Blocking of human causal learning involves learned changes in stimulus processing

Several theories of associative learning propose that blocking reflects changes in the processing devoted to learning about cues. The results of the only direct test of this suggestion in human learning (Kruschke & Blair, 2000) could equally well be explained in terms of, among others, interference in learning or memory. The present study tested this suggestion in a situation in which processing-change and interference accounts predict opposing results. Results support the idea that blocking in human learning can reflect a change in processing of the cues involved.     

Blocking of human causal learning involves learned changes in stimulus processing

Several theories of associative learning propose that blocking reflects changes in the processing devoted to learning about cues. The results of the only direct test of this suggestion in human learning (Kruschke & Blair, 2000) could equally well be explained in terms of, among others, interference in learning or memory. The present study tested this suggestion in a situation in which processing-change and interference accounts predict opposing results. Results support the idea that blocking in human learning can reflect a change in processing of the cues involved.     

Perceptual learning in flavor aversion: evidence for learned changes in stimulus effectiveness

Rats were exposed to the compound flavors AX and BX, presented in alternation, and to CX on a separate block of trials. Generalization to BX after aversion conditioning with AX was less than to CX. An equivalent effect was found when the nature of the common element was changed after preexposure but not when the common element was omitted during preexposure, during conditioning and test, or both. Rats conditioned with X alone again showed less aversion to BX than to CX; similarly, rats conditioned with a novel flavor (Y) showed less aversion to BY than to CY. These effects support the proposal that intermixed preexposure to AX and BX enhances the perceptual effectiveness of their unique features, A and B.     

The influence of irrelevant stimulus changes on stimulus and response repetition effects

In this study the influence of irrelevant stimulus changes from one trial to another in a serial reaction time task was investigated. Two experiments were performed in which subjects were required to respond to stimulus colour. Four colours were mapped on two response keys, so that colour and response repetition effects could be dissociated. In Experiment 1, the irrelevant stimulus dimension was location and in Experiment 2 it was shape. Both experiments were performed with a short and a long response-stimulus interval (RSI)-condition. In both experiments, the irrelevant dimension influenced the response repetition effect but not the colour repetition effect. In the reaction times, a response alternation effect was observed only when the irrelevant location of the stimulus changed in the long RSI-condition. The error rates showed a response alternation benefit for both irrelevant dimensions, in the short and the long RSI-condition. The benefit for response alternations is explained in terms of a response bias towards change that is triggered by a changing stimulus feature. We assume that the response bias is stronger for location than for colour and that accuracy is more sensitive to this bias than response latencies.     

Effects of a brief stimulus accompanying reinforcement on instrumental responding in pigeons

The responding of pigeons on a variable interval schedule of reinforcement was investigated in four experiments. In some conditions in each experiment reinforced keypecks were accompanied by a brief (0.5-sec) flash of the houselight. This procedure resulted in a low rate of response in comparison with that found in conditions when response-contingent light flashes occurred uncorrelated with reinforcement (Experiments 1 and 2) or when no light flash was presented (Experiment 3). Experiment 4 allowed a comparison between the effects of a signal accompanying the reinforced response and one accompanying the delivery of “free” food. Signaling the delivery of earned food produced a lower rate of response than did signaling the delivery of free food. The role of stimulus-reinforcer and response-reinforcer associations in producing these effects is discussed.     

Assessments of changes in the effective salience of stimulus elements as a result of stimulus preexposure

Rats received exposure to 3 flavor compounds, AX and BX, presented in alternation, and CX, presented on a separate block of trials. The hypothesis that this treatment would leave B effectively more salient than C was tested in 3 ways. Experiment 1 showed that the unconditioned response evoked by B was stronger than that evoked by C. Experiment 2 showed that B was more effective than C when used as a reinforcer in a sensory preconditioning procedure. Experiment 3 showed that B was learned about more readily than C as a conditioned stimulus in flavor aversion conditioning. Alternating preexposure to 2 similar stimuli may protect their distinctive features from the loss of salience normally produced by nonreinforced exposure to a stimulus.     

The effect of a masked stimulus on the response to the masking stimulus

Four experiments are reported in which the subjects had to respond to a target that masked a preceding prime via metacontrast masking. In one part of Experiment l, the subjects discriminated the target's shape (square or diamond) by a motor-choice reaction, and in another part they had to respond with a simple reaction. The prime was neutral (circular) with respect to the target's shape. The data showed a facilitation effect. In both tasks the reaction time was reduced by the masked prime. However, the reduction was more pronounced with simple reaction than with choice reaction. In the other experiments, additional primes were used with the same angular shapes as the targets. In Experiments 2 and 3, after discriminating the target's shape by a choice reaction, the subjects had to judge the prime's shape in a signal-detection task. While neither the d' value for discriminating the angular primes from the circular ones (Exp. 2) nor the d' value for distinguishing between the angular primes (Exp. 3) was different from zero, the choice-reaction data showed a congruency effect. With a congruent prime (i.e., a prime that had the same shape as the target), the reaction times were reduced. With an incongruent prime, the reaction times grew. In Experiment 4 the errors were investigated. The facilitation effect was present in the RT, but not in the number of errors, whereas the congruency effect was present in the number, but not in the RT of errors.While the facilitation effect can be attributed either to an unspecific activation by the masked prime or to an influence of the prime on attentional processes, the congruency effect can be explained by the assumption that the masked prime directly activates the specific response, which corresponds to the prime's shape.     

Rate of response as a visual social stimulus

In Exp. 1, a high rate of responding (chain pulling) of a stimulus monkey was established as a visual positive discriminative stimulus for the operant behavior (bar pressing) of an observer monkey. The terminal performance of the observer under conditions in which a high rate of response of the stimulus monkey alternated in a variable temporal arrangement with a zero rate of response of the stimulus monkey (negative discriminative stimulus) was essentially the same as when nonbehavioral stimuli are correlated with the availability of reinforcement. By manipulating the schedule of reinforcement to change the rate of responding of the stimulus subject without changing its rate of reinforcement, Exp. 2 showed that the effective behavioral stimulus for the observer was the rate of chain pulling by the stimulus subject. A novel intermediate rate of responding by the stimulus monkey resulted in an intermediate rate (generalization) on the part of the observer during an extinction test. These experiments demonstrated that the rate of responding of one organism can function as a discriminative stimulus to control the rate of responding of another organism; and that the rate of responding is similar to other physical stimuli in terms of discrimination and generalization.     

Avoidance response rates during a pre-food stimulus in monkeys

Free-operant avoidance responding was maintained by a shock avoidance schedule in three monkeys. The frequency of avoidance responses during a stimulus terminated by response-independent food pellet presentation was dependent upon the method of pellet delivery. Avoidance rates were relatively increased when food retrieval responses followed pellet delivery. Avoidance rates were decreased when retrieval responses preceded pellet delivery.     

Learned changes in stimulus representations (a personal history)

Almost 40 years ago I began what turned out to be a programme of research on the way in which experience can change the effectiveness of the events used as stimuli in standard associative learning procedures. In this personal history I will describe my early (failed) attempts to find evidence for the acquired distinctiveness of cues, and my conclusion that experience tends to reduce, not enhance the associability of stimuli. I then go on to describe my attempts to square this conclusion with the stubborn empirical fact that, in some circumstances, pretraining with (or preexposure to) stimuli, can facilitate subsequent discrimination between them. I describe experiments (conducted mostly with rats as the subjects) showing how some of these effects can be explained in associative terms. Others, however, seemed to demand an explanation in terms of a new learning process that modulates the effective salience of stimuli. I go on to describe attempts to specify the nature of this process, and (bringing the story up to date) to describe recent experiments investigating the effects of salience modulation in human perceptual learning.     

Comparing stimulus preference and response force in a conjugate preparation

Researchers used a conjugate preparation to evaluate how response force changed based on participants' preferences for visual stimuli. First, researchers used a verbal preference assessment to evaluate each participant's preference for viewing for five object categories of visual stimuli; this process identified high preference (HP) stimuli and low preference stimuli for most participants. Thereafter, researchers exposed each participant to the five stimulus categories in a randomized order while using a force dynamometer to measure their response force to increase visual clarity of each stimulus. Results indicate the majority of participants' HP stimuli corresponded to the stimulus category for which they exerted the highest mean force. These preliminary findings suggest conjugate preparations involving response force may provide another option for measuring the relative value of some stimulus events.     

Potentiation of the acoustic startle response by a conditioned stimulus paired with acoustic startle stimulus in rats

The hypothesis that the standard acoustic startle habituation paradigm contains the elements of Pavlovian fear conditioning was tested. In a potentiated startle response paradigm, a startle stimulus and a light conditioned stimulus (CS) were paired. A startle stimulus then was tested alone or following the CS. Freezing behavior was measured to index conditioned fear. The startle response was potentiated on CS trials, and rats froze more in CS than in non-CS periods. In Experiment 1, response to a previously habituated, weak startle stimulus was potentiated. In Experiment 2, response to the same stimulus used as the unconditioned stimulus (US) in training was potentiated. This CS-potentiated response retarded the course of response decrements over training sessions as compared with an explictly unpaired control group. Conditioned fear is a standard feature of this habituation paradigm, serves to potentiate the startle response, and provides an associative dimension lacking in the habituation process per se.     

Effect of preconditioning unconditioned stimulus experience on learned taste aversions.

One taste-aversion study using male Long-Evans rats in which ethanol was the unconditioned stimulus (UCS) and six studies in which lithium chloride (LiCl) was the UCS demonstrate that (a) exposure to the UCS prior to conditioning retards subsequent acquisition of learned taste aversions; (b) a single preconditioning UCS exposure is sufficient to attenuate conditioning; (c) the preconditioning UCS exposure must occur within a limited period prior to conditioning to attenuate learning; (d) repeated conditioning trials will override the effect of prior exposure to the UCS; (e) tolerance to the UCS is not a necessary condition for the attenuation effect to occur; (f) pairing the preconditioning UCS with a novel flavor other than the CS does not remove the preexposure effect, although it may reduce its magnitude; and (g) the degree of disruption is a positive function of preconditioning UCS dosage and an inverse function of conditioning UCS dosage.