Some determinants of changes in preference over time

Results of longitudinal studies suggest that the stability of preferences varies across individuals, although it is unclear what variables account for these differences. We extended this work by conducting periodic assessments of preference for leisure activities over 3 to 6 months with 10 adults with developmental disabilities. Although previous research has collectively shown that preferences identified via repeated assessment are highly variable, our results showed that preferences were relatively stable for the majority (80%) of participants. In an attempt to identify some environmental determinants of shifts in preference, we provided extended daily access to high-preference items (preference-weakening manipulation) and paired access to low-preference items with social and edible putative reinforcers during brief sessions (preference-strengthening manipulation). Preference assessments continued over the course of these manipulations with 2 participants. Results showed that changes in preference across time could be produced systematically and suggest that naturally occurring changes in establishing operations or conditioning histories contribute to temporal shifts in preference. Implications for preference assessments, reinforcer usage, and planned attempts to change preferences are discussed.     

Chronic stress impairs recall of extinction of conditioned fear

Chronic restraint stress produces retraction of apical dendrites of pyramidal neurons in medial prefrontal cortex. To begin to examine the functional significance of this dendritic reorganization, we assessed the effects of chronic restraint stress on a prefrontally mediated behavior, extinction of conditioned fear. After bar press training to obtain a baseline of activity against which to measure freezing, rats were either unstressed or stressed via placement in a plastic restrainer (3 h/day for 1 week). After an additional day of bar press training, rats underwent fear conditioning and extinction. Rats received five habituation trials to a 30-s tone (4.5 kHz, 80 db) followed by seven pairings of tone and footshock (500 ms, 0.5 mA). One hour later, rats received tone-alone extinction trials to criterion. The next day, rats received 15 additional extinction trials. Percent freezing was assessed during all phases of training. Stress did not significantly affect unconditioned responding to tone, acquisition of conditioned fear, or initial extinction, but significantly increased freezing on extinction day 2. Thus, consistent with the regressive dendritic changes seen in medial prefrontal cortex, one week of restraint stress specifically impaired recall of extinction, a pattern of deficits typical of animals with impaired medial prefrontal function.     

On the role of covarying functions in stimulus class formation and transfer of function

This experiment investigated whether directly trained covarying functions are necessary for stimulus class formation and transfer of function in humans. Initial class training was designed to establish two respondent-based stimulus classes by pairing two visual stimuli with shock and two other visual stimuli with no shock. Next, two operant discrimination functions were trained to one stimulus of each putative class. The no-shock group received the same training and testing in all phases, except no stimuli were ever paired with shock. The data indicated that skin conductance response conditioning did not occur for the shock groups or for the no-shock group. Tests showed transfer of the established discriminative functions, however, only for the shock groups, indicating the formation of two stimulus classes only for those participants who received respondent class training. The results suggest that transfer of function does not depend on first covarying the stimulus class functions.     

Expectancy-learning and evaluative learning in human classical conditioning: affective priming as an indirect and unobtrusive measure of conditioned stimulus valence

It has been argued that in classical conditioning two processes might be operative. First, one may learn that the conditioned stimulus (CS+) is a valid predictor for the occurrence of the biologically negative or positive event (US; expectancy-learning). Second, one may learn to perceive the conditioned stimulus itself as a negative or positive stimulus, depending on the valence of the event it has been associated with (evaluative learning). Until the present, however, both forms of learning have been investigated using rather different conditioning procedures. Using a differential aversive conditioning preparation with pictures of human faces as CSs and an electrocutaneous stimulus as US, we were able to demonstrate that both forms of learning can co-occur. Moreover, the extent of evaluative learning in this aversive conditioning procedure did not significantly differ from the amount of evaluative learning in an evaluative conditioning procedure with positive and negative adjectives as USs, which was administered to the same participants. In the present study evaluative learning was not only indexed by direct evaluative ratings, but we introduced affective priming as an indirect and unobtrusive, reaction time based measure of stimulus valence. Finally, imagery instructions during acquisition did not facilitate expectancy-learning nor evaluative learning.     

Lack of phenotype for LTP and fear conditioning learning in calpain 1 knock-out mice

We previously proposed the hypothesis that calpain activation played an important role in long-term potentiation (LTP) of synaptic transmission in hippocampus. Two forms of calpain are predominant in brain tissues, calpain 1 (mu-calpain), activated by micromolar calcium concentration and calpain 2 (m-calpain), activated by millimolar calcium concentration in vitro. In the present study, we tested the role of calpain 1 in LTP and in learning and memory using calpain 1 knock-out mice. Changes in learning and memory were assessed using both context and tone fear conditioning. No differences in freezing responses were observed between the knock-out and the wild-type animals during the acquisition phase of the training, eliminating the possibility that the knock-out animals could be differentially affected by the foot shock. Likewise, no differences in freezing responses elicited by either the context or the tone were observed during the retention phase. No differences in short-term potentiation (STP) or LTP were observed in hippocampal slices from the knock-out and matched wild-type mice. Several interpretations might explain these negative results. First, it is conceivable that calpain 2 plays a more dominant role in neurons, and that calpain 1 makes a minor contribution as opposed to its suspected predominant role in the hematopoietic system. Alternatively, it is conceivable that some as yet unknown compensatory mechanisms take effect, and that calpain 2 or another calpain isoform substitutes for the missing calpain 1.     

Social reinforcement of somatic versus psychological description of depressive events

The cultural construction of illness model explains the clinical observation that psychological correlates of depression such as sadness and worthlessness are frequently reported in Western cultures, whereas somatic or vegetative complaints are often associated with depression in non-Western cultures. This study employed Portnoy and Salzinger's (J. Gen. Psychol. 70 (1964) 311) verbal conditioning paradigm to determine whether social contingencies, in the form of verbal and non-verbal reinforcement contingent upon verbal responses, affect the reporting of experiences related to depressive events. Using an ABA design, this experimental study (N=36) demonstrated that one's reporting of experiences of hypothetical depressive events, via psychological, somatic, or neutral verbal response classes, can be conditioned by verbal and non-verbal positive social reinforcement, and therefore socially constructed through environmental contingencies. This suggests one pathway by which culture can shape symptom presentation. Implications for clinical interviewing and behavioral assessment, especially in the cross-cultural context, are discussed.     

Classical fear conditioning in the anxiety disorders: a meta-analysis

Fear conditioning represents the process by which a neutral stimulus comes to evoke fear following its repeated pairing with an aversive stimulus. Although fear conditioning has long been considered a central pathogenic mechanism in anxiety disorders, studies employing lab-based conditioning paradigms provide inconsistent support for this idea. A quantitative review of 20 such studies, representing fear-learning scores for 453 anxiety patients and 455 healthy controls, was conducted to verify the aggregated result of this literature and to assess the moderating influences of study characteristics. Results point to modest increases in both acquisition of fear learning and conditioned responding during extinction among anxiety patients. Importantly, these patient-control differences are not apparent when looking at discrimination studies alone and primarily emerge from studies employing simple, single-cue paradigms where only danger cues are presented and no inhibition of fear to safety cues is required.     

Cannabinoid agonists and antagonists modulate lithium-induced conditioned gaping in rats

Considerable evidence indicates that conditioned gaping in rats reflects nausea in this species that does not vomit. A series of experiments evaluated the potential of psychoactive cannabinoid agonists, Δ-9-THC and HU-210, and non-psychoactive cannabinoids, Cannabidiol (CBD) and its dimethylheptyl homolog (CBD-dmh), to interfere with the establishment and the expression of conditioned gaping in rats. All agents attenuated both the establishment and the expression of conditioned gaping. Furthermore, the CB1 antagonist, SR-141716, reversed the suppressive effect of HU-210 on conditioned gaping. Finally, SR-141716 potentiated lithium-induced conditioned gaping, suggesting that the endogenous cannabinoid system plays a role in the control of nausea.     

Variable-ratio conditioning history produces high- and low-rate fixed-interval performance in rats

Four rats were exposed to an A-B-A-B series of 30 sessions each of variable-ratio 20 (A) and fixed-interval 30-s (B) schedules. Four other rats received 120 sessions of fixed-interval 30 s. The rats with a history of variable-ratio responding subsequently showed primarily high or low response-rate patterns on the fixed-interval schedule without evidence of classical scalloping (i.e., increased rates of responding throughout the interreinforcement interval), except infrequently in 1 rat. The rats exposed to only the fixed-interval 30-s schedule displayed the expected sequence of scalloping giving way to lower rate break-run or simply low-rate responding over time. This experiment shows that when naive rats are exposed to even a simple history of reinforcement (in this case, a variable-ratio 20), their subsequent fixed-interval performance is very different from comparable performance in naive rats, and might be said to be more similar to the responding of adult humans. The argument is made that care should be taken in comparing the fixed-interval performance of humans and nonhumans because humans have a complex history of reinforcement, whereas laboratory nonhumans are typically naive.     

Signaled avoidance in the eye withdrawal reflex of the green crab

Learning in a signaled avoidance procedure was studied in the eye withdrawal reflex of the green crab, Carcinus maenas. A puff of air to the eye, which causes eye retraction, was used as the unconditioned stimulus (US). A mild vibration on the carapace, which has no effect on untrained animals, was used as a warning (conditioned) stimulus (CS). Eye withdrawal during the CS led to the omission of the otherwise scheduled US. Acquisition was rapid, reaching about 75% avoidance after 30 trials. Extinction occurred slowly over the course of 40 CS-only trials. Yoked controls did not perform as well. The behavior of experimental animals in the avoidance procedure was found to be essentially identical to the performance of animals subjected to a classical conditioning paradigm in which CS responses had no effect on US presentation. Additional groups of animals were subjected to experiments in which (a) avoidance conditioning (60 trials) was followed by classical conditioning (40 trials) or (b) classical conditioning was followed by avoidance. The behavior of these groups was, again, essentially identical. The results suggest that there may be an underlying Pavlovian mechanism for the learned response, although the contribution of an operant process is not excluded. The results expand the range of invertebrate animals in which fundamental conditioning phenomena can be demonstrated, and may provide a neuronal model for learning in a signaled avoidance procedure.