Latent inhibition in the developing rat: an examination of context-specific effects

Latent inhibition (LI) refers to the reduction in conditioned responding when the conditioned stimulus (CS) is preexposed prior to CS-unconditioned stimulus pairings. Experiment 1a demonstrated that preexposure to an odor CS prior to odor-shock pairings markedly reduced conditioned freezing in 25-day-old rats; however, this LI effect was observed only if odor preexposure and odor-shock pairings occurred in the same context (i.e., LI was context-specific at this age). The results of Experiment 1b showed that 18-day-olds also exhibited LI, but this effect was not context-specific at this age. In Experiment 2, rats were preexposed to the odor at 18 days of age and given odor-shock pairings at 25 days of age. These rats exhibited context-specific latent inhibition, suggesting that 18-day-old rats encoded the preexposure context. In Experiment 3, all parameters were identical to Experiment 2, with the exception that odor-shock pairings were given at approximately PN18 and testing occurred at approximately PN25. These rats exhibited latent inhibition at test, but this effect was not context-specific. The results of this study suggest that (a) PN18 rats can exhibit latent inhibition, and (b) the expression of context-specific latent inhibition depends on the age at which conditioning occurs.     

Cingulate cortical coding of context-dependent latent inhibition

Neuronal activity of the auditory thalamus, amygdala, cingulate cortex, and substantia nigra was recorded during the administration of a behavioral test for latent inhibition (LI) or the retardation of behavioral conditioning because of preexposure of the conditional stimulus (CS). Following CS preexposure, both the preexposed CS and a control CS predicted avoidable footshock. LI occurred as significantly fewer avoidance conditioned avoidance responses after the preexposed CS than after the control CS. Attenuation of neuronal responses to the preexposed CS, or neural LI, occurred in all monitored areas. One group of subjects (Oryctolagus cuniculus) then received context extinction, and additional groups experienced novel context exposure or handling. Context extinction enhanced behavioral responding to the preexposed CS, eliminating LI. Context extinction also eliminated cingulate cortical neural LI by enhancing posterior cingulate cortical responses to the preexposed CS and attenuating anterior cingulate cortical responses to the control CS. Present and past results are interpreted to indicate that LI is (a) a failure of response retrieval and/or expression mediated by interfering CS-context associations and (b) a product of interactions of the posterior cingulate cortex and the hippocampus.     

Effects of conditioned stimulus pre-exposure on human electrodermal conditioning to fear-relevant and fear-irrelevant stimuli

The effects of conditioned stimulus (CS) pre-exposure and fear-relevance of the CS on human Pavlovian electrodermal conditioning were investigated. A differential delayed conditioning paradigm was used with a CS-unconditioned stimulus (US; shock) interval of 8 s. In Experiment 1, 64 subjects were randomized into four groups, two of which received fear-relevant stimuli and the other two fear-irrelevant stimuli. Half of the subjects were pre-exposed to the to-be-CSs and the other half to two not-to-be-CSs, with 15 exposure of each stimulus. During acquisition, subjects received 8 reinforced and 8 nonreinforced CS+ and CS- trials, and during the extinction phase 15 nonreinforced trials of each CS. Pre-exposure to the to-be-CSs retarded conditioning for the first and second interval anticipatory responses (FIRs and SIRs); that is, a latent inhibition effect was demonstrated, although the results for the FIR were inconclusive. The expected effects of fear-relevance were not revealed. Experiment 2 addressed the question whether the long pre-exposure period interfered with the frequently observed "preparedness effect" of higher resistance to extinction to fear-relevant stimuli. The design was similar to that of Experiment 1, but for half of the subjects the acquisition phase was initiated immediately after a short rest period, and for the other half acquisition started after an extended rest period, equal to the duration of the pre-exposure phase in Experiment 1. Twenty extinction trials of each CS were presented. A reliable difference in arousal in terms of spontaneous fluctuations was produced by the rest periods, but although differential conditioning was observed, no effect of fear-relevance was seen during extinction.     

The ontogeny of interstimulus interval (ISI) discrimination of the conditioned eyeblink response in rats

Discrimination of the eyeblink conditioned response (CR) between conditioned stimuli (CSs) of different durations and modalities was examined across development in rats. Interstimulus interval (ISI) discrimination was evident at Postnatal Days 23-34 in Experiment 1, and earlier CR peak latencies and enhanced CR amplitudes were seen to the long CS in the ISI discrimination group relative to a control group receiving the short CS without reinforcement. Experiment 2 showed that early CR peak latencies and enhanced CR amplitudes to the long CS in the ISI discrimination group were due to associative pairing of the short CS and unconditioned stimulus. Experiment 3 demonstrated ISI discrimination in adults that was improved relative to younger subjects, but with no enhancement of CR amplitude to the long CS in the ISI discrimination group. Cerebellar cortical maturation may influence the ontogeny of CR timing.     

Conditioned hippocampal cellular response to a behaviorally silent conditioned stimulus.

Multiunit activity (MUA) was chronically recorded in the hippocampal CA3 field of rats using a blocking paradigm with conditioned suppression of lever pressing for food as the measure of conditioning. In Experiment 1, a classical blocking paradigm demonstrated the good conditionability of 2 stimuli (a light and a tone) and their respective ability to block each other. In Experiment 2, MUA was recorded in CA3 cells in rats submitted to a similar paradigm. Four groups received either tone (groups B and B1) or click (groups BC and B1C) conditioned stimulus (CS) presentations that were followed immediately by an electrical footshock (unconditioned stimulus, US). The rats were then given either 40 (groups B and BC) or 1 (groups B1 and B1C) tone + light-footshock presentations. During test sessions, the animals showed MUA responses to the added CS (light), with no conditioned suppression of lever presses occurring during CS presentations. The results of Experiment 3 strongly suggest that hippocampal increase in cellular activity to the light appeared at the first compound trial presentation. Conditioning to the light obtained by increasing the US intensity after the single compound trial suggests that the hippocampal response reflects a redundant CS-US association (light-shock). Long-term retention tests given 45 days after the end of conditioning revealed that behavioral and hippocampal responses could still be detected but only in response to the stimulus that had elicited a behavioral response during learning.     

Latent Inhibition and Conditioning in Rat Strains Which Show Differential Prepulse Inhibition

Latent inhibition (LI) is the retardation of associative conditioning resulting from preexposure of the conditioned stimulus (CS) alone prior to conditioning. Schizophrenic patients show deficient prepulse inhibition (PPI) and, at least acutely, deficient LI as well. We recently found that Brown Norway (BN) rats show a PPI deficit compared to Wistar-Kyoto (WKY) rats. If PPI and LI depend on neural processes with common genetic substrates, then LI should be deficient in BN rats as well. Here, LI of a conditioned taste aversion was examined in BN and WKY rats. One group from each strain was preexposed to a saccharin-flavored solution (CS) the day prior to conditioning. For taste aversion conditioning, these two groups again consumed saccharin and were injected with lithium chloride (unconditioned stimulus) 10 min later. A second group from each strain was not preexposed to the CS and was treated identically during conditioning, while a third group was not conditioned (injected with sodium chloride). To test for taste aversion conditioning, saccharin was offered for 20 min/day for 3 days. Nonconditioned BN and WKY rats consumed equal amounts of saccharin on test days. In both strains, conditioned rats showed a saccharin aversion. However, conditioning was less robust in BN than in WKY rats. WKY rats showed good LI of the conditioned taste aversion in that preexposed WKY rats consumed significantly more saccharin on test days than conditioned, nonpreexposed WKY rats. Preexposed BN rats did not consume significantly more saccharin on test days than conditioned, nonpreexposed BN rats. The previously reported deficiency in PPI in the BN rats was confirmed here 1 week after the taste aversion experiment. These results suggest that BN rats show deficient LI as well as PPI and display poor associative learning, a trait also reported in schizophrenia.     

Age-related disruption of trace but not delay classical conditioning of the rabbit's nictitating membrane response

Young (6 months of age) and old (36-60 months) New Zealand albino rabbits underwent classical conditioning of the nictitating membrane response in either a delay conditioning (Experiment 1) or a trace conditioning (Experiment 2) paradigm. There was no difference between old and young animals in acquisition of the conditioned response in the delay paradigm, nor were there any age-related differences in generalization to the tone conditioned stimulus (CS) or in sensitivity to the tone CS or eye shock unconditioned stimulus. In the trace conditioning paradigm, however, old animals acquired the conditioned response significantly more slowly than young rabbits. Because the same stimulus parameters and the same response were used in both experiments, it is unlikely that age-related differences in trace conditioning were due to stimulus sensitivity, motivation, or fatigue. The results are discussed in terms of how brain changes that accompany aging could differentially affect these two types of classical conditioning.     

Ontogenetic comparison of memory for Pavlovian conditioned aversions to temperature, vibration, odor, or brightness

Ontogenetic differences in forgetting of Pavlovian conditioning were tested in four experiments with 550 rats of ages 12, 18, or 60 days postpartum. The experiments had in common the same unconditioned stimulus (US; footshock) and procedures for conditioning and testing, but the conditioned stimulus (CS) differed. The CS was a particular vibrotactile stimulus (Experiment 1), ambient temperature (Experiment 2), odorant (Experiment 3), or brightness (Experiment 4). The intent of this study was to assess, in more comparable circumstances than previously tested, the possibility of substantial differences in infantile amnesia for different sensory modalities. Particular attention was given those modalities that mature early and had not been tested in previous studies of infantile amnesia. The results verified some degree of infantile amnesia for even those sensory modalities used adaptively from birth.     

Early Pavlovian conditioning impairs later Pavlovian conditioning

Four experiments tested the effects in the rat of very early experience with stimuli to be used later for Pavlovian conditioning. Beginning on postnatal Day 12, prior to the development of substantial detection and effective perception of visual and auditory stimuli, rats were given five daily experiences with either lights or tones and a footshock known to be an effective unconditioned stimulus at these ages. Twenty-four hours after the last of these experiences, pairings of either the light or tone and the unconditioned stimulus were given with parameters established to yield a moderate degree of conditioning in untreated preweanlings (Experiment 1). Experiment 2 determined that early experience with paired or unpaired presentations of either the light or tone and the unconditioned stimulus resulted in a failure to condition to these same lights or tones on postnatal Day 17, although nontreated pups from the same litters conditioned quite effectively. Experiment 3 determined that this early conditioning experience with either paired or unpaired presentations of the lights or tones and the unconditioned stimulus yielded impaired conditioning on postnatal Day 17 in the alternative sensory modality as well, although again nontreated siblings conditioned quite effectively. Experiment 4 replicated the results of each of Experiments 2 and 3 and determined in addition that despite the impairment in conditioning that resulted from early paired or unpaired experience with the stimuli of conditioning, early experience with the individual stimuli of conditioning—with only the CS, the US, or the context—did not result in a similar impairment in conditioning. Although the results were unexpected, they may be understood in part in terms of intersensory competition during development, and there is precedent in the literature for similar interfering effects of early learning on later learning in a variety of species. © 1996 John Wiley & Sons, Inc.     

Disruption of contextual freezing, but not contextual blocking of fear-potentiated startle, after lesions of the dorsal hippocampus

The role of the dorsal hippocampus in contextual fear conditioning was investigated with a contextual blocking paradigm. In Experiment 1, rats were given pairings of a light conditioned stimulus (CS) and footshock after preexposure either to footshock or to the context alone. The group preexposed to footshock showed poorer fear conditioning to the light CS, as measured by the fear-potentiated startle reflex. In Experiment 2, a group preexposed to footshock in the same context showed poorer fear conditioning to the light CS than did a group preexposed to footshock in a different context, indicating contextual blocking of fear-potentiated startle. In Experiment 3, lesions of the dorsal hippocampus had no effect on contextual blocking, even though contextual freezing was disrupted. The sparing of contextual blocking indicated that contextual memory was intact following hippocampal lesions, despite the disruption of contextual freezing.     

Lesions of the entorhinal cortex disrupt behavioral and neuronal responses to context change during extinction of discriminative avoidance behavior

 Rabbits given either electrolytic lesions of the entorhinal cortex or sham-lesions were trained to prevent a foot-shock by stepping in an activity wheel after one tone, a positive conditioned stimulus (CS+), and to ignore a different tone, a negative conditioned stimulus (CS–). Neuronal activity was recorded simultaneously in the basolateral nucleus of the amygdala, the CA1 cell field of hippocampus, anterior cingulate cortical area 24b and posterior cingulate cortical area 29c/d. The activity of neurons in the entorhinal cortex was recorded in the controls. Acquisition of conditioned avoidance responses (CRs) was not affected by lesions of the entorhinal cortex. Discriminative neuronal activity (greater neuronal responses to the CS+ than to the CS–) during CR acquisition was significantly enhanced in hippocampal area CA1 and attenuated in the basolateral amygdala in rabbits with lesions. Following acquisition to a criterion, two counterbalanced extinction tests were administered, one in the original context and the other in the presence of novel contextual stimuli. CR frequency was significantly reduced in controls but not in rabbits with lesions, during extinction with novel contextual stimuli, relative to performance in the original context. The rabbits with lesions also showed fewer inter-trial responses than controls during extinction in the original context but inter-trial response frequency in rabbits with lesions did not differ from the frequency in controls during extinction in the novel context. Neurons in the basolateral amygdala in controls showed discriminative activity during extinction in the original context but not in the novel context. Amygdalar neurons in the rabbits with lesions did not show discriminative activity during extinction in either context. Posterior cingulate cortical neurons in control rabbits did not show discriminative activity during extinction in the original context but these neurons exhibited robust discriminative activity in the novel context. Posterior cingulate cortical neurons in rabbits with lesions showed discriminative activity in both extinction sessions. The results indicated that the entorhinal cortex does not play a significant role in the acquisition of discriminative avoidance behavior, under the employed conditions of training. However, the interactions of neurons in the entorhinal cortex, amygdala and cingulate cortex are essential for contextual modulation of CRs during extinction. Received: 17 September 1996 / Accepted: 14 January 1997     

Acquisition of conditioned associations in Hermissenda: additive effects of contiguity and the forward interstimulus interval

Conditioned suppression of photokinesis by the marine mollusc Hermissenda was examined in 3 experiments. In each experiment, groups of animals received light (the conditioned stimulus, CS) that was paired with high-speed orbital rotation (the unconditioned stimulus, UCS), light and rotation explicitly unpaired, or no exposure to these stimuli. Twenty-four hours after training, all animals were tested for suppression of photokinesis in the presence of the light. To establish the effectiveness of our conditioning procedure, in Experiment 1 individual groups of animals received either 50, 100, or 150 CS-UCS pairings. Fifty pairings resulted in a marginal suppression of photokinesis, whereas 100 and 150 pairings produced strong suppression. In Experiment 2, the delay between CS onset and UCS onset was varied between 1 and 10 s. The 10-s interstimulus interval (ISI) did not support conditioning, whereas 1-s and 2-s ISIs were effective. As predicted by the current understanding of Hermissenda's neural network, in Experiment 3 it was found that CS-UCS pairings in which the CS preceded the onset of the UCS and terminated with the offset of the UCS evoked stronger conditioned suppression than either a CS that preceded the UCS and terminated with its onset or a CS that was paired in simultaneous compound with the UCS. This result indicates that CS-UCS contiguity as well as the forward ISI act additively to establish the CS-UCS association. In none of the 3 experiments were any differences observed between groups that were untreated and that received the CS and UCS unpaired. In total, these experiments suggest strong similarities in the temporal characteristics of associative learning in Hermissenda and vertebrate species.     

The effects of task type and task requirements on the dissociation of skin conductance responses and secondary task probe reaction time

Although task-irrelevant events elicit smaller skin conductance responses (SCRs) than do task-relevant events, secondary task probe reaction time (RT) is often slower during the former. Three experiments (N = 48 in each) examined the effects of task demands, instructions, and stimulus discriminability on this dissociation effect. SCRs were larger to task-relevant stimuli in all experiments regardless of experimental manipulation. Subjects in Experiment I counted either all tones of one pitch (high/low group) or longer-than-usual tones of one pitch (longer group). There was more RT slowing during task-irrelevant tones at a 250-ms probe position in the high/low group and at a 150-ms probe position in the longer group. Experiment 2 employed differential Pavlovian conditioning in which the offset of task-relevant stimuli (CS+) coincided with the onset of a shock stimulus. Half the subjects were told which stimulus would be followed by shock (information group), whereas the others received no information (no-information group). Increased RT slowing during CS? was restricted to the no-information group. Experiment 3 employed visual conditioned stimuli that were easy or difficult to discriminate. RT slowing at 4,000 ms was greater during CS+, whereas there was a tendency for more RT slowing during CS? at 150 ms. There was no effect for CS discriminability. The results suggest that during both simple discrimination and during Pavlovian conditioning, task-irrelevant stimuli are more actively processed than task-relevant stimuli within the first 250 ms of stimulus presentation.     

Spontaneous recovery but not reinstatement of the extinguished conditioned eyeblink response in the rat

Reinstatement--the return of an extinguished conditioned response (CR) after reexposure to the unconditioned stimulus (US)--and spontaneous recovery--the return of an extinguished CR with the passage of time--are 2 of 4 well-established phenomena that demonstrate that extinction does not erase the conditioned stimulus (CS)-US association. However, reinstatement of extinguished eyeblink CRs has never been demonstrated, and spontaneous recovery of extinguished eyeblink CRs has not been systematically demonstrated in rodent eyeblink conditioning. In Experiment 1, US reexposure was administered 24 hr prior to a reinstatement test. In Experiment 2, US reexposure was administered 5 min prior to a reinstatement test. In Experiment 3, a long, discrete cue (a houselight), present in all phases of training and testing, served as a context within which each trial occurred to maximize context processing, which in other preparations has been shown to be required for reinstatement. In Experiment 4, an additional group was included that received footshock exposure, rather than US reexposure, between extinction and test, and contextual freezing was measured prior to test. Spontaneous recovery was robust in Experiments 3 and 4. In Experiment 4, context freezing was strong in a group given footshock exposure but not in a group given eye shock US reexposure. There was no reinstatement observed in any experiment. With stimulus conditions that produce eyeblink conditioning and research designs that produce reinstatement in other forms of classical conditioning, we observed spontaneous recovery but not reinstatement of extinguished eyeblink CRs. This suggests that reinstatement, but not spontaneous recovery, is a preparation- or substrate-dependent phenomenon.     

Effects of fear-relevance on electrodermal safety signal learning

Safety signal learning in human subjects was investigated using a conditioned inhibition procedure with a shock unconditioned stimulus (US). The conditioned stimulus (CS) duration, and thus the CS-US interval, varied randomly from 20 to 50 s. Conditioning was assessed by change in tonic skin conductance level (SCL). Experiment 1 demonstrated reliable learning of a conditioned inhibition (A+/AB-) discrimination, in terms of both self-reported shock expectancy and change in SCL, in subjects who were able to report correctly the stimulus contingency. There was, however, no evidence of transfer of inhibitory properties from the safety signal B to a separately trained excitor. Experiment 2 compared two groups in which the safety signal was either fear-relevant (picture of snake or spider) or fear-irrelevant (flower or mushroom). As in previous research, there was no effect of fear-relevance on safety-signal learning. The results are discussed in terms of preparedness theory and excitatory conditioning with fear-relevant stimuli.     

The forgetting of stimulus attributes in latent inhibition

Numerous studies have demonstrated that the forgetting of stimulus attributes is a common occurrence; that is, organisms forget the specific characteristics of training stimuli over long retention intervals, while retaining general information of the training stimuli themselves. However, most studies have examined this effect after a learning episode, and there have been virtually no accounts to test whether the forgetting of attributes occurs for stimuli presented prior to training. Therefore, this experiment was designed to test that possibility, and it examined whether the forgetting of stimulus attributes occurred prior to training for the flavor stimulus in a conditioned taste aversion (CTA) procedure. Specifically, a latent inhibition (LI) procedure was used to measure the extent of forgetting for a pre-exposed flavor over short and long retention intervals. The results indicate that rats forgot the specific characteristics of the flavor stimulus (CS) while retaining memory for pre-exposure sessions over a long retention interval. That is, subjects pre-exposed and conditioned with different concentrations of sucrose showed no LI effect with a 1-day delay between pre-exposure and training, but demonstrated a generalized LI with an 8-day delay between pre-exposure and conditioning. This experiment provides further evidence for the robustness of the forgetting of stimulus attributes, and demonstrates that this specific type of forgetting also occurs prior to the learning of a CTA task.     

Potentiation of conditioned freezing following dorsomedial prefrontal cortex lesions does not interfere with fear reduction in mice

In Experiment 1, an auditory conditioned stimulus (CS) was paired with footshock, except when it was preceded by another stimulus (a visual conditioned inhibitor [CI]). After conditioning, all mice displayed less CS-evoked freezing when the CI-CS compound was presented than when the CS was presented alone. However, lesions of the dorsomedial prefrontal cortex (dmPFC) potentiated CS-evoked freezing on each of the 2 sessions (i.e., CI CS and CS alone). In Experiment 2, mice were submitted to fear extinction (CS-alone presentation for 3 days). Lesioned mice exhibited a higher level of freezing behavior than controls on each of the 3 sessions. However, lesioned mice and controls displayed the same rate of reduction of freezing over the 3 days of extinction. These data in mice support previous studies in rats, which suggests that the dmPFC is not critical for either conditioned inhibition or extinction of acquired freezing behavior.     

Effects of conditioned fear on responsiveness to pain: long-term retention and reversibility by naloxone

The effect of a conditioned fear stimulus (CS) on responsiveness to pain was examined in three experiments. In Experiment 1, a CS that signaled shock attenuated freezing in response to shock, with the attenuation occurring several minutes after the shock. Naloxone blocked the effect of the CS. The effect of the CS, including its reversibility by naloxone, was retained over an interval of 90 days. Experiment 2 showed that this effect on freezing is due to associative fear conditioning, rather than blocking of conditioning to context by a novel cue. In Experiment 3, presenting a fear CS just prior to administering a tail-flick (radiant heat) test of nociception increased the tail-flick latencies; that is, the fear CS apparently induced hyperalgesia rather than analgesia. Because this result makes it difficult to interpret the change in freezing seen in the first experiment as reflecting antinociception, it raises questions about how pain might differentially affect different measures of pain responsiveness. A memory hypothesis is advanced to resolve the different effects obtained with the freezing and tail-flick tests.     

An auditory conditioned stimulus modulates unconditioned stimulus-elicited neuronal activity in the cerebellar anterior interpositus and dentate nuclei during nictitating membrane response conditioning in rabbits.

The effects of the presentation of an auditory conditioned stimulus (CS) on unconditioned stimulus (US)-elicited neuronal activity in the anterior interpositus (AIPN) and dentate (DN) nuclei of the cerebellum were investigated during the initial stages of classical conditioning of the nictitating membrane (NM) response in rabbits. In Experiment 1, a 500-ms CS (but not a 30-ms CS) facilitated US-elicited single-unit activity in the AIPN and depressed US-elicited activity in the DN during training. In Experiment 2, lesions of the AIPN but not of the DN prevented acquisition of conditioned NM responses. The results are interpreted within the framework of a model of classical conditioning that proposes that conditioned neuronal activity that underlies behavioral plasticity develops from the modulation of US-elicited neuronal activity by the CS.     

Changes in Spatial Patterns of Rabbit Olfactory EEG with Conditioning to Odors

Electrode arrays (8 × 8, 3.5 × 3.5 mm) were implanted epidurally on the olfactory bulbs of rabbits for EEG recording. The rabbits were trained to give a conditioned response to a warning odor paired with an electric shock. EEGs were recorded and edited, and representative ERG bursts with odor and preceding the odor were selected for measurement. Each burst was displayed in a contour map of amplitude. The contour maps revealed active EEG foci in the bulb with size, shape and location unique to each rabbit. Changes in shape and location took place only during familiarization and during training, when a warning odor was paired with the aversive stimulus. The EEG spatial patterns did not change when visual or auditory stimuli were used as CS. EEG spatial patterns did not reflect conformal mapping of odor stimulus to neural activity response, but were determined by state variables of the animal related to olfactory conditioning history. The implications for human EEG are briefly discussed.