Classical conditioning,sensitization and habituation in the spinal cat

Changes in the flexion reflex of the tibialis anterior muscle of acute spinal cats were examined during conditioning, sensitization and habituation paradigms. Experimental animals were classically conditioned by pairing electrical stimulation of the saphenous nerve (CS) with stimulation of the superficial peroneal nerve (US). Recordings from these nerves assured known and constant stimulus inputs. The response observed was an increase in the magnitude of the reflex response to the CS over training. Habituation (CS only) and sensitization (CS and US presentations, unpaired) control animals exhibited no such reflex facilitation. The results of post-tetanic potentiation studies indicated that the intertrial intervals used were not a factor in the differences observed between experimental and control groups. The results give positive support to the concept of spinal conditioning and emphasize the potential of this model system for the study of neural correlates of learning.     

Fear-potentiated startle conditioning in humans: Explicit and contextual cue conditioning following paired versus unpaired training

Conditioned fear in response to explicit and contextual cues was examined using the startle reflex in three groups of participants over two sessions separated by 4–5 days. The conditioned stimulus (CS) was paired with an aversive unconditioned stimulus (US) (shock) during conditioning in the paired but not in the unpaired group. In the reaction time (RT) group, the US was a nonaversive visual signal for an RT task. In the paired group, the CS potentiated startle in the postconditioning phase. This conditioned response was fully retained over the retention interval. There was no substantial change in baseline startle (startle delivered in the absence of CS). By contrast, startle was not potentiated by the CS in the unpaired group, but baseline startle was increased from Session 1 to Session 2. In the RT group, startle was not affected by the CS, and baseline startle was reduced from Session 1 to Session 2. These results suggest that paired presentations of a CS and an aversive US result in conditioned fear in response to the CS but little contextual fear, whereas unpaired presentations of a CS and US leads to poor explicit cue conditioning but substantial contextual fear.     

Naloxone induces multiple effects on aversive Pavlovian conditioning in rabbits

A series of experiments examined the effects of intravenous naloxone treatment on aversive Pavlovian conditioning of eye-blink and heart rate responses, and related unconditioned behaviors, in rabbits. Naloxone treatment before testing attenuated bradycardiac orienting responses to tones used as conditioning stimuli. Naloxone also attenuated conditioned bradycardia when administered either before or after training sessions, but it potentiated conditioned bradycardia during extinction of discriminative conditioning. Naloxone did not influence acquisition or extinction of discriminative eye-blink conditioning or somatic or cardiac responses to shocks used as unconditioned stimuli, but it did decrease locomotor activity. Naloxone treatment immediately after training sessions facilitated acquisition of eye-blink responses. It was concluded that naloxone influences aversive Pavlovian conditioning in more than one way: (a) During training, it appears to alter reception and processing of signals but does not affect subsequent development of somatic responses to the Pavlovian conditioning contingency. (b) After training sessions, naloxone apparently affects consolidation of both somatic and autonomic conditioning. (c) Naloxone also appears to delay extinction of Pavlovian conditioning; this effect may similarly involve changes in a stimulus-processing mechanism or in memory functions, but it apparently does not involve changes in somatomotor responsitivity.     

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.     

Appetitive-aversive interactions in Pavlovian fear conditioning

The existence of value coding and salience coding neurons in the mammalian brain, including in habenula and ventral tegmental area, has sparked considerable interest in the interactions that occur between Pavlovian appetitive and aversive conditioning. Here we studied these appetitive-aversive interactions at the behavioral level by assessing the learning that occurs when a Pavlovian appetitive conditioned stimulus (conditional stimulus, CS) serves as a CS for shock in Pavlovian fear conditioning. A Pavlovian appetitive CS was retarded in the rate at which it could be transformed into a fear CS (counterconditioning), but the presence of the appetitive CS augmented fear learning to a concurrently presented neutral CS (superconditioning). Retardation of fear learning was not alleviated by manipulations designed to restore the associability of the appetitive CS before fear conditioning but was alleviated by manipulations designed to increase the aversive quality of the shock unconditioned stimulus (US). These findings are consistent with opponent interactions between the appetitive and aversive motivational systems and provide a behavioral approach for assessing the neural correlates of these appetitive-aversive interactions.     

Ontogenetic differences in the effects of unpaired stimulus preexposure on eyeblink conditioning in the rat

Learned irrelevance (LIr) is a Pavlovian conditioning phenomenon in which random or unpaired preexposure to a conditional stimulus (CS) and to an unconditional stimulus (US) retards subsequent paired conditioning involving these stimuli. A previous developmental study of eyeblink conditioning in the rat suggested that LIr is not present on postnatal Day 20. Stanton, Fox, and Carter (1998) showed that unpaired preexposure to a CS and a US on postnatal Day 17 failed to retard (and, in fact, facilitated) subsequent paired conditioning involving these stimuli on postnatal Day 20. The present experiments were designed to further characterize the ontogeny of this phenomenon. In Experiment 1, LIr was observed when rat pups were tested for eyeblink conditioning as described in Stanton et al. (1998), except that preexposure occurred on postnatal Day 27, and acquisition testing occurred on postnatal Day 30. In Experiment 2, preexposure and acquisition both occurred on postnatal Day 30, and four types of preexposure were compared: chamber only, CS alone, US alone, or unpaired presentation of CS and US. Unpaired preexposure impaired acquisition relative to that of the remaining three groups, which did not differ. Experiment 3, showed that under the conditions of Experiment 2, LIr failed to appear on postnatal Day 20, but was observed on postnatal Days 25 and 30. These findings suggest that learning that events are unrelated emerges between postnatal Days 20 and 25 in the rat. Possible behavioral and neural mechanisms underlying this effect are discussed.     

Arousal-related associative response characteristics of dorsal lateral geniculate nucleus neurons during acoustic Pavlovian fear conditioning

This research determined whether fear-conditioned, acoustic stimuli induce thalamic arousal reflected in associative responses in dorsal lateral geniculate nucleus (dLGN) neurons. Rabbits received a Pavlovian discriminative fear conditioning procedure in which one tone conditioned stimulus (CS +) was always paired with an aversive unconditioned stimulus (US) and another tone (CS-) was never paired with the US. Responses of single dLGN neurons to random CS+ and CS- presentations were then recorded. Nine of 15 recorded neurons demonstrated significantly greater firing during the CS+ versus the CS-. Their spontaneous activity demonstrated tonic firing during increased neocortical arousal and burst firing during decreased neocortical arousal. The results demonstrate that dLGN neurons show associative responses to fear-conditioned, acoustic stimuli and present a model for investigating the neural circuits by which such stimuli affect sensory processing at the thalamic level.     

Aversive learning in patients with unilateral lesions of the amygdala and hippocampus

The present study applied a visual half field paradigm with emotional facial expressions in patients with selective unilateral amygdalo-hippocampectomy (AHE) to elucidate the contributions of the left and right medial temporal lobe and amygdala to emotional learning. Electrodermal indicators of aversive learning were studied in 14 left AHE and 12 right AHE patients, as well as 13 controls matched in sex and age. In a differential conditioning paradigm with negative (CS+) and positive (CS-) facial expressions, CS+ were associated with an aversive vocalization (US, 95 dB, 3 s). During extinction, stimuli were presented laterally and preattentively using backward masking. Appropriate CS durations yielding preattentive presentation were individually determined prior to conditioning. In contrast to controls, both left and right AHE patients failed to show an autonomic conditioning effect following left visual field presentations of masked negative CS+ during extinction. AHE patients also showed no clear differential acquisition. Moreover, right AHE patients poorly recognised that negative valence was an affiliating dimension of the CS-US compound.     

Impact of continuous versus intermittent CS-UCS pairing on human brain activation during Pavlovian fear conditioning

During Pavlovian fear conditioning a conditioned stimulus (CS) is repeatedly paired with an aversive unconditioned stimulus (UCS). In many studies the CS and UCS are paired on every trial, whereas in others the CS and UCS are paired intermittently. To better understand the influence of the CS-UCS pairing rate on brain activity, the experimenters presented continuously, intermittently, and non-paired CSs during fear conditioning. Amygdala, anterior cingulate, and fusiform gyrus activity increased linearly with the CS-UCS pairing rate. In contrast, insula and left dorsolateral prefrontal cortex responses were larger during intermittently paired CS presentations relative to continuously and non-paired CSs. These results demonstrate two distinct patterns of activity in disparate brain regions. Amygdala, anterior cingulate, and fusiform gyrus activity paralleled the CS-UCS pairing rate, whereas the insula and dorsolateral prefrontal cortex appeared to respond to the uncertainty inherent in intermittent CS-UCS pairing procedures. These findings may further clarify the role of these brain regions in Pavlovian fear conditioning.     

Associative and non-associative blinking in classically conditioned adult rats

Over the last several years, a growing number of investigators have begun using the rat in classical eyeblink conditioning experiments, yet relatively few parametric studies have been done to examine the nature of conditioning in this species. We report here a parametric analysis of classical eyeblink conditioning in the adult rat using two conditioned stimulus (CS) modalities (light or tone) and three interstimulus intervals (ISI; 280, 580, or 880 ms). Rats trained at the shortest ISI generated the highest percentage of conditioned eyeblink responses (CRs) by the end of training. At the two longer ISIs, rats trained with the tone CS produced unusually high CR percentages over the first few acquisition sessions, relative to rats trained with the light CS. Experiment 2 assessed non-associative blink rates in response to presentations of the light or tone, in the absence of the US, at the same ISI durations used in paired conditioning. Significantly more blinks occurred with longer than shorter duration lights or tones. A higher blink rate was also recorded at all three durations during the early tone-alone sessions. The results suggest that early in classical eyeblink conditioning, rats trained with a tone CS may emit a high number of non-associative blinks, thereby inflating the CR frequency reported at this stage of training.     

Activity profiles of single neurons in caudal anterior cingulate cortex during trace eyeblink conditioning in the rabbit

Acquisition of trace eyeblink conditioning involves the association of a conditioned stimulus (CS) with an unconditioned stimulus (US) separated by a stimulus-free trace interval. This form of conditioning is dependent upon the hippocampus and the caudal anterior cingulate cortex (AC), in addition to brain stem and cerebellar circuitry. Hippocampal involvement in trace eyeblink conditioning has been studied extensively, but the involvement of caudal AC is less well understood. In the present study, we compared neuronal responses from rabbits given either paired (trace conditioning) or unpaired (pseudoconditioning) presentations of the CS and US. Presentation of the CS elicited significant increases in neuronal activity at the onset of both trace conditioning and pseudoconditioning. A robust CS-elicited neuronal response persisted throughout the first 2 days of trace conditioning, declining gradually across subsequent training sessions. In contrast, the magnitude of the CS-elicited excitatory response during pseudoconditioning began to decline within the first 10 trials. Neurons exhibiting excitatory responses to the CS during trace conditioning also exhibited excitatory responses to the US that were significantly greater in magnitude than US-elicited responses during pseudoconditioning. CS-elicited decreases in neuronal activity became more robust over the course of trace conditioning compared to pseudoconditioning. Reductions in activity during the CS interval consistently preceded excitation in both training groups, suggesting that the CS-elicited decreases in neuronal activity may serve to increase the signal-to-noise ratio of the excitatory response to the tone. Taken together, these data suggest that the caudal AC is involved early in trace eyeblink conditioning and that maintenance of the CS-elicited excitatory response may serve to signal the salience of the tone.     

The use of an unpleasant sound unconditional stimulus in an aversive conditioning procedure with 8- to 11-year-old children

The study of aversive Pavlovian conditioning in children can contribute to our understanding of how fears are acquired and extinguished during development. However, methodological issues hamper further research because of ethical and procedural concerns regarding the use of traditional aversive unconditional stimuli (USs) and no established method to measure trial-by-trial changes in the child's expectancy of the US. The present experiment used geometric shape conditional stimuli (CSs) and an unpleasant sound US with 8- to 11-year-old children. Reliable acquisition and extinction were observed with first, second, and third interval skin conductance responses, on-line expectancy judgments, and post-conditioning subjective ratings of pleasantness and arousal. The experiment confirms the novel use of an unpleasant sound of metal scraping on slate as a US in aversive conditioning with children. The methods have the potential to facilitate the ethical conduct of aversive conditioning research in children using psychophysiological, affective, and self-report expectancy measures.     

Learning to fear suffocation: a new paradigm for interoceptive fear conditioning

The present study aimed to establish a new interoceptive fear conditioning paradigm. The conditioned stimulus (CS) was a flow resistor that slightly obstructs breathing; the unconditional stimulus (US) was a breathing occlusion. The paired group (N = 21) received 6 acquisition trials with paired CS–US presentations. The unpaired group (N = 19) received 6 trials of unpaired CS–US presentations. In the extinction phase, both groups were administered 6 CS‐only trials. Measurements included startle eyeblink response, electrodermal responses, and self‐reported US expectancy. In the paired group, startle blink responses were larger during CS compared to intertrial interval during acquisition and extinction. Electrodermal and US expectancies were larger for the paired than for the unpaired group during acquisition, but not during extinction. The present paradigm successfully established interoceptive fear conditioning with panic‐relevant stimuli.     

Amygdaloid unit activity during classical conditioning of the nictitating membrane response in rabbit

Single and multiple unit activity was recorded from the amygdaloid nuclei in awake unanesthestized rabbits during classical conditioning of the nictitating membrane (NM) response. Over half o the unit recordings from the amygdaloid complex demonstrated changes in firing rate following the presentation of the tone conditioned stimulus (CS) or corneal air-puff unconditioned stimulus (US). Of the unit records that were responsive to the training stimuli, 58% showed responses to the CS and 73% showed responses to the US. Amygdaloid responses to either the CS or the US tended to be a long latency (greater than 70 msec), long duration (greater than 250 msec), modest increase (less than 2 fold) in unit firing. There were no statistically significant differences between the spontaneous firing rates, response latencies, response magnitudes, or response type distributions seen during unpaired (control) and paired stimulus presentations. However, four of 18 single and multiple unit groups that were tested during both unpaired and paired training developed new or enhanced responses following the onset of paired training. All four of these records were from the basolateral or lateral amygdaloid nuclei. Although these few records did develop response alterations after CS-US pairing, the majority of the records indicated that essentially the same amygdaloid response patterns occur during unpaired training as during paired training. it is therefore unlikely that the critical neuronal changes that underlying NM conditioning occur within the amygdaloid complex. Other possible roles for the amygdaloid complex during conditioning are considered.     

The effect of emotional and attentional load on attentional startle modulation

The interactive effects of emotion and attention on attentional startle modulation were investigated in two experiments. Participants performed a discrimination and counting task with two visual stimuli during which acoustic eyeblink startle-eliciting probes were presented at long lead intervals. In Experiment 1, this task was combined with aversive Pavlovian conditioning. In Group Attend CS+, the attended stimulus was followed by an aversive unconditional stimulus (US) and the ignored stimulus was presented alone whereas the ignored stimulus was paired with the US in Group Attend CS−. In Experiment 2, a non-aversive reaction time task US replaced the aversive US. Regardless of the conditioning manipulation and consistent with a modality non-specific account of attentional startle modulation, startle magnitude was larger during attended than ignored stimuli in both experiments. Blink latency shortening was differentially affected by the conditioning manipulations suggesting additive effects of conditioning and discrimination and counting task on blink startle.     

Effect of naloxone on conditioned suppression in rats

Painful stimuli are known to engage an endorphin analgesic system that can be reversed by the opiate antagonist, naloxone. Naloxone, then, should increase the effectiveness of aversive unconditioned stimuli (USs) in Pavlovian fear conditioning. Consistent with this hypothesis, naloxone administered during the acquisition of conditioned suppression in rats enhanced posttrial suppression and preconditioned stimulus (pre-CS; context-controlled) suppression. Furthermore, it enhanced CS-elicited suppression during extinction when administered during acquisition but not when administered only during extinction. Thus naloxone does not enhance an already existing fear nor enhance the memory of previous conditioning; instead, it enhances the conditioning of fear presumably by making the aversive US more painful.     

Conditioned turning behavior: a Pavlovian fear response expressed during the post-encounter period following aversive stimulation

Rats were trained to fear an auditory conditioned stimulus (CS) by pairing it with a mild electric shock (the unconditioned stimulus, or US) delivered to one eyelid. After training, the CS elicited two different conditioned fear responses from rats: a passive freezing response, and an active turning response. The balance between these two modes of conditioned responding depended upon the rat's recent history of encounters with the US. If rats had not recently encountered the US, then they responded to the CS by freezing. But after recently encountering the US, rats exhibited CS-evoked turning responses that were always directed away from the trained eyelid, even if the US had recently been delivered to the opposite (untrained) eyelid. This post-encounter turning behavior was not observed in rats that had been trained with unpaired presentations of the CS and US, indicating that even though CS-evoked turning was selectively expressed after recent encounters with the US, it was nonetheless a conditioned Pavlovian fear response that depended upon a learned association between the CS and US. Further supporting this conclusion, pharmacological inactivation experiments showed that expression of both freezing and turning behaviors depended upon lateralized circuits in the amygdala and periaqueductal gray (PAG) that are known to support expression of Pavlovian fear responses. These findings indicate that even though the ability of a CS to elicit Pavlovian fear responses depend upon the long-term history of CS–US pairings, the mode of conditioned responding (freezing versus turning in the present experiments) can be modulated by short-term factors, such as the recent history of US encounters. We discuss neural mechanisms that might mediate such short-term transitions between different modes of defensive responding, and consider how dysregulation of such mechanisms might contribute to clinical anxiety disorders.     

Eyeblink conditioning using cochlear nucleus stimulation as a conditioned stimulus in developing rats

Previous studies demonstrated that the development of auditory conditioned stimulus (CS) input to the cerebellum may be a neural mechanism underlying the ontogenetic emergence of eyeblink conditioning in rats. The current study investigated the role of developmental changes in the projections of the cochlear nucleus (CN) in the ontogeny of eyeblink conditioning using electrical stimulation of the CN as a CS. Rat pups were implanted with a bipolar stimulating electrode in the CN and given six 100-trial training sessions with a 300 ms stimulation train in the CN paired with a 10 ms periorbital shock unconditioned stimulus (US) on postnatal days (P) 17-18 or 24-25. Control groups were given unpaired presentations of the CS and US. Rats in both age groups that received paired training showed significant increases in eyeblink conditioned responses across training relative to the unpaired groups. The rats trained on P24-25, however, showed stronger conditioning relative to the group trained on P17-18. Rats with missed electrodes in the inferior cerebellar peduncle or in the cerebellar cortex did not show conditioning. The findings suggest that developmental changes in the CN projections to the pons, inferior colliculus, or medial auditory thalamus may be a neural mechanism underlying the ontogeny of auditory eyeblink conditioning.     

The effect of androgen on the retention of extinction memory after conditioned taste aversion in mice

Conditioned taste aversion (CTA) induced by the application of a novel taste such as sodium saccharin (Sac) as the conditioned stimulus (CS) and a malaise-inducing agent as the unconditioned stimulus (US), results in acquisition of CTA memory to Sac. In contrast, CTA is extinguished by repeated presentations of the CS without the US, resulting in acquisition of the extinction memory. We examined the effects of androgenic hormones on acquisition and retention of extinction memory in mice. We gonadectomized sexually immature mice and continuously administered androgens to these animals. After sexual maturation, the mice underwent a conditioning period followed by an extinction period. Retrieval tests revealed that the androgen-treated group showed significantly greater retention of extinction memory than the non-treated group 5 weeks later, whereas such significant difference was not observed in acquisition of extinction memory. These results demonstrate the enhancing effect of androgens on retention of extinction memory.     

The use of an unpleasant sound as an unconditional stimulus in a human aversive Pavlovian conditioning procedure

Ethical considerations can limit the use of traditional unconditional stimuli (US), such as electric shock and loud tones, when used in a human aversive Pavlovian conditioning procedure. The risk of the US causing pain or excessive anxiety is a particular concern when testing sensitive populations such as children, the elderly, and those with psychological or neurodevelopmental disorders. Two experiments used a differential conditioning procedure to determine whether an unpleasant sound (metal scraping on slate) could support the acquisition and extinction of conditioned responses to the same extent as either electric shock or a 100 dB(A) tone US. Experiment 1 (N=48) demonstrated equivalent or superior conditioning effects for the signal-based learning measures of US expectancy, skin conductance responses, and heart rate. Experiment 2 (N=57) yielded similar outcomes in the affective-based learning measures of startle blink modulation and pleasantness ratings. The results support the use of an unpleasant sound as a US in human Pavlovian conditioning experiments.