Associative transfer and stimulus selection in classical conditioning of the rabbit's nictitating membrane response to serial compound CSs

Four experiments were conducted to determine whether in conditioning to a serial compound, CS1-CS2-UCS, there are (a) associative mechanisms operating to extend conditioning beyond the bounds of a CS-UCS contiguity gradient and (b) stimulus selection processes acting to attenuate the potency of CS-UCS contiguity. In Experiments 1 and 2, the CS2-UCS interval was held at .35 sec while the CS1-UCS interval was varied across groups from .75 to 2.75 sec. CS1 test trials revealed substantial CR acquisition at all CS1-UCS intervals. Moreover, Experiment 2 indicated that when the contribution of cross-modal generalization from CS2 to CS1 was factored out, there still remained a substantial level of conditioning, which Experiment 3 indicated was attributable to an associative mechanism like higher-order or sensory conditioning. The observation of CR acquisition at CS1-UCS intervals of 4.75, 8.75, and 18.75 sec in Experiment 4 suggested that serial compound training yields conditioning to CSs located well beyond the single CS contiguity gradient for the rabbit's nictitating membrane response. Experiments 1 and 2 also indicated the presence of stimulus selection processes because, at the shorter CS1-UCS intervals (.75 and 1.25 sec), the levels of test-trial responding to CS2 fell below those observed to the less contiguous CS1.     

Pavlovian conditioning with proximal stimuli

This experiment was conducted with the objective of demonstrating that the effective stimuli in Pavlovian Conditioning are not environmental stimuli but internal physiological processes elicited by environmental input (proximal stimuli). In order to achieve the objective, afterimages in color vision were used: looking at a diffuse lightened circle after seeing a red circle yields an image of a green circle. A differential conditioning paradigm with two sequential compounds was run. In one group (G⁺B^?: n₁=10), a red circle followed by a green circle was paired with shock, whereas a red circle followed by a blue circle remained unpaired. A second group (G^?B⁺: n₂=10) received red-blue paired trials and unpaired red-green trials. Immediately after that training, subjects were tested with a new, never trained sequential compound: a red circle followed by a diffuse lightened circle. Furthermore, they were tested with the already trained compounds. Taking the environmental point of view, the never trained stimulus should elicit an orienting response lying inbetween the excitatory reaction to the paired stimulus and the inhibitory reaction to the unpaired stimulus. From the proximal point of view, the diffuse light should elicit an excitatory reaction in group G⁺B^? and an inhibitory reaction in group G^?B⁺. Electrodermal conditioned anticipatory and omission responses were measured. The results supported the proximal hypothesis. Hence, defining input in environmental terms may be the wrong way. Instead, in conceptualizing the stimulus in conditioning, the following should be considered: the processing organism itself is creating the effective stimuli.     

Stimulus and temporal cues in classical conditioning

In 2 experiments, separate groups of rats were given stimulus conditioning, temporal conditioning, untreated control and (in Experiment 2) learned irrelevance control procedures, followed by a compound with both stimulus and temporal cues. Stimulus conditioning consisted of a random 15-s duration conditioned stimulus (CS) followed by food; temporal conditioning consisted of food-food intervals of fixed 90 s (Experiment 1) or fixed 75 + random 15 s (Experiment 2). The stimulus group abruptly increased responding after CS onset, and the temporal group gradually increased responding over the food-food interval. When the food-food interval was fixed 90 s, the temporal cue exerted stronger control in the compound, whereas when the food-food interval was fixed 75 + random 15 s, the stimulus cue exerted stronger control. The strength of conditioning, temporal gradients of responding, and cue competition effects appear to reflect simultaneous timing of multiple intervals.     

UCS revaluation and conditioning models of acquired fears

Recent developments in the study of Pavlovian conditioning in humans have suggested that the strength of a conditioned response (CR) is affected not only by associative processes that link the conditioned stimulus (CS) with the unconditioned stimulus (UCS), but also by processes which lead the individual to revalue the UCS itself. These latter processes can frequently lead to dramatic changes in CR strength independently of any experience with the CS-UCS contingency. This paper discusses a number of ways in which UCS revaluation can be effected in human subjects, and discusses their implications for conditioning models of acquired fears.     

Acquisition and discrimination of appetitively and aversively conditioned heart rate responses in rhesus monkeys

This report describes the acquisition of a conditional heart rate response to both classical aversive and appetitive conditioning in eight rhesus monkeys(Macaca mulatta). The behavioral paradigm consisted of two discrete one minute tones followed by the corresponding unconditional stimuli,i.e., electric shock or Purina monkey chow. A third tone followed by neither food nor shock served as a discriminative stimulus (DS). The conditional heart rate responses developed in two phases. The first phase was characterized by similar responses to both conditional stimuli and the DS. Control heart rate rose during this period. During the second phase, control heart rate decreased (five animals), the DS response disappeared, and different conditional heart rate patterns to food and shock emerged. The existence of distinct conditional response patterns indicates probable differences in the autonomic nervous regulation of the heart during aversive and appetitive conditioning.     

Two models for psychophysical judgment: Scale invariance with changes in stimulus range

Two scaling models are investigated, each of which includes parameters which reflect variation in the use of judgmental categories. In one model, response categories are represented as successive intervals on an underlying sensory continuum. In the other, they are represented as points. Psychophysical scales based upon either of these models are invariant up to a linear transformation when the range of stimulus magnitudes is manipulated. Successive intervals scale values obtained from magnitude estimates are also linearly invariant with changes in stimulus range. Similarities in the proximity functions for the two judgment models are examined.     

Changing the rate and hippocampal dependence of trace eyeblink conditioning: slow learning enhances survival of new neurons

Trace eyeblink conditioning in which a conditioned stimulus and unconditioned stimulus are separated by a gap, is hippocampal dependent and can rescue new neurons in the adult dentate gyrus from death (e.g., Beylin et al., 2001; Gould et al., 1999). Tasks requiring more training trials for reliable expression of the conditioned response are most effective in enhancing survival of neurons (Waddell & Shors, 2008). To dissociate hippocampal dependence from acquisition rate, we facilitated hippocampal-dependent trace eyeblink conditioning in two ways: a shorter trace interval and signaling the intertrial interval with a post-US cue. Trace conditioning with a shorter trace interval (250ms) requires an intact hippocampus, and acquisition is faster relative to rats trained with a 500ms trace interval (e.g., Weiss et al., 1999). Using excitotoxic hippocampal lesions, we confirmed that eyeblink conditioning with the 250 or 500ms trace interval is hippocampal dependent. However, training with the post-US cue was not hippocampal dependent. The majority of lesion rats in this condition reached criterion of conditioned responding. To determine whether hippocampal dependence is sufficient to rescue adult-generated neurons in the dentate gyrus, rats were injected with BrdU and trained in one of the three trace eyeblink arrangements one week later. Of these training procedures, only the 500ms trace interval enhanced survival of new cells; acquisition of this task proceeded slowly relative to the 250ms and post-US cue conditions. These data demonstrate that rate of acquisition and not hippocampal dependence determines the impact of learning on adult neurogenesis.     

Effects of awareness and motor involvement on autonomic conditioning in chronic schizophrenics

Chronic schizophrenics under neuroleptic medication were compared with normals, matched for age and education, in two conditioning experiments. Both experiments employed a differential paradigm with long conditional stimulus-unconditional stimulus (CS-UCS) intervals. Skin resistance (SR) and finger pulse or heart rate (HR) were recorded. In Experiment 1, half of the subjects were trained in categorizing the to-be conditional stimuli. In a further step, half of the subjects were informed about the CS-UCS contingency. The UCS was an electric shock. Information improved discrimination of SR responses only of normals, and discrimination training had no effect at all on autonomic responses. In Experiment 2, only electrodermal responders were included. Each subject was tested in two sessions, using a loud tone as UCS in one, and a reaction time signal in the other. Again, half of the subjects were informed about the contingency. Information improved discrimination of SR responses of both diagnostic groups. Decelerations of HR following CS onset showed informed schizophrenics to discriminate better with the loud tone UCS than with the reaction time signal. General autonomic responsivity seems to determine not only discriminative conditioning of schizophrenic patients but also the reports of awareness and the effects of manipulations of awareness. However, when subjects are matched for electrodermal responsivity and are equivalent in terms of SR response conditioning, patients react to information about the contingency with enhanced HR deceleration to the CSs, possibly reflecting a heightened sensitivity of the cardiovascular system.     

The content of compound conditioning

In three experiments using Pavlovian conditioning of magazine approach, rats were trained with a compound stimulus, AB, and were concurrently trained with stimulus B on its own. The reinforcement rate of B, rB, was either 1/2, 2/3, or 2/5 of rAB. After extended training, the conditioning strength of A was assessed using probe trials in which A was presented alone. Responding during A was compared with that during AB, B, and a third stimulus, C, for which rC = rAB - rB. In each experiment, the rats' response rate during A was almost identical to that during C (and during B, when rB = 1/2rAB). This suggests that, during AB conditioning, the rats had learned about rA as being equal to [rAB - rB], and implies that the content of their learning was a linear function of r. The findings provide strong support for rate-based models of conditioning (e.g., Gallistel & Gibbon, 2000). They are also consistent with the associative account of learning defined in the Rescorla and Wagner (1972) model, but only if the learning rate during reinforcement equals that during nonreinforcement.     

Optimal Stimulus Parameters In Avoidance Conditioning Of Inbred Strains Of Mice

Highly inbred mice of fast and slow conditioning strains (N = 228) were trained in avoidance conditioning and 180 were continued to extinction. The principal goal was to determine optimal stimulus conditions for fast learning, small variances, and maximal strain differences. Conclusions are: a) the major effect of change in CS-US and intertrial intervals occurs, respectively, for intervals less than 3 sec. and 60 sec.; b) change in amperage (150-400 pa) or voltage (250-400 v.) caused no significant differences; c) strain differences in extinction occurred after nearly optimal conditioning, and d) strain differences were altered considerably as stimulus conditions became optimal. Optimal levels of parameters are: 3 sec. CS-US interval, 120-sec. intertrial interval, and 400 v. (220 K ohm impedance circuit).     

Concomitant Pavlovian conditioning of heart rate and leg flexion responses in the rat

Pavlovian conditioning was studied in male Fischer 344 rats using tones as the conditioned stimulus (CS) and footshock as the unconditioned stimulus (UCS). Different groups of animals received (a) contiguous CS-UCS pairings with a 0.5 sec CS, (b) contiguous CS-UCS pairings with a 4.0 sec CS, or (c) a random sequence of noncontiguous tones and shocks using either a 0.5 sec or a 4.0 sec CS. Heart rate (HR) and leg flexion (LF) responses were recorded. Leg flexion conditioning occurred only in the 0.5 sec contiguous group. Decelerative HR CRs occurred only in the 4.0 sec contiguous group. Accelerative HR changes occurred in the other two groups but were significantly greater in the 0.5 sec contiguous group. These results are similar to but not identical to those obtained during eyeblink or nictitating membrane conditioning in rabbits, and suggest that the topography of the Pavlovian HR CR is dependent on the simultaneous occurrence of other classically conditioned responses.     

Effects of psychoactive agents on the conditioning of the microcirculation in the rat

Normal vascular tonus and permeability of the microcirculatory network is mediated by catecholamines, histamine and serotonin. Following electroconvulsive treatment (ECT), histamine did not provoke local vasodilatation. The inhibition was further potentiated by drugs which prevent the enzymatic or physical inactivarion of free catecholamines and by locally administered norepinephrine. This system became conditioned to a visual stimulus when rats were presented in six consecutive occasions with the visual stimulus preceedingly coincided with the administration of ECT. In the conditioned rats, light alone inhibited the histamine-induced vasodilatation, and this inhibition was potentiated by sub-effective doses of injected norepinephrine and by drugs which prevent the enzymatic or physical inactivation of catecholamines. Sedatives, antidepressants, MAO inhibitors and the chlorpromazine-type agents as well as the psychotomimetic drugs characteristically influence either the acquisition or the extinction of the conditioned reflex. Results suggest that the obtained conditioning of the microcirculatory functions are mediated by tie centrally released catecholamines.     

The strength of the orienting response during blocking

In two experiments rats initially received appetitive Pavlovian conditioning with an auditory conditioned stimulus (CS). Subsequently this stimulus was presented in compound with a novel light and paired with the same appetitive reinforcer. In keeping with the outcome of many such experiments on blocking, there was very little evidence of appetitive conditioning during subsequent independent presentations of the light. Of main concern in the present experiments, however, was the influence of this training on the strength of the orienting response directed towards the light. When the light was first presented it elicited a strong orienting response. The strength of this response declined rapidly when the light was presented in compound with the previously trained auditory CS but more slowly when it was paired with the reinforcer either by itself or in conjunction with an initially neutral auditory stimulus. It is suggested that the extent to which the events following the light are accurately predicted determines the strength of orientation towards this stimulus.     

Neuronal unit activity in the abducens nucleus during classical conditioning of the nictitating membrane response in the rabbit (Oryctolagus cuniculus).

Neuronal unit activity was recorded from the abducens (6th nerve) nucleus, the "final common path," during classical conditioning of the nictitating membrane (NM) response in the rabbit, with the use of a tone conditioned stimulus, an air puff unconditioned stimulus (UCS), 250-msec interstimulus interval, and 60-sec intertrial interval. Animals were given 2 days of conditioning training (104 trials in eight blocks per day) and 1 day of extinction. Control animals were given comparable periods of stimulus presentations, explicitly unpaired. Activity of small clusters of units--"multiple unit" recording--was compared with the amplitude-time course of the NM response. Between-blocks comparisons of neural and behavioral responses indicated an essentially perfect correlation during acquisition of the conditioned response (Day 1, r = .99; Day 2, r = .98) and a slightly lower correlation during extinction (r = .93) for the conditioning animals. Within-blocks comparisons indicated a close correspondence between the histograms of unit activity and the amplitude-time course of the NM response for the conditioning animals in all phase of training and for the control animals in the UCS trial blocks.     

Cardiac arrhythmias in the monkey during classically conditioned fear and excitement

Rhesus monkeys(Macaca mulatto) were tested in classic aversive and appetitive conditioning paradigms following complete coronary artery occlusion (CO) to test the hypothesis that “emotional stress” induces ventricular arrhythmias. Findings were based upon conditioning trials conducted for one or more weeks after occlusion in 13 animals. When all data from each animal for the week following CO were considered, there was no demonstrable tendency for arrhythmias to increase during “fear” conditioned to unavoidable electric shock or during “excitement” in anticipation of food. However, selected trials from six monkeys did reveal instances when changes in the frequency of occurrence of arrhythmias were coupled with behavioral conditioning. While analysis of these trials did not reveal any simple relationship between emotional stress and the development of ventricular arrhythmias after myocardial infarction, certain of the behavioral situations may be more potentially arrhythmogenic than others. For these selected trials, with respect to control, the number of arrhythmias may have increased or decreased upon presentation of the conditional stimulus; the exact response appears to depend upon the immediate physiologic status of the animal as well as on the behavioral condition. “More stressful” situations, such as aversive conditioning, are not necessarily associated with greater numbers of arrhythmias than were “less stressful” situations, such as appetitive conditioning. Arrhythmias appear to occur more frequently when an animal’s heart rate is within a given range; this may reflect underlying cardiac sympathetic and parasympathetic nerve activity.     

"Pseudoconditioned" jaw movements of the rabbit reflect associations conditioned to contextual background cues.

Two experiments employing 180 rabbits and involving tone conditioned stimuli (CSs) and intraoral water unconditioned stimuli (UCSs) investigated pseudoconditioning of jaw movement. CS-alone, UCS-alone, paired CS-UCS, and four explicitly unpaired CS-UCS treatments were compared to no stimulus presentation. UCS-alone presentations were sufficient to produce pseudo-CR (conditioned response) acquisition. Pseudo-CRs were retained and gradually extinguished over 30 days of CS-alone presentations. Random sequences of unpaired CSc and UCSs produced higher pseudo-CR frequencies than fixed sequences. A pseudo-CR partial reinforcement extinction effect was observed. Background extinction, that is, simply confining the subject in the experimental environment without stimulation, was effective in extinguishing both pseudo-CRs and CRs. Pseudo-CR results could not be attributed to CS-UCS trace conditioning, sensory preconditioning, second-order conditioning, or intra-analyzer conditioning. Results indicate that the associative mechanisms underlying pseudoconditioning phenomena involve conditioning of associations to contextual background (apparatus, trace, temporal, and sequential aftereffect) cues by UCS-alone and unpaired UCS presentations.     

Cardiac activity in a simple reaction time task

Twenty subjects, half having high extraversion and the other half having low extraversion scores on the Eysenck Personality Inventory, performed a simple reaction time task in which the warning stimulus was a light and the imperative stimulus was a tone. Beat-by-beat analysis showed significantly different response patterns between the two groups in time of onset, magnitude, and duration of the cardiac responses. The extraversion measures were found to relate to central-nervous-system-strength types and to indicate relative strength of central inhibitory processes, as reflected by cardiac activity.     

Cognitive factors in the concurrent differential conditioning of eyelid and skin conductance responses

The objectives of the study were to (1) determine if differential conditioning of eyelid responses ocurs only among subjects who accurately report knowledge of the CS-UCS relations, (2) assess whether differentially conditioned eyelid responding occurs only after the initial accurate interrial report, and (3) explore characteristics of differentially conditioned skin conductance responses (SCRs) obtained in an eyelid conditioning paradigm, with special attention to whether eyelid and SCR conditioning are similarly related to the subjects’ knowledge of the stimulus relations. Fifty-one male subjects were administered 80 differential eyelid conditioning trials, the CSs consisting of the 1,200-msec illumination of slides containing either grammatically correct or grammatically incorrect phrases. Significant differential SCR and eyelid conditioning was obtained for both V- and C-form eyelid responders, but only for subjects who accurately reported the CS-UCS relations. The initial appearance of SCR and eyelid differentiation was related within the trial sequence to subjects’ recognition of the stimulus relations. Eyelid and SCR conditioning were related similarly to knowledge of the stimulus relations, the exception being that subjects who recognized the stimulus relations late in the trial sequence did not develop reliable eyelid differentiation.     

Delayed escape from light by the albino rat

Two albino rats were trained to terminate an aversive light for 1 min by pressing a bar. After 19 hr of conditioning they were exposed to successive delays of 1, 2, 5, and 10 sec imposed between occurrence of the escape response and light termination. No stimulus change accompanied the delay interval, and any additional responses made at this time reset the delay timer. For both rats the relative frequency of escape responses with very long latencies increased as the delay interval increased. The modal escape latency, however, remained essentially unchanged for all delay values of greater than 1 sec. “Superstitious” responding was observed during the delay interval.     

Concurrent performances: a baseline for the study of conditioned anxiety

Three rats were trained to lever press on concurrent random interval 2-min random interval 2-min schedules of milk reinforcement. With a 5-sec changeover delay, relative response rate matched the relative reinforcement duration associated with each lever. A stimulus, during which unavoidable shocks occurred at random intervals, was superimposed on this concurrent baseline, and shifts in preference were found. However, data from this procedure were ambiguous, apparently confounded by shock-elicited response bursts. Termination of the shocks during the stimulus resulted in a rapid recovery of matching, which was preceded by a brief facilitation of responding on the less-preferred lever. The procedure was then changed to a conventional conditioned anxiety paradigm with a variable duration pre-shock stimulus. A marked shift in relative response rate towards the preferred lever was found in all three rats; that is, responding on the preferred lever was far less suppressed during the pre-shock stimulus than responding on the less-preferred lever.