Hippocampal synaptic plasticity during instrumental learning

present research examined the role of hippocampal NMDA-dependent synaptic potentiation on appetitive instrumental conditioning under a continuous reinforcement schedule. In the first experiment, low (.025 mg.kg) or moderate (.05 mg/kg) dosages of the NMDA receptor antagonist, MK801, failed to increase the number of training days required to reach acquisition criterion; number of training days required to reach criterion for extinction were also unaffected. In the second experiment, a higher dosage (.10 mg/kg) of MK801 or induction of long-term potentiation failed to alter the number of responses occurring during acquisition. These data suggest that hippocampal synaptic potentiation does not play a prominent role in instrumental learning with simple contingency conditions. It is suggested that hippocampal LTP reflects a perceptual process that contributes differentially to spatial cognition, classical and instrumental conditioning.     

Gender bias and individualism: Responses to case vignettes among psychologists, professors, and students

This study examined the extent to which psychologists, when compared to university professors and evening school students, employ individualist values differentially in their responses to vignettes describing males and females. The participants responded to 6 vignettes by choosing initial hypotheses that reflected either utilitarian (tradition-ally masculine) or expressive (traditionally feminine) themes. Two vignettes involved a sex of client manipulation, but neither resulted in a sex of client effect. An analysis of the responses to the remaining 4 vignettes indicated a marked preference for the masculine form of individualism for males, but were evenly divided for females in all three groups. These results suggest that psychologists were just as likely to engage in bias as others.     

Effects of a variable-ratio conditioning history on sensitivity to fixed-interval contingencies in rats.

We investigated the possibility that human-like fixed-interval performances would appear in rats given a variable-ratio history (Wanchisen, Tatham, & Mooney, 1989). Nine rats were trained under single or compound variable-ratio schedules and then under a fixed-interval 30-s schedule. The histories produced high fixed-interval rates that declined slowly over 90 sessions; differences as a function of the particular history were absent. Nine control animals given only fixed-interval training responded at lower levels initially, but rates increased with training. Despite differences in absolute rates, rates within the intervals and postreinforcement pauses indicated equivalent development of the accelerated response patterns suggestive of sensitivity to fixed-interval contingencies. The finding that the histories elevated rates without retarding development of differentiated patterns suggests that the effective response unit was a burst of several lever presses and that the fixed-interval contingencies acted on these units in the same way as for single responses. Regardless of history, the rats did not manifest the persistent, undifferentiated responding reported for humans under comparable schedules. We concluded that the shortcomings of animal models of human fixed-interval performances cannot be easily remedied by including a variable-ratio conditioning history within the model.     

The effects of cocaine on behavior maintained by timeout from avoidance.

Rats were trained on concurrent schedules under which responses on one lever postponed shock (avoidance) and responses on the other lever produced brief (2-min) periods of signaled timeout from avoidance. For 6 rats, timeout from avoidance was programmed on a variable-interval 45-s schedule that generally resulted in rates that were lower than those on the avoidance lever. For another 6 rats, timeout was arranged on a variable-ratio 15 schedule that produced higher baseline rates. Cocaine (3 to 40 mg/kg) produced large, dose-dependent increases in behavior maintained by timeout in both groups of rats. Avoidance responding was also generally increased by cocaine, but the increases were of lesser magnitude. Increases in response rates were seen across a broad range of doses on behavior maintained by either interval or ratio schedules, an outcome that was unexpected on the basis of most studies of cocaine on food-maintained behavior. These results were similar to those of previous studies of the effects of amphetamine on behavior maintained by timeout from avoidance and suggest that stimulant drugs affect behavior maintained under a shock-postponement schedule differently than they affect behavior maintained by timeout from avoidance.     

Food-deprivation effects on punished schedule-induced drinking in rats.

Food-deprived rats (at 80% of their free-feeding weights) were exposed to a fixed-time 60-s schedule of food-pellet presentation and developed schedule-induced drinking. Lick-dependent signaled delays (10 s) to food presentation led to decreased drinking, which recovered when the signaled delays were discontinued. A major effect of this punishment contingency was to increase the proportion of interpellet intervals without any licks. The drinking of yoked control rats, which received food at the same times as those exposed to the signaled delay contingency (masters), was not consistently reduced. When food-deprivation level was changed to 90%, all master and yoked control rats showed decreases in punished or unpunished schedule-induced drinking. When the body weights were reduced to 70%, most master rats increased punished behavior to levels similar to those of unpunished drinking. This effect was not observed for yoked controls. Therefore, body-weight loss increased the resistance of schedule-induced drinking to reductions by punishment. Food-deprivation effects on punished schedule-induced drinking are similar to their effects on food-maintained lever pressing. This dependency of punishment on food-deprivation level supports the view that schedule-induced drinking can be modified by the same variables that affect operant behavior in general.     

Behavioral economics of concurrent ethanol-sucrose and sucrose reinforcement in the rat: effects of altering variable-ratio requirements.

These experiments examined the own-price and cross-price elasticities of a drug (ethanol mixed with 10% sucrose) and a nondrug (10% sucrose) reinforcer. Rats were presented with ethanol-sucrose and sucrose, both available on concurrent independent variable-ratio (VR) 8 schedules of reinforcement. In Experiment 1, the variable ratio for the ethanol mix was systematically raised to 10, 12, 14, 16, 20, and 30, while the variable ratio for sucrose remained at 8. Five of the 6 rats increased ethanol-reinforced responding at some of the increments and defended baseline levels of ethanol intake. However, the rats eventually ceased ethanol-reinforced responding at the highest variable ratios. Sucrose-reinforced responding was not systematically affected by the changes in variable ratio for ethanol mix. In Experiment 2, the variable ratio for sucrose was systematically increased while the ethanol-sucrose response requirement remained constant. The rats decreased sucrose-reinforced responding and increased ethanol-sucrose-reinforced responding, resulting in a two- to 10-fold increase in ethanol intake. Experiment 3 examined the substitutability of qualitatively identical reinforcers: 10% sucrose versus 10% sucrose. Increases in variable-ratio requirements at the preferred lever resulted in a switch in lever preference. Experiment 4 examined whether 10% ethanol mix substituted for 5% ethanol mix, with increasing variable-ratio requirements of the 5% ethanol. All rats eventually responded predominantly for the 10% ethanol mix, but total amount of ethanol consumed per session did not systematically change. In Experiment 5, the variable-ratio requirements for both ethanol and sucrose were simultaneously raised to VR 120; 7 of 8 rats increased ethanol-reinforced responding while decreasing sucrose-reinforced responding. These data suggest that, within this ethanol-induction procedure and within certain parameters, demand for ethanol-sucrose was relatively inelastic, and sucrose consumption was independent of ethanol-sucrose consumption. Demand for sucrose, on the other hand, was relatively elastic, and ethanol-sucrose readily substituted for it. The results are discussed in terms of applying a behavioral economic approach to relationships between drug and nondrug reinforcers.