust, 1960). That this nucleus seems to be implicated in visual performance rather than the adjacent midbrain structures (red nucleus, medial lemniscus, oculomotor nucleus, mammillary peduncle, mammillo-tegmental tract) is indicated by two lines of evidence. First, lesions placed immediately lateral, superior, or posterior to the IPN are without effect on retention. In the second place, the degree of memory loss in percentage of error savings was found to be highly correlated with the extent of damage to the IPN (rho = .85).The present study reports the results of three experiments which were designed to provide further information concerning the role of the IPN in retention. In Experiment 1, an attempt was made to determine whether damage to the IPN impairs retention of a brightness discrimination when the hunger motive is employed. The initial report by Thompson and Massopust utilized avoidance of shock to the feet as a motive. Experiments 2 and 3 were aimed at ascertaining whether or not the IPN functions in memory of nonvisual habits. Two different problems involving different sense modalities were investigated. In one, rats were trained on a kinesthetic discrimination in an incline box (Experiment 2), while the other involved conditioning rats to make a jumping response to the onset of a buzzer (Experiment 3).