Conditioning-specific reflex modification of the rabbit (Oryctolagus cuniculus) nictitating membrane response is sensitive to context

Conditioning-specific reflex modification occurs when an unconditioned response is modified in theabsence of the conditioned stimulus as a result of pairings of the conditioned stimulus and an unconditioned stimulus. In two experiments, we assessed conditioning-specific reflex modification in either a novel context (Experiment 1) or a context different from, but equally familiar in relation to, the training context (Experiment 2). Conditioning-specific reflex modification did not demonstrate sensitivity to a novel context but did demonstrate sensitivity to a change in familiar context. The data cannot be explained by unconditioned stimulus preexposure, overtraining, or context insensitivity. The results suggest that conditioning-specific reflex modification models normal stress and may be used to evaluate theories of and treatments for posttraumatic stress disorder.     

Compound-component differentiation as a function of CS-US interval and CS duration in the rabbit’s conditioned nictitating membrane response

The acquisition of the rabbit’s nictitating membrane response to a tone + light compound and its components was examined as a function of the CS-US interval (Experiment 1) and CS duration (Experiment 2). In Experiment 1, responding to the compound attained high levels in all groups, but responding on component test trials declined to low levels as the CS-US interval increased across values of 300, 800, and 1,300 msec. Further disparities between the compound and components appeared when the animals were shifted to a positive patterning schedule. In Experiment 2, disparities between the compound and components increased as the duration of the CS was increased across values of 50, 200, and 800 msec within a fixed CS-US interval of 800 msec. The results are discussed with respect to distributive processes, configuration, and speed-accuracy tradeoffs.     

The CS-US interval (ISI) function in rabbit nictitating membrane response conditioning with very long intertriai intervals

In Experiment 1, four values of conditioned stimulus-unconditioned stimulus interval, or interstimulus interval (ISI) (200, 500, 800, and 1,100 msec) were studied in one trial/day acquisition of the conditioned nictitating membrane response of the rabbit. The 1,100-msec ISI group was superior to the other groups, contrary to the usual findings of ISI studies in the conditioning literature. In Experiment 2, effective conditioning levels were attained with an ISI as long as 2,200 msec, when the intertriai interval was set at either 24 or 48 h. Results are interpreted in terms of the role of orienting responses in the conditioning process.     

Temporal specificity in patterning of the rabbit nictitating membrane response

Rabbits were trained in either positive patterning (AX+, A−, X−) or negative patterning (A+, X+, AX−) using one of four intervals between the onset of A and the onset of X on AX trials. These intervals were 0, 800, 2,400, and 5,600 msec. In each task, all groups acquired an appropriate pattern of discriminative responding. Following acquisition, all rabbits were tested with the four different A-X intervals. All positive patterning groups showed an excitatory gradient, in which the highest level of responding occurred at the interval used in training. Conversely, all but one of the negative patterning groups showed an inhibitory gradient, in which the lowest level of responding occurred at the interval used in training. The one exception was the negative patterning group trained with simultaneous AX stimuli (0 msec), which showed a low, broad gradient, indicating transfer of inhibition across all the intervals. The results are discussed with respect to temporal encoding mechanisms and accounts of conditional discriminations.     

Stimulus specificity of concurrent recovery in the rabbit nictitating membrane response

Three experiments demonstrated that, following the extinction of an established conditioned stimulus (CS; e.g., tone), the pairing of an orthogonal stimulus from another modality (e.g., light) with the unconditioned stimulus (US) results in strong recovery of responding to the extinguished CS. This recovery occurred to about an equal degree regardless of whether or not initial training contained unambiguous stimulus—reinforcer relationships—that is, consistent CS—US pairings—or some degree of ambiguity, including intramodal discrimination training, partial reinforcement, or even cross-modal discrimination training (tone vs. light). Experiments 1 and 2 demonstrated that this recovery of responding was largely specific to the extinguished CS, but moderate generalization to other stimuli from the same modality did appear. The results are discussed with reference to alternative mechanisms applicable to learning-dependent generalization between otherwise distinct CSs. These models assume that such generalization is mediated by either a shared response, shared reinforcer, shared context, or shared hidden units within a layered neural network. A specific layered network is proposed to explain the present results as well as other types of savings seen previously in conditioning of the rabbit nictitating membrane response.     

Second-order conditioning of the rabbit’s nictitating membrane response as a function of the CS2-CS1 and CS1-US intervals

Three experiments investigated conditioning of the rabbit’s nictitating membrane response (NMR) in a second-order conditioning procedure which intermixed first-order trials (CS1-US) and second-order trials (CS2-CS1) from the outset of training. Experiment 1 provided a controlled demonstration that substantial levels of second-order conditioning can be obtained with the NMR preparation. Experiment 2 showed that the level of CR acquisition to CS2 was an inverse function of the CS2-CS1 interval over the values of 400, 800, and 2,400 msec. Experiment 3 found that CR acquisition to CS2 and CS1 in second-order conditioning varied in a parallel fashion across CS1-US intervals. Similarly, Experiment 3A found that the level of CR acquisition to the two components of a serial compound (CSA-CSB-US) varied in a parallel fashion as a function of the CSB-US interval. The results of the CS2-CS1 and CS1-US interval manipulations were all predictable from the known CS-US interval effects in NMR conditioning with a single CS. The present results are discussed with regard to their implications for accounts of serial compound conditioning and second-order conditioning.     

Conditioning-specific reflex modification of the rabbit (<Emphasis Type="Italic">Oryctolagus cuniculus</Emphasis>) nictitating membrane response: US intensity effects

Conditioning-specific reflex modification (CRM) of the rabbit’s nictitating membrane response (NMR) describes changes in responding to an unconditioned stimulus (US) when the rabbit is tested in the absence of the conditioned stimulus. Specifically, after at least 3 days of tone-electrical stimulation pairings, responses to the US increase in size, especially at intensities weaker than the training intensity. CRM is similar to classical conditioning in that it is a function of the strength of conditioning, it can be extinguished, and it can be generalized from one stimulus to another. To compare CRM and classical conditioning further, we conducted three experiments to examine the effects of US intensity (1.0, 2.0, and 4.0 mA) on CRM. CRM was weak following conditioning with 1.0 mA and extremely strong following conditioning with 2.0 mA and 4.0 mA. The data suggest that CRM is a function of US intensity and have implications for posttraumatic stress disorder, a disorder potentially modeled by CRM.     

Serial discrimination reversal learning in pigeons as a function of intertrial interval and delay of reinforcement

Pigeons learned a series of reversals of a simultaneous red-green visual discrimination. Delay of reinforcement (0 vs. 2 sec) and intertrial interval (ITI; 4 vs. 40 sec) were varied across blocks of reversals. Learning was faster with 0-sec than with 2-sec delays for both ITI values and faster with 4-sec ITIs than with 40-sec ITIs for both delays. Improvement in learning across successive reversals was evident throughout the experiment, furthermore, even after more than 120 reversals. The potent effects of small differences in reinforcement delay provide evidence for associative accounts and appear to be incompatible with accounts of choice that attempt to encompass the effects of temporal parameters in terms of animals’ timing of temporal intervals.     

Amplitude of the rabbit’s unconditioned nictitating membrane response in the presence of a conditioned inhibitor

Using a light or backshock as the reinforced CS (A) and a tone or backshock as the conditioned inhibitor (X), rabbits experienced conditioned inhibition training in an A+/AX? paradigm. Following training, the amplitude of the unconditioned nictitating membrane response elicited by a mild (.5-mA) paraorbital shock was measured in the presence of X and AX and expressed as a percentage of the amplitude of the UR to the shock presented alone. In Experiment 1, the effect of X and AX on UR amplitude for conditioned inhibition animals was compared with that of control animals treated to a variety of pretest procedures. In general, UR amplitude in the presence of X exceeded that observed to the US presented alone. There was no consistent difference between the experimental and control groups. In Experiments 2–5, A test trials were added as an alternative reference point. Again, UR amplitude increased rather than decreased UR amplitude. In addition, X as a conditioned inhibitor enhanced the facilitating effect of A on UR amplitude in four out of five experiments. These findings have implications for theories of the “locus of action” of conditioned inhibitors.     

US preexposures retard excitatory and facilitate inhibitory conditioning of the rabbit’s nictitating membrane response

Two experiments were conducted to examine the effects of US preexposure on differential conditioning of the rabbit’s nictitating membrane response. Both experiments consisted of three phases: a 10-day US preexposure phase, a 7-day differential conditioning phase, and a 3-day retardation of learning test for inhibition. In Experiment 1, US preexposures retarded the development of excitation to CS+ but facilitated the development of inhibition to CS?. In Experiment 2, half of the preexposed subjects received the preexposures in one experimental environment and differential conditioning in a second environment. The remaining preexposed subjects received both phases in a single environment. Retarded excitatory and facilitated inhibitory conditioning were observed only in the preexposed subjects that received both phases in the same environment. Rabbits that received a context shift performed at control levels. The results are discussed in terms of current theories of US preexposure effects, and the best account was provided by a modified associative theory.     

Real-time factors in the rabbit’s nictitating membrane response to pulsed and serial conditioned stimuli

Conditioning theories and recent real-time models commonly postulate that a reinforcer is signaled by a series of stimuli. In both Pavlovian and operant procedures, serial stimuli have been shown to control the likelihood and timing of responses over intervals of seconds and minutes. The present experiments were conducted to determine whether serial stimuli exercise similar effects over stimulus-reinforcer intervals in the order of hundreds of milliseconds. Such intervals typify those used in conditioning of the rabbit nictitating membrane response. A sequence of four tone pulses (50–100 msec) was used as the CS to assess the effectiveness of serial stimuli. After training with this CS, tests were conducted in which one or more of the pulses were removed. These perturbations of the sequence of stimuli over a 400-msec interval produced large deficits in CR likelihood and smaller alterations in CR timing. The results are discussed with respect to their implications for current real-time models of conditioning, and particularly with respect to their assumptions about the source of internal stimuli, rules for learning, and rules for generating CRs.     

Classical conditioning of the rabbit nictitating membrane response: Effects of reinforcement schedule on response maintenance and resistance to extinction

Two experiments were conducted to assess the effects of the introduction of schedules of partial reinforcement (PRF), subsequent to continuous reinforcement training, on the maintenance and resistance to extinction of the rabbit’s nictitating membrane CR. Substantial response levels were maintained by schedules of reinforcement as lean as 15%, and the performance decrements, when observed to be reliable, could not be localized to the immediate effects of one, two, or three consecutive nonreinforced trials by the examination of conditional response probabilities. Moreover, reliable PRF extinction effects were obtained. The relevance of these findings to the purported empirical divergence of PRF effects on classical and instrumental conditioning were discussed.     

Temporal discrimination using different feature-target intervals in classical conditioning of the rabbit’s nictitating membrane response

In a typical conditional discrimination, a target stimulus (X) is reinforced during one feature cue (A→X+), but not during another feature cue (B→X−). The present experiments used only a single “feature” cue (a 66-sec tone). On half of the trials, the target stimulus (a 400-msec light) was paired with the reinforcer when the feature-target interval was one duration (e.g., 5 sec). On the remaining trials, the interval was different (e.g., 45 sec), and the target stimulus was presented without the reinforcer. All the animals acquired this temporal discrimination, and subsequent testing with other feature-target intervals yielded generalization-like gradients. These results provide solid evidence that each portion of a feature cue is encoded in a distinctive fashion. Had temporal encoding not occurred, the feature cue would have been just as ambiguous a predictor of the reinforcer as was the target stimulus, and discrimination would not have been possible. The integration of real-time temporal encoding mechanisms into models of conditional discrimination is discussed.     

Temporal primacy overrides prior training in serial compound conditioning of the rabbit’s nictitating membrane response

Conditioning of the rabbit’s nictitating membrane response was conducted with a serial compound CSA-CSB-US. In the present experiments, prior training of CSB was pitted against the temporal primacy of CSA. Prior training of CSB was able to only weakly block CR acquisition to the added CSA, but CSA caused a pronounced decline in responding to the pretrained CSB. By the end of training, high levels of responding were sustained only in the final portion of the serial compound in which CSA or its traces coincided with CSB. These results provide support for real-time models as exemplified by Sutton and Barto (1981).     

Motivational vs. associative role of the US in classical conditioning of the rabbit’s nictitating membrane response

Two experiments tested the motivational role of the US in classical conditioning of the rabbit’s nictitating membrane (NM) response. In Experiment 1, subjects were trained to an intermediate performance level and then given a series of (1) CS-US trials, (2) “backwards” US-CS trials, (3) CS-alone trials, (4) US-alone trials, or (5) no-stimulus presentations. Interpolated presentations of the US, either alone or in a backwards contingency, tended to produce an impairment of subsequent acquisition. In Experiment 2, subjects were trained with strong or weak US intensity on paired or interpolated trials. US intensity on interpolated trials had only a very small effect, whereas the effect of US intensity on paired trials was quite large. Shifts in paired-trial US intensity produced corresponding shifts in performance, but shifts in the intensity of the interpolated US produced no apparent effect. We conclude that the arousal of motivation is not sufficient to maintain performance in classical NM conditioning.     

Retardation of rabbit nictitating membrane conditioning following US preexposures depends on the distribution and number of US presentations

Retarded conditioned response (CR) acquisition produced by unconditioned stimulus (US) preexposures has been attributed either to interference resulting from contextual conditioning or to habituation of the US. Both perspectives assume that the amount of retardation is directly related to the number of US preexposures. This assumption was examined in two experiments. In Experiment 1, separate groups of rabbits received 200 paraorbital shock US preexposures either in one session or spread equally over 10 daily sessions. Subsequently, all subjects received 150 CS-US pairings. Acquisition of nictitating membrane CRs was retarded relative to a naive control group only in the group that received the preexposures over 10 sessions. Thus, the number of US preexposure sessions, rather than the number of US preexposures, determined whether or not retarded acquisition was observed. In Experiment 2, four groups of rabbits received 1, 5, 20, or 40 shock US preexposures in each of 10 sessions. Over the subsequent 150 CS-US pairings, similar levels of retarded CR acquisition were observed in groups that received 20 and 40 US preexposures per session, a weak retardation effect was observed with 5 preexposures per session, and no retardation was observed with 1 preexposure per session. Thus, Experiment 2 suggested that retarded CR development was not greatly influenced by increasing the number of US preexposures above some minimum threshold number of exposures per session. Implications for current theories were discussed.     

One-trial context fear conditioning as a function of the interstimulus interval

In two experiments, we examined the effects of a wide range of interstimulus intervals (2.5, 15, 45, 120, 135, and 405 sec) on one-trial context fear conditioning with rats. Here, the interstimulus interval (ISI) denotes the time between placement in a conditioning chamber and the onset of a single footshock. On the conditioning day, we observed that the rats’ behavior at the time of shock onset varied systematically across ISI values. On the subsequent test day, we used context-evoked freezing as a measure of context conditioning and found the well-known inverted U-shaped ISI function. We also found that conditioned freezing for the shortest ISI values was concentrated early in the test session, whereas freezing at longer ISIs was distributed more evenly throughout the test session. The freezing results found here are more consistent with the literature on conditioning with punctate cues than are previously described results from one-trial context fear-conditioning procedures.     

The effect of feature-target intervals in conditional discriminations on acquisition and expression of conditioned nictitating membrane and heart rate responses in the rabbit

Nictitating membrane (NM) and heart rate (HR) responses were investigated in a conditional discrimination (A→X+ vs. B→X−), using feature-target intervals of 0, 5, 15, and 45 sec. Conditional control of NM responses, but not of HR responses, was acquired to the 400-msec X stimulus in all the groups tested. However, differential conditioning of both the NM and the HR responses to A versus B feature cues appeared for the three shorter intervals. Following acquisition, all the rabbits were tested with the four different feature-target intervals. All the groups showed a gradient of NM responding to X, in which the highest level of responding occurred at or near the interval used in training. The results are discussed with respect to the relationship of simple conditioning of the feature cues to their control over responding during presentation of the target stimulus, the putative role of HR as an index of preparatory processes during presentation of feature cues, and mechanisms of temporal specificity in conditional discriminations.     

Amnesia induced by hypothermia as a function of treatment-test interval and recooling in rats

In Experiment I, a repeated tests procedure was employed to assess hypothermia-induced amnesia of a footshock experience. Rats tested 4 h after training treatment showed no memory loss, but amnesia was present at 24 h. Although recovery of memory was obtained when the same animals were cooled 2 h prior to a 50-h test, repeated testing also tended to attenuate amnesia. In Experiment II, independent groups were tested at 6 or 50 h after training treatment. Again, memory of the footshock was present at the short, but not at the long, interval. Recooling shortly prior to the 50-h test eliminated amnesia. Experiment III indicated that the return of memory produced by recooling did not persist if testing was delayed. These findings suggested that hypothermia may function as an important contextual cue for memory retrieval.