Septal driving of the hippocampal theta rhythm produces a long-term, proactive and non-associative increase in resistance to extinction

Previous experiments employing electrical or pharmacological induction, or instrumental reinforcement, of hippocampal theta rhythm in rats have reported subsequent performance changes in a number of learning situations. In all these studies theta induction took place either concurrent with or following the behaviour under investigation. In the present two experiments, a treatment phase of electrically-induced hippocampal theta (by septal stimulation at 7·7 Hz) preceded acquisition of a discrete trial, barpress response on a fixed ratio 5 reinforcement schedule. In one of three treatment conditions animals received electrical stimulation (1) on its own, or (2) in a classical conditioning relationship with food delivery as unconditioned stimulus, or (3) temporally uncorrelated with food delivery; controls were implanted with electrodes but not stimulated. After 15 days' acquisition of barpressing, all animals were extinguished over the subsequent 12 days. Results indicated that theta-driving stimulation, independent of any association with food, resulted in increased resistance to extinction of barpressing. In addition, conditions (1) and (3) enhanced the speed of response during early acquisition. These findings cast doubt on a “memory consolidation” hypothesis of hippocampal theta function and demonstrate a non-associative, long-lasting, proactive effect of theta induction on behaviour.     

Survival of the partial reinforcement extinction effect after contextual shifts

The effects of contextual shifts on the partial reinforcement extinction effect (PREE) were studied in autoshaping with rats. Experiment 1 established that the two contexts used subsequently were easily discriminable and equally salient. In Experiment 2, independent groups of rats received acquisition training under partial reinforcement (PRF) or continuous reinforcement (CRF). Significant PREEs were observed whether extinction occurred in the same or in a different context. In Experiment 3, after PRF or CRF acquisition training, each group received extinction of the same conditioned stimulus in both the same and different contexts. PREEs were observed under both conditions, although extinction was accelerated after a contextual shift. These results were discussed in relation to the role of mediational states in the PREE.     

A partial reinforcement extinction effect despite equal rates of reinforcement during Pavlovian conditioning

In 4 experiments rats received appetitive Pavlovian conditioning followed by extinction. Food accompanied every trial with the conditioned stimulus (CS) for the continuously reinforced groups and only half of the trials for the partially reinforced groups. In contrast to previous experiments that have compared the effects of partial and continuous reinforcement, the rate at which food was delivered during the CS was the same for both groups. The strength of the conditioned response during extinction weakened more rapidly in the continuously than in the partially reinforced groups. The results demonstrate that the partial reinforcement extinction effect is a consequence of the nonreinforced trials with the CS, rather than the rate at which the unconditioned stimulus is delivered during the CS.     

Perseveration of the partial reinforcement effect in extinction with rats over two phases of extinction and two stages of continuous reinforcement

One group of 10 male albino rats was given partial reinforcement while the other 10 rats received continuous reinforcement in a straight alley. Subjects then experienced five consecutive stages of Extinction 1, Continuous Reinforcement 1, Extinction 2, Continuous Reinforcement 2, and finally, Extinction 3. Analysis showed the partial reinforcement effect in extinction was sustained over two stages of extinction and two stages of continuous reinforcement, since subjects receiving partial reinforcement ran faster than rats given continuous reinforcement throughout all three of the extinction periods. The results seem to support those of Amsel's (1967) and Cabpaldi's (1967) theoretical formulations of the partial reinforcement effect in extinction.     

A sequential analysis of the partial reinforcement extinction effect in children*

In a series of studies employing children between the ages of 7.6 and 10.6 years of age guessing on a modified Humphreys’s board, extinction training was administered following either continuous reinforcement or various schedules of partial reinforcement training. Besides the observation of a partial reinforcement extinction effect, it was found that resistance to extinction appeared to be regulated by those sequential variables specified by extensions of Capaldi’s theory of instrumental learning.     

The effects of early handling on the partial reinforcement extinction effect and the partial punishment effect in male and female rats

Handled (Day 1-22) and non-handled infantile Wistar rats were tested in maturity for the partial reinforcement extinction effect (PREE) and the partial punishment effect (PPE). In Experiments 1 and 2, mature male and female rats were trained to run in an alley for food reward on a 1-trial/day schedule. In the PREE paradigm (Experiment 1), the partially reinforced group (PRF) received reinforcement on a quasi-random 50% schedule, while the continuously reinforced group (CRF) received reinforcement on every trial. In the test stage, both groups were given extinction training. In the PPE paradigm (Experiment 2), the partially punished (PP) group received, together with continuous reinforcement, shocks on a quasirandom 50% schedule, while the continuously reinforced group was reinforced on every trial. In test, all animals were given both reinforcement and shock on every trial. In Experiment 1, PREE—i.e. increased resistance to extinction in the PRF as compared to the CRF group—was more pronounced in the handled animals. More specifically, no PREE was obtained in the non-handled males, and in the non-handled females the PREE was reduced compared to the handled females. The results of Experiment 2 revealed no effect of handling or sex on PPE, that is, increased resistance to punishment in the PP group as compared to the CRF group was evident in all four conditions. In Experiment 3, handled and non-handled male rats were tested for the PREE using a multi-trial procedure in an operant chamber. PREE was obtained in the handled but not in the non-handled animals. The implications of these results for the differential effects of handling on male and female rats and the distinction between the PREE and PPE paradigms are discussed.     

Effects of anxiolytic drugs on the partial reinforcement extinction effect in runway and skinner box

The partial reinforcement extinction effect (PREE) in the runway is reduced by anxiolytics in a non-state-dependent manner when a 24 h inter-trial interval is used, but there is some doubt as to the nature of the drug effects when shorter intervals are used. Experiment 1 repeated a study by Gray (1969), in which ambiguous results were obtained using eight trials/day. It demonstrated that the anxiolytic barbiturate, sodium amylobarbitone, given both in acquisition and extinction does not reduce the PREE. It confirmed Gray's observation that the PREE is abolished if the drug is given in acquisition but not in extinction. This suggests that a 24 h inter-trial interval is one critical factor in non-state-dependent reduction of the PREE. Experiments 2 and 3 tested the effects of the anxiolytic benzodiazepine, chlordiazepoxide, on the PREE with a 24 h inter-trial interval in the Skinner box. The basic task was a single FR5 sequence terminating in delivery of 10 (Experiment 2) or 20 (Experiment 3) reward pellets each day. There were 10 acquisition trials and partially reinforced rats received either three (Experiment 2) or four (Experiment 3) non-rewarded trials. The drug abolished the PREE in Experiment 2 and effectively reversed it in Experiment 3. These results confirm previous work with this drug in the runway (Feldon and Gray, 1981) and extend them to a very different experimental situation. These results support the idea that the PREE depends on different processes with different acquisition parameters; and that when a 24 h inter-trial interval is used the PREE is largely produced by some general process, probably the counterconditioning of conditioned frustration, which is sensitive to anxiolytic drugs. They also demonstrate very clearly the paradoxical effects of the anxiolytic drugs when given in both acquisition and extinction: they generally increase resistance to extinction in continuously reinforced animals, but block the increase in resistance (the PREE) produced by behavioural schedules.     

The partial punishment effect following minimal acquisition training: Sodium amobarbital and the stimulus properties of early punished trials

In a runway investigation, six groups of rats received limited runway training such that partial punishment, partial reinforcement, or continuous reinforcement was accompanied by sodium amobarbital or saline. Following an interpolated phase of continuous reinforcement without injections, all groups were given punished extinction. The entire experiment was conducted under widely spaced conditions (ITI = 24 hr). It was found that partial punishment increased resistance to punished extinction relative to partially and continuously reinforced controls when acquisition was given under saline. When partial punishment training was accompanied by amobarbital this effect was eliminated. The drug was observed to have no effect on the punished extinction performance of the partial reinforcement and continuous groups, respectively. Moreover, the partial reinforcement effect (PRE) did not generalize to punished extinction. These data provide information concerning the difference between the stimuli associated with the early trials of punishment and nonreward and indicate that the former but not the latter contain emotional elements.     

Effects of medial and lateral septal lesions on the partial reinforcement extinction effect at short inter-trial intervals

Rats sustained electrolytic lesions either in the medial septal (MS) area (of a kind known to eliminate the hippocampal theta rhythm) or in the dorso-lateral septal (LS) area (of a kind known to spare theta) and were compared to sham-operated controls in three experiments in the straight alley with food reward on continuous (CRF) or partial (PRF) reinforcement and inter-trial intervals of 3-8 min. With 6 acquisition trials MS lesions increased resistance to extinction and enhanced the partial reinforcement extinction effect (PREE). With 48 acquisition trials MS lesions did not alter resistance to extinction after either CRF or PRF training, but LS lesions abolished the PREE by increasing resistance to extinction in rats trained with CRF and decreasing it in rats trained with PRF. With 96 acquisition trials LS lesions were without effect on resistance to extinction after either CRF or PRF training, as previously reported by Henke (1974) using total septal lesions. Thus the impairment in the PREE previously shown after large septal lesions is due to damage to the lateral, not the medial, septal area.     

Transfer of the partial reinforcement extinction effect between escape (shock) and appetitive (food) conditioning

In the first experiment rats were given partial reinforcement or continuous reinforcement in either an escape or an appetitive paradigm. Subsequently, the rats received continuous reinforcement training under motivational conditions opposite those experienced earlier. Finally, responses were extinguished according to the motivational conditions experienced in the second phase. The results indicated that partial reinforcement in the initial phase operated to increase resistance to extinction in the last. In a second experiment this intermotivational partial reinforcement extinction effect was shown to survive interpolated experiences with extinction, a 1-week rest period, and continuous reinforcement reacquisition. A third experiment examined the influence of intramodal versus intermodal nonreinforcement-reinforcement sequences on the intermotivational partial reinforcement extinction effect. Interactive effects between similarity of aversive outcome (escape nonreinforcement, appetitive nonreinforcement) and reinforcement type (negative, positive) were found. The theoretical implications of the data from all three experiments are discussed.     

Effects of medial and lateral septal lesions on the partial reinforcement extinction effect at one trial a day

Rats sustained electrolytic lesions of either the medial septal (MS) area (of a kind known to eliminate the hippocampal theta rhythm) or the dorso-lateral septal (LS) area (of a kind known to spare theta) or both (a “total septal”, TS, lesion). They were compared to sham-operated controls in three experiments in the straight alley with food reward on continuous (CRF) or partial (PRF) reinforcement at one trial a day. MS lesions either left the partial reinforcement extinction effect (PREE) unchanged or enhanced it; LS lesions substantially reduced the PREE. The latter effect was due to a fall in resistance to extinction in PRF animals with no change in CRF animals. MS lesions greatly increased resistance to extinction in both CRF and PRF animals in one experiment but increased resistance to extinction only marginally and only in PRF animals in a second experiment. The TS lesion acted like the LS lesion. These results demonstrate a clear double dissociation between the effects of MS and LS lesions, especially in the PRF condition (LS lesions reduce resistance to extinction, MS lesions increase it).