A stimulus-compounding assay of conditioned inhibition and excitation

Rats were trained in an operant discrimination involving two panel lights, A and B. Group CI (Conditioned Inhibition) was trained to lever press in the presence of A (S+), and not in the presence of the compound AB (S?). The opposite contingencies held in Group CE (Conditioned Excitation). Response rates were higher in Group CE than in Group CI, especially to the positive stimulus. However, the groups acquired the discrimination at comparable rates. In a separate phase of the experiment, a tone (C) was separately trained in the absence of the lights to control responding. All seven possible combinations of A, B, and C were then presented randomly during extinction. The inhibitory effect of B on C in Group CI was comparable in magnitude to the excitatory effect of B on C in Group CE. B's effect on C was not altered by the presence or absence of A, regardless of whether A was inhibitory or excitatory. The symmetry between CI and CE revealed by the stimulus-compounding test may be partly artifactual.     

Inhibition as a "slave" process: deactivation of conditioned inhibition through extinction of conditioned excitation

Rats were used in a conditioned-suppression paradigm to investigate why a conditioned inhibition (CS-) does not extinguish when presented alone. Experiment 1 assessed the role of blocking by excitatory contextual cues and/or an evoked representation of the conditioned excitor (CS+), which had been nonreinforced in compound with the CS-. When the CS+ and context were extinguished prior to presentations of the CS- alone, the CS- showed a retardation effect, evidently reflecting latent inhibition, because no inhibition was detected in controls for which presentations of the CS- alone had been omitted. Experiment 2 showed that the loss of conditioned inhibition (CI) was due to excitatory extinction and not to time since conditioning. Furthermore, when excitation was reconditioned to the extinguished CS+ (Experiment 1), or to a novel CS in the same context (Experiment 2), CI was restored. Two other experiments evaluated whether the maintenance of CI depended upon excitation that was generic in form or associatively tied to the training context. They showed no loss of CI when groups received CS+ extinction in that context, along with concomitant presentations in a different context of the US by itself, for a novel CS, or correlated either positively or negatively with the original CS+. Collectively, the findings argue that CI is a "slave" to excitation, for when excitation is extinguished, CI is deactivated; and yet when excitation is reconditioned to the original or a new CS+ in the same or a different context, CI is restored.     

Time of day-dependent latent inhibition of conditioned taste aversions in rats

We have determined that the temporal context of drinking can modulate latent inhibition of learned saline aversions in Wistar rats by changing the time of day of drinking of the preexposure and conditioning phases. Latent inhibition was absent in the group preexposed and conditioned to saline at different times of the day, but not in the group that was preexposed and conditioned at the same time of day. The results confirm a previous report that the time of day can modulate taste aversion learning independently of other environmental cues. It is proposed that the features and duration of the habituation training to the temporal contexts used may be critical for time-dependent latent inhibition to appear.     

Blockade of GABAA receptors in the interpositus nucleus modulates expression of conditioned excitation but not conditioned inhibition of the eyeblink response

The cerebellum and related brainstem structures are essential for excitatory eyeblink conditioning. Recent evidence indicates that the cerebellar interpositus and lateral pontine nuclei may also play critical roles in conditioned inhibition (CI) of the eyeblink response. The current study examined the role of GABAergic inhibition of the interpositus nucleus in retention of CI. Male Long-Evans rats were implanted with a cannula positioned just above or in the anterior interpositus nucleus before training. The rats were trained with two different tones and a light as conditioned stimuli, and a periorbital shock as the unconditioned stimulus. CI training consisted of four phases: 1) excitatory conditioning (8 kHz tone paired with shock); 2) feature-negative discrimination (2 kHz tone paired with shock or 2 kHz tone concurrent with light); 3) summation test (8 kHz tone or 8 kHz tone concurrent with light); and 4) retardation test (light paired with shock) After reaching a criterion level of performance on the feature-negative discrimination (40% discrimination), 0.5 μl picrotoxin (a GABA_A receptor antagonist) was infused at one of four concentrations, each concentration infused during separte test sessions. Picrotoxin transiently impaired conditioned responses during trials with the excitatory stimulus (tone) in a dose-dependent manner, but did not significantly impact responding to the inhibitory compound stimulus (tone-light). The results suggest that expression of conditioned inhibition of the eyeblink conditioned response does not require GABAergic inhibition of neurons in the anterior interpositus nucleus.     

Distractor effect on the latent inhibition of conditioned flavor aversion in rats

Using a conditioned flavor aversion procedure with rats as subjects, the effect of the addition of a distractor stimulus on the magnitude of the latent inhibition effect was examined. Experiment 1 showed that latent inhibition to vinegar was attenuated by the addition of sucrose during preexposure. On the other hand, sucrose added during conditioning to vinegar did not attenuate latent inhibition. It was also found that the degree of latent inhibition to the vinegar-sucrose compound solution was less when vinegar alone was preexposed (i.e., when sucrose was added only during conditioning) than when the compound solution was preexposed (i.e., when sucrose was added both during preexposure and during conditioning). Experiment 2 gave similar results but with sucrose assigned as the target flavor and vinegar as the distractor. These findings are in full agreement with the generalization decrement account of latent inhibition.     

CAMKII inhibition in the parabrachial nuclei elicits conditioned taste aversion in rats

The conditioned taste aversion (CTA) paradigm was used to assess the role of Ca(2+)/calmodulin-dependent protein kinase (CAMKII) in associative learning. KN62, a specific inhibitor of CAMKII, was injected into the parabrachial nuclei (PBN) either immediately after saccharin drinking (CS) or after saccharin drinking and i.p. injection of LiCl (US). Injection of KN62 into the PBN after saccharin drinking elicited clear CTA (Exp. 1). This effect was dosage-dependent and site-specific (Exp. 2). The results are discussed in relation with an earlier report showing that CTA acquisition is disrupted by injection of Ca(2+)/phospholipid-dependent protein kinase (PKC) inhibitor chelerythrine into the PBN during CS-US interval. It is suggested that the principal serine/threonine kinases play different roles in CTA learning: whereas PKC activity is necessary for the gustatory short-term memory formation, CAMKII acts similarly to the US itself-an unexpected role of CAMKII in associative learning.     

Associations in Pavlovian conditioned inhibition

Two experiments with rat subjects used a variety of transfer tests to examine the associations learned when Pavlovian inhibition is established by an A+, AX− paradigm. Experiment 1 found in a conditioned suppression situation that inhibition conditioned to X with one exciter (A) readily transferred to another exciter (B) which had been paired with the same shock US. Transfer occurred even when the response to A had been extinguished prior to testing with B. However, X did not inhibit a general activity response produced by a B which had been subsequently paired with a food US. Experiment 2 employed a Pavlovian conditioning situation in which A and B, when separately paired with the same food US, evoked dissimilar responses. Nevertheless, an inhibitor trained in an A+, AX− paradigm successfully inhibited the different response evoked by B. However, such an X did not inhibit the behaviors acquired by A or B when they were subsequently paired with a shock US. The transfer of Pavlovian inhibition across conditioned stimuli and responses but not across unconditioned stimuli is consistent with the notion that a conditioned inhibitor acts to prevent activation of a US representation which would normally be activated by conditioned exciters.     

Simultaneous and sequential conditioned inhibition in autoshaping

Two experiments examined sequential and simultaneous conditioned inhibition in an autoshaping preparation. In Experiment 1, a 5-sec coloured keylight (A) was reinforced when presented alone. However, reinforcement was withheld from trials on which A was preceded by one grid pattern or presented simultaneously with another pattern. A third (control) pattern was presented separately without reinforcement. Inhibition developed most strongly to the grid pattern presented simultaneously with A. However, a transfer test with another excitatory coloured keylight found both inhibitors to be equally strong whether they were tested in simultaneous or in sequential compound. Experiment 2 compared grid patterns that consistently had sequential or simultaneous relations to the excitor with one that was sometimes presented in sequential and sometimes in simultaneous compound. The mixed treatment pattern showed levels of inhibition like the simultaneous or sequential pattern, depending on the temporal relation it bore to A on a particular trial. These results suggest that sequential and simultaneous relations between inhibitors and excitors yield similar amounts of inhibition. However, that inhibition is better exhibited when tested in simultaneous compound with the excitor.     

Conditioned inhibition is not the symmetrical opposite of conditioned excitation: A test of the Rescorla-Wagner model

Rescorla and Wagner's (1972) linear associative model of learning predicts that a neutral stimulus will become weakly excitatory if it is paired with a conditioned inhibitor and the compound is not reinforced. If the inhibitor is not allowed to extinguish during these pairings, the model predicts that the neutral stimulus will become strongly excitatory. Rescorla (1971) has presented some evidence to support the former prediction of a weak effect. Experiment I tested the latter prediction of a strong effect using rats in a conditioned emotional response paradigm, and found no evidence that the pairings made the neutral stimulus excitatory. Experiment II replicated Rescorla's results, but showed that even on a relearning test the experimental group learned no faster than a novel stimulus control group.     

Forgetting of conditioned fear inhibition

Three experiments assessed retention of conditioned fear and of fear inhibition over long-term retention intervals (35 days in two experiments, 25 days in the third). Although excitatory conditioning was retained well, conditioned inhibition was not. This effect occurred when inhibition was assessed by compounding stimuli on an appetitive baseline and when it was assessed by testing cues on a baseline of unsignalled avoidance responding. Inhibition was also forgotten when the initial conditioning treatment was designed to reduce the role of possible proactive interference processes.     

Stimulus generalization of excitation and inhibition

Five sign-tracking experiments with pigeons investigated the breadth of stimulus generalization of excitatory and inhibitory conditioning. Using a compound stimulus test procedure, these experiments found evidence for a narrower generalization of associative strength in excitation than in inhibition. They also found a narrower associative gradient with a more strongly conditioned excitatory stimulus, although this did not seem to account for the difference between exciters and inhibitors. These results confirm earlier findings comparing generalization after acquisition and extinction and raise various theoretical issues about generalization.     

Stimulus generalization of excitation and inhibition

Five sign-tracking experiments with pigeons investigated the breadth of stimulus generalization of excitatory and inhibitory conditioning. Using a compound stimulus test procedure, these experiments found evidence for a narrower generalization of associative strength in excitation than in inhibition. They also found a narrower associative gradient with a more strongly conditioned excitatory stimulus, although this did not seem to account for the difference between exciters and inhibitors. These results confirm earlier findings comparing generalization after acquisition and extinction and raise various theoretical issues about generalization.     

Transfer of negative occasion setting and conditioned inhibition across conditioned and unconditioned stimuli

The transfer of negative occasion setting and conditioned inhibition across conditioned stimuli (CSs) and unconditioned stimuli (USs) was examined in four experiments that used Pavlovian appetitive feature negative discrimination training procedures with rats. After training with simultaneous compounds (A+, XA-), X inhibited conditioned responding (CRs) elicited by other CSs and CRs supported by other appetitive USs that had not been involved in discrimination training. After training with serial compounds (A+, X----A-), X's power to set the occasion for nonresponding transferred across CSs and USs only if those events had also been involved in serial feature negative discrimination training. The results supported the suggestion that the acquisition of negative occasion setting involves the representation of individual events in a higher order memory system, separate from that involved in simple association, and that negative occasion setters act only on events that are represented in that system.