Trace fear conditioning is reduced in the aging rat

Auditory trace fear conditioning is a hippocampus-dependent learning task that requires animals to associate an auditory conditioned stimulus (CS) and a fear-producing shock-unconditioned stimulus (US) that are separated by an empty 20-s trace interval. Previous studies have shown that aging impairs learning performance on hippocampus-dependent tasks. This study measured heart rate (HR) and freezing fear responses to determine if aging impairs hippocampus-dependent auditory trace fear conditioning in freely moving rats. Aging and Young rats received one long-trace fear conditioning session (10 trials). Each trial consisted of a tone-CS (5 s) and a shock-US separated by an empty 20-s trace interval. The next day rats received CS-alone retention trials. Young rats showed significantly larger HR and freezing responses on the initial CS-alone retention trials compared to the Aging rats. A control group of aging rats received fear conditioning trials with a short 1-s trace interval separating the CS and US. The Aging Short-Trace Group showed HR and freezing responses on the initial CS alone retention trials that were similar to the Young Long-Trace Group, but greater than the Aging Long-Trace Group. A second aging control group received unpaired CSs and USs, and showed no HR or freezing responses on CS-alone retention trials. These data show that HR and freezing are effective measures for detecting aging-related deficits in trace fear conditioning.     

Pavlovian heart rate and jaw movement conditioning in the rabbit: effects of medial prefrontal lesions

An experiment was conducted in which jaw movements (JM) and heart rate (HR) were concomitantly assessed in rabbits during simple Pavlovian conditioning. A 2-s 1200-Hz tone was the conditioned stimulus (CS) and an intraoral 1-cc pulse of 0.5 M sucrose-water solution was the unconditioned stimulus (US). Sham and medial prefrontal (mPFC)-lesioned animals received paired CS/US training with a 70- to 75-dB CS and were compared with sham- and mPFC-lesioned animals that received explicitly unpaired CS/US presentations. The percentages of JM CRs were significantly greater in the paired than the unpaired groups, but mPFC lesions had no effect on this measure. Conditioned HR decelerations occurred only in the paired groups and then only during the first session of training. Moreover, these CS-evoked cardiac decelerations were somewhat attenuated by the mPFC lesion. CS-evoked HR accelerations, which were significantly greater in unpaired than in paired animals, occurred during the four subsequent sessions. These results suggest that a CS-evoked cardioinhibitory process, mediated by the mPFC, is engendered by Pavlovian appetitive conditioning, as has been previously demonstrated for aversive conditioning. However, during JM conditioning these inhibitory changes are quickly replaced by tachycardia, possibly related to increased nonspecific somatomotor activity, since the tachycardia was somewhat greater in the unpaired animals.     

Posttraining prefrontal lesions impair jaw movement conditioning performance,but have no effect on accompanying heart rate changes

This experiment examined the role of the medial prefrontal cortex (mPFC) in regulating learned autonomic and somatomotor responses in rabbits using appetitive Pavlovian conditioning. Interstimulus interval (ISI) duration [i.e., the time between the onset of the conditioned stimulus (CS) and unconditioned stimulus (US)] was manipulated in order to determine whether ISI duration was related to the heart rate (HR) responses obtained during conditioning. Two groups received either a 1- or a 4-s ISI, with a tone as the CS and an intraoral pulse of water as the US. Another two groups received explicitly unpaired presentations of either the 1- or 4-s tone CS and water US. Few conditioned jaw movement (JM) or HR conditioned responses (CRs) were observed in the unpaired conditions. Significant JM conditioning was, however, elicited by the paired conditions, especially to the 4-s ISI. Consistent CS-evoked HR accelerations were observed in both ISI conditions. After five sessions of training, the mPFC was lesioned in half the animals. A separate group of paired animals received sham lesions. After surgical recovery, all animals received 3 days of postoperative training. During the first postoperative training session, JM CRs significantly declined in both groups with mPFC lesions in comparison to the groups with sham lesions. The mPFC lesions, however, did not affect the CS-evoked cardiac accelerations, which again occurred during postoperative training.     

Conditioned taste aversion is enhanced when the unconditioned stimulus is presented in a different context

Using a conditioned taste aversion procedure with rats as the subjects, two experiments examined the effect of presenting a conditioned stimulus (CS saccharin solution) in one context followed by an unconditioned stimulus (US LiCl) in a different context. Experiment 1 showed that animals which received the above-mentioned procedure (Group D) showed a more marked conditioned aversion to the CS than animals which were given both the CS and the US in the same context (Group S). Experiment 2 found that in both Group D and Group S, aversion to the CS increased when the subjects were exposed to the conditioned context after the conditioning. These findings supported the argument that the strength of the CS-US association acquired during conditioning is compared with that of the context-US to determine the magnitude of aversion revealed to the CS.     

Undernutrition during the brain growth period of the rat significantly delays the development of processes mediating Pavlovian trace conditioning

Pavlovian trace conditioning procedures allow one to assess the ability of animals to associate events that are temporally separated because the conditioned stimulus terminates prior to the occurrence of the unconditioned stimulus. We report that undernutrition during Postnatal Days 2-18 significantly delays the development of trace conditioning to a visual stimulus. Previously undernourished 20-day-old rats conditioned when no temporal interval separated the termination of the CS and US onset (0-s trace interval). However, it was not until the undernourished pups were 30 days old that they conditioned when the trace interval was 10 or 30 s. In contrast, control pups only 20 days old were able to condition when a 10-s trace interval separated the CS and US events, and 25-day-old control pups conditioned when the interval was either 10 or 30 s. These results suggest that undernutrition delays the development of processes that enable the rat to sustain a representation of a visual CS during the trace interval.     

Similarity of cardiac CR forms in the rhesus monkey during several experimental procedures

Cardiac rates of rhesus monkeys were observed in a variety of different conditioning procedures, each of which involved a visual stimulus (CS) followed by an electric shock (US). With a 30-sec CS, cardiac rate accelerated rapidly after CS onset, reached a maximum in the middle of CS, and decelerated thereafter, with a terminal CS rate often at the level of, or below, pre-CS levels. A similar biphasic cardiac rate response in CS was also observed under subsequent exposure to intermittent pairings of CS and US, avoidance of US, response-produced termination of US, and when CS-US pairings were superimposed upon an avoidance baseline, even when CS duration was varied from 12 to 60 seconds. The unusual regularity of cardiac rate responses in several different procedures may result from one or more of these factors: (a) characteristics of the rhesus monkey, (b) initial exposure to Pavlovian conditioning, or (c) the uniformity of measurement of cardiac rate employed in this study.     

Total sleep deprivation impairs the encoding of trace-conditioned memory in the rat

Sleep may help consolidate the information of certain memories, though its benefits in the consolidation of trace-conditioned memory still remain elusive. We investigated the effect of sleep deprivation on trace learning in male wistar rats. Rats were trained for trace conditioning and the number of head entries into liquid dispenser was accounted as an outcome measure of trace-learning. For training and testing, 75 presentations of conditioned stimulus (CS) (light) and unconditioned stimulus (US) (juice) were offered in five sessions (15 presentations/session; with 5 min inter-session gap). The duration of CS and US stimuli were 15 and 20s respectively, with 5s trace delay between stimuli and 20s condition delay between each presentation. The animals were divided randomly into three groups soon after training, sleep deprived (SD) (n=8), non-SD (NSD) (n=8) and stress control (n=5) groups. The animals of NSD and control groups were left undisturbed, while SD animals were sleep deprived for 6h after training. The learning of trace-conditioned task was examined on following days. We observed that SD rats poked approximately 63% less than NSD and control groups (p<0.001) to obtain juice on testing day. Also, the NSD rats exhibited significant positive correlation in number of head entries during the training and testing days; while the SD rats showed no significant correlation. The results demonstrate that SD animals had difficulties to associate CS with US and suggest that sleep deprivation soon after training impairs the encoding of trace memory.     

The effects of lesions in the central nucleus of the amygdala on appetitive classical conditioning in rats

Abstract: Lesions in the central nucleus of the amygdala (cAMY) have been known to interfere with the acquisition of fear classical conditioning when footshock is used as an unconditioned stimulus (US). The present study examined whether or not a similar interference would occur with an appetitive US. Five rats with lesions in the cAMY (the cAMY group), and eight unoperated control rats were trained in an appetitive classical conditioning paradigm, which did not include elements of operant learning, using a visual conditioned stimulus (CS) (5 W of light for 10 s duration) paired with a food pellet US (45 mg, cheese flavor). The behavioral index of appetitive conditioning was an increase in rearing approach behavior to the CS after CS and US pairings. During CS and US pairings, the movement of the rat was limited so that this approach behavior could not occur. As a result, all control rats showed an increase in rearing, but the cAMY group did not. These results suggest that the cAMY is critical for appetitive as well as fear classical conditioning.     

Sign- and goal-tracking in Atlantic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>)

When animals associate a stimulus with food, they may either direct their response towards the stimulus (sign-tracking) or towards the food (goal-tracking). The direction of the conditioned response of cod was investigated to elucidate how cod read cue signals. Groups of cod were conditioned to associate a blinking light (conditioned stimulus, CS) with a food reward (unconditioned stimulus, US), with the CS and the US located at opposite sides of the tank. Two groups were trained in a delay conditioning procedure (CS = 60 s, interstimulus interval = 30 s) and two groups were trained in a trace conditioning procedure (CS = 12 s, trace interval = 20 s). The response pattern was similar for the delay- and trace-conditioned groups. The initial main response at the onset of the CS was approaching the blinking lights, i.e. sign-tracking. In the early trials, the fish did not gather in the feeding area before the arrival of food. In the later trials, the fish first approached the blinking lights, but then moved across the tank and gathered below the feeder before the food arrived, i.e. sign-tracking followed by goal-tracking within each trial. These two responses are interpreted as reflecting two learning systems, i.e. one rapid, reflexive response directed at the signal (sign-tracking) and one slower, more flexible response based on expectations about time and place for arrival of the food (goal-tracking). The ecological significance of these two learning systems in cod is discussed.     

Pavlovian conditioning with ethanol and lithium: effects on heart rate and taste aversion in rats

Rats received paired injections of either ethanol or saline as the conditioned stimulus and lithium chloride as the unconditioned stimulus (US) in a Pavlovian differential conditioning paradigm. Lithium chloride evoked a large deceleration in heart rate (80-100 beats per minute) as an unconditioned response. As a result of 10 conditioning trials, the substance paired with LiCl elicited a lower average heart rate than that elicited by the unpaired substance. Moreover, animals that received ethanol-LiCl injections subsequently were more averse to the taste of ethanol than animals receiving saline-LiCl pairings. However, there were no differences in ethanol's ability to serve as the US to induce an aversion to a novel flavor solution (i.e., the Avfail phenomenon was not observed). The overall pattern of results underscores the value of using multiple indexes of learning in drug-drug conditioning paradigms.     

Failure to develop a conditional reflex by stimulation of the cephalad portion of the severed vagosympathetic trunk: Its significance in terms of the central connections of the vagus

Stimulation of the central stump of either vagosympathetic trunk in the dog, the contralateral nerve remaining intact, regularly provoked deep respiratory movements with forceful expiration, followed by a period of apnea, and a fall in blood pressure, systolic and diastolic, of 20–60 mm/Hg. Stimulation of the cephalad portion of the left nerve provoked brief acceleration of heart rate during the period of hyperventilation, followed by bradycardia; when the stimulus was applied to the central stump of the right nerve heart rate remained relatively unchanged. When a 12-second tone as a conditional stimulus (CS) was reinforced during its last six seconds with such stimulation of the vagosympathetic trunk as an unconditional stimulus (US), despite the striking visceromotor responses elicited by the US, no conditional reflex was established even after more than 3,400 trials in 16 dogs (34–781 trials per animal).     

Timed excitatory conditioning under zero and negative contingencies

Rats (rattus norvegicus) anticipated the arrival of a food pellet unconditioned stimulus (US) even when the conditioned stimulus (CS) signaled no overall change or a substantial decrease in the overall rate of US occurrence. Pellet USs were scheduled probabilistically in the intertrial interval at either an equivalent rate (Experiment 1) or a four times higher rate (Experiments 2 and 3) than in the CS, which included one fixed-time target US. Conditioning has been said to involve learning "whether" (contingency) the CS signals a change in the US, and if so, "when" (contiguity) the US is scheduled to arrive. Our results suggest that "when" trumps "whether," challenging the received view that a positive CS-US contingency is necessary for successful conditioning.     

A role for CS-US contingency in Pavlovian conditioning

Two experiments evaluated the role of conditioned stimulus-unconditioned stimulus (CS-US) contingency in appetitive Pavlovian conditioning in rats. In both experiments, some groups received a positively contingent CS signaling an increased likelihood of the US relative to the absence of the CS. These groups were compared with control treatments in which the likelihood of the US was the same in the presence and absence of the CS. A trial marker served as a trial context. Experiment 1 found contingency sensitivity. There was a reciprocal relationship between responding to the CS and the trial marker. Experiment 2 showed that this result was not stimulus or response specific. These results are consistent with associative explanations and the idea that rats are sensitive to CS-US contingency.     

Response rate and reinforcement rate in Pavlovian conditioning

Four experiments used delay conditioning of magazine approach in rats to investigate the relationship between the rate of responding, R, to a conditioned stimulus (CS) and the rate, r, at which the CS is reinforced with the unconditioned stimulus (US). Rats were concurrently trained with four variable-duration CSs with different rs, either as a result of differences in the mean CS-US interval or in the proportion of CS presentations that ended with the US. In each case, R was systematically related to r, and the relationship was very accurately characterized by a hyperbolic function, R = Ar/(r +c). Accordingly, the reciprocal of these two variables-response interval, I (= 1/R), and CS-US interval, i (= 1/r) - were related by a simple affine (straight line) transformation, I = mi+b. This latter relationship shows that each increment in the time that the rats had to wait for food produced a linear increment in the time they waited between magazine entries. We discuss the close agreement between our findings and the Matching Law (Herrnstein, 1970) and consider their implications for both associative theories (e.g., Rescorla & Wagner, 1972) and nonassociative theories (Gallistel & Gibbon, 2000) of conditioning. (PsycINFO Database Record (c) 2011 APA, all rights reserved).     

Conditioning the unconditioned response: modification of the rabbit's (Oryctolagus cuniculus) unconditioned nictitating membrane response

Conditioning-specific reflex modification (CRM) occurs when classical conditioning modifies responding to an unconditioned stimulus (US) in the absence of a conditioned stimulus (CS). Three experiments monitored rabbit nictitating (Oryctolagus cuniculus) membrane unconditioned responses to 5 intensities and 4 durations of periorbital electrical stimulation before and after CS or US manipulation. CRM occurred after 12 days of CS-US pairings but not following unpaired CS/US presentations or restraint. CRM survived CS-alone and CS/US-unpaired extinction of the conditioned response (CR) but not presentations of the US alone, although CRs remained intact. Thus, CRs could be weakened without eliminating CRM and CRM could be weakened without eliminating CRs. Data indicate CRM is a reliable, associative effect that is more than a generalized CR and may not be explained by habituation, stimulus generalization, contextual conditioning, or bidirectional conditioning.     

Timing of fear expression in trace and delay conditioning measured by fear-potentiated startle in rats

In two experiments, the time course of the expression of fear in trace (hippocampus-dependent) versus delay (hippocampus-independent) conditioning was characterized with a high degree of temporal specificity using fear-potentiated startle. In experiment 1, groups of rats were given delay fear conditioning or trace fear conditioning with a 3- or 12-sec trace interval between conditioned stimulus (CS) offset and unconditioned stimulus (US) onset. During test, the delay group showed fear-potentiated startle in the presence of the CS but not after its offset, whereas the trace groups showed fear-potentiated startle both during the CS and after its offset. Experiment 2 compared the time course of fear expression after trace conditioning with the time course in two delay conditioning groups: one matched to the trace conditioning group with respect to CS duration, and the other with respect to ISI. In all groups, fear was expressed until the scheduled occurrence of the US and returned to baseline rapidly thereafter. Thus, in both trace and delay fear conditioning, ISI is a critical determinant of the time course of fear expression. These results are informative as to the possible role of neural structures, such as the hippocampus, in memory processes related to temporal information.     

Signal-centered action patterns of dogs in appetitive classical conditioning

The movements of five unrestrained dogs were monitored during discriminative Pavlovian conditioning. An auditory-visual stimulus from one source (CS+) was followed by meat morsels (US); the same stimulus from another source (CS?) was not. Sources were equidistant from the site of US delivery. Before each trial, animals were required to position themselves at a starting location equidistant from both sources and removed from the US site. The stable behavior pattern in most subjects included approach to and contact or near contact with CS+, followed by approach to the US site. Dogs showed individually distinctive action patterns to CS+, in some cases suggestive of soliciting, in another of sight-pointing. Similar actions were not evoked by CS?. In postacquisition tests, patterns were generally unaffected by increased deprivation or by relocating the starting position next to the US site. The auditory component of CS+ was more effective than the visual. With a response-contingent procedure animals were successfully trained to approach the US site directly, although many trials, and in one case a special procedure, were required. It was proposed that the experimental signaling arrangement mimics a natural signaling sequence and induces appetitive behavior to the CS that corresponds to the behavior induced by certain natural signals of the US.     

Changes in pain reactivity induced by unconditioned and conditioned excitatory and inhibitory stimuli

In three experiments we investigated the effects of aversive-conditioning components on the reactivity of rats to pain. After training in Experiment 1 with a discrete conditioned stimulus (CS) for a shock unconditioned stimulus (US), different groups were exposed to the CS, US, CS/Us compound, just the training context, or none of those immediately prior to a hot-plate test assessing the latency of a paw-lick response. Relative to no exposure and context alone, the CS produced a shorter latency--that is, an apparent sensitization effect--whereas the US produced a longer latency--that is, a hypoalgesic effect--that was actually augmented by the CS/US compound. Furthermore, whereas the US-induced hypoalgesia was unaffected by the opiate antagonist, naloxone, hypoalgesia produced by the CS/US compound was appreciably decremented by the drug. Experiment 2 showed the same effects with parameters more typical of conditioning research. Experiment 3 compared signals for the presence (CS+) and absence (CS-) of the US. The CS- did not itself affect pain reactivity, but in inhibited the effects of the CS+, US, and CS+/US compound. Collectively, the results suggest that a CS+sensitizes the animal to imminent events and also potentiates an opioid reaction that supplants the less effective nonopioid hypoalgesia induced by the US. In contrast, a CS- functions as a general moderator of excitation, inhibiting both sensitization and hypoalgesic effects, whether opioid or nonopioid.     

Learned Irrelevance Is Not the Sum of Exposure to CS and US

In three experiments hungry rats received appetitive conditioning trials with a light that signalled the delivery of sucrose solution. In Experiment 1, prior exposure to uncorrelated presentations of the conditioned stimulus (CS) and unconditioned stimulus (US) retarded conditioning significantly more than did prior exposure to the CS alone. In Experiments 2 and 3, groups exposed to uncorrelated presentations of the CS and US within the same session conditioned significantly more slowly than groups given separate sessions of exposure to the CS followed by sessions of exposure to the US (or vice versa). Some part of the learned irrelevance effect depends on exposure to a zero correlation between the CS and US, perhaps because this promotes learning that the CS predicts no change in the probability of the US.     

Goal-tracking behavior in the pond snail, Lymnaea stagnalis

The occurrence of goal-tracking, an unconditioned stimulus (US)-directed autoshaping behavior, was studied in open-field tests with control and classically conditioned pond snails, Lymnaea stagnalis. In an appetitive classical conditioning paradigm with a tactile stimulus as conditioned stimulus (CS) and a localized food stimulus as US a conditioned feeding response built up in the experimental but not in the control animals. In the post-training open-field tests the experimental group alone showed an enhanced attraction toward the source of water current in the environment which previously signalled the arrival of the US but did not act as CS in the classical conditioning procedure. We suggest that this stimulus-directed goal-tracking behavior in Lymnaea is the result of a classical-operant interaction, described so far only in vertebrate animals, and that neurophysiological analysis of this behavior is possible in this snail.