Rats were trained on a contextual discrimination after nonreinforced

preexposure to both contexts. In Experiment 1, where contexts differed in terms of

tactile and visual stimuli, preexposure retarded subsequent discrimination by

comparison with non preexposed controls (latent inhibition). In Experiment 2, where

contexts differed only in terms of visual cue, discrimination was facilitated in

preexposed animals (perceptual learning). Food was used as reinforcer and anticipatory

activity as dependent measure. These results suggest that contextual similarity

influences the outcome of nonreinforced preexposure.

Rats received training in the consummatory successive negative contrast (cSNC) situation in which access to 32% sucrose solution during ten daily trials is followed by a downshift to 4% sucrose. Separate groups were exposed to the hot plate test for pain sensitivity immediately after either the first or the second downshift trial (Trials 11 and 12, respectively). Rats exhibited hypoalgesia after Trial 12 downshift, but not after Trial 11. These results suggest that cSNC induces the release of endogenous opioids that cause hypoalgesia, but only after some experience with the downshifted solution. This interpretation is supported by experiments demonstrating that opioid agonists reduce cSNC, whereas opioid antagonists enhance it.

Human and animal studies have shown the long-lasting impact of early life experiences on the development of individual differences in stress responsiveness in later life. Despite the numerous works that evaluate the effect of early experiences on different behavioral paradigms, which for the most part are related to aversive situations, there are few studies that assess the effects on the unexpected downshift or omission of positive rewards. The purpose of this article is to present several independent lines of research into how frustration responses during adulthood may be influenced by early experiences. Very few works have been found on the subject, and in most cases the results were negative or controversial. However, more recent investigations suggest that the responses in adults to frustration or euphoria may be modulated by early experiences.

Context learning was studied in the Red Opossum, the marsupial, Lutreolina crassicaudata . In Experiment 1 the animals received four trials per day in two different boxes (contexts): X and Y. Half of the animals received periodic deliveries of a sugar solution ( + ) in one box (X+), but not in the other (Y-); the rest received the opposite training (X-), (Y+). Several behavioral categories were recorded during the final trial in each context. Animals approached the feeder significantly more in the positive context. Experiment 2 was designed to determine the extent to which the number of trials per day affected acquisition. Two groups of animals received differential training with either four or one trial per day. No differences between groups were observed, although in both of them, approach to the feeder was significantly higher in the positive context. The results are discussed in relation to both the role of practice distribution on learning in marsupials, and their potential value of this species for the study of learning processes.

Studies with several mammalian species show that the surprising omission of an appetitive reinforcer invigorates aggressive behavior. In the present experiment, dominant and nondominant individuals within pairs of male rats were identified in pretests sessions. The dominant males were then randomly assigned to one of two groups and trained in a consummatory contrast situation. Group 32-W received exposure to 32% sucrose solution and was then shifted to water; Group W-W received exposure to water throughout the experiment. Immediately after a shift to water, nondominant males were introduced in the training box for a 5-min-long session. The previously dominant males of Group 32-W exhibited a significant decrease in aggressive attacks to nondominant pairmates, compared to the W-W dominant rats. Consummatory training in a situation involving surprising nonreward inhibits aggressive behavior. The potential connection between the present results and those obtained in experiments with inescapable shocks is discussed.

Two experiments with rats studied the relationship between reinforcer magnitude and frequency, and extinction rate in a consummatory situation with rats. In Experiment 1, groups received access to either a 2% or a 32% sucrose solution during twenty 5-min sessions and were subsequently shifted to extinction (access to an empty sipper tube). Goal tracking time (time spent near the sipper tube) was the dependent measure. Extinction was faster after training with 2% solution than with 32% solution. In Experiment 2, extinction was faster after training with 50% partial reinforcement than with continuous reinforcement. In both experiments, extinction was gradual and rats exhibited spontaneous recovery of goal-tracking behavior. Results are discussed in the context of evidence pointing to a dissociation of consummatory and instrumental behavior.

Frustration can be defined as an emotional state generated by the omission or devaluation in the quantity or quality of an expected appetitive reward. Thus reactivity to a reward is affected by prior experience with the different reinforcer values of that reward, a phenomenon known as incentive relativity, which can be studied by different paradigms to induce frustration. In this work we will focus in successive negative contrast (SNC), involving a downshift of the reward, and in the complete omission of the reward, a phenomenon known as extinction. Also, we will discuss the role of the neuroendocrine mechanisms involved in these processes. Specifically, we analyze the action of monoamines, adrenal and sexual hormones.

Posttraining administration of cholinergic drugs modulates the consolidation of memory processes in several learning tasks. We studied the effect of the administration of atropine (cholinergic antagonist, Experiment 1) and physostigmine (acetylcholinesterase inhibitor, Experiment 2) immediately after the first session of reward downshift, and immediately after the last preshift session (Experiment 3) on a consummatory successive negative contrast procedure. Animals were given access to a high-value reward (32% sucrose solution), and surprisingly shifted to a low-value reward (4% sucrose solution) in a second phase. The results indicate that atropine and physostigmine have no effects on contrast. The role of cholinergic neurotransmission in the memory of surprising reward changes is discussed.