Retention of a geotaxic discrimination task following septal lesions in rats

Abstract

Septal lesions did not impair retention of a simple discrimination task that was based on geotaxic cues, but it did disturb the ability of the animals to make fine discriminations and prolonged extinction when the animals were not preloaded with water.Several recent studies have investigated the importance of the septal area in retention. Reports on the effects of lesions in this region range from complete loss of memory to slight or no impairment at all. Deficits in memory have been demonstrated on tasks that require active avoidance from shock (McCleary et aI, 1965;Moore, 1964;Rich & Thompson, 1965), on a complex maze (Carey, 1968), by means of an extinction test (Carey, 1967), and to a lesser extent on a brightness discrimination task (Schwartzbaum et aI, 1964). A number of other studies have reported that septal lesions disrupt preoperatively conditioned behavior (e.g., Burkett & Bunnell, 1966; Schwartzbaum et aI, 1964). However, it is uncertain whether these effects represent a retention loss or are only secondary to some motivational and motor disinhibitory effects of septal damage. Septal lesions failed to impair retention of a thirst habit in a shuttle-box situation (Breen & Thompson, 1966), and of a black-white discrimination that was based on shock (Kleiner et aI, 1967; Meyer et aI, 1968). Rats were even found to improve their performance in a linear runway after destruction of the septal area (Raphelson et aI, 1966).The present experiment extends the inquiry to the effects of septal lesions on the retention of a discrimination based on geotaxic cues. An added incentive for using this task was that it presented a novel experimental procedure for tests of discrimination. METHOD Preoperative Tests Twelve male Sprague-Dawley rats (250-300 g) were taught to discriminate between geotaxic cues on the basis of water reinforcement. The data of two animals had to be discarded due to illness and death during the experiment. Following 23-h water deprivation, each animal was initially trained to press a lever for 0.1 ml of water while the test chamber was in a level position. Discrimination training usually Psychon. Sci., 1969, Vol. 15 (3) started on the third session and introduced tilting the cage 20 deg. The apparatus was adapted in such a way that the side of the cage housing the lever was free to move through various degrees of tilt, while the opposite side was fixed to a hinge. Each daily session during discrimination consisted of 18 trials, 60 sec in duration. On nine trials the cage was level and all responses were reinforced (S+). On the other nine trials the cage was tilted 20 deg and responses were never reinforced (S-). The session always started with a reinforced trial. The remaining S+ trials were distributed randomly among the S-trials, with the restriction of no more than three consecutive trials with either condition. Between trials the cage was raised and lowered a few times so as not to condition the animals to the direction of movement. Training on the discrimination continued for each S unti...