Hippocampus and trace conditioning of the rabbit's classically conditioned nictitating membrane response

Rabbits received classical conditioning of the nictitating membrane response (NMR) in a trace conditioning paradigm. In this paradigm, a 250-ms tone conditioned stimulus (CS) occurs, after which there is a 500-ms period of time in which no stimuli occur (the trace interval), followed by a 100-ms air puff unconditioned stimulus (UCS). In Experiment 1, lesions of the hippocampus or cingulate/retrosplenial cortex disrupted acquisition of the long-latency or adaptive conditioned response relative to unoperated controls and animals that received neocortical lesions that spared the cingulate/retrosplenial areas. When animals with hippocampal or cingulate/retrosplenial lesions were switched to a standard delay paradigm in which the CS and UCS were contiguous in time, they acquired in about the same number of trials as naive rabbits. In a second experiment multiple-unit activity in area CA1 of the hippocampus was examined during acquisition of the trace conditioned response (CR). Three groups of animals were tested: animals that had a 500-ms trace interval (Group T-500), animals that received explicitly unpaired presentations of the CS and UCS (Group UP), and animals that underwent conditioning with a 2,000-ms trace interval (Group T-2000). Animals in Group T-500 acquired the CR in about 500 trials. Early in training, and well before any CRs occurred, there was a substantial increase in neuronal activity in the hippocampus that began during the CS and persisted through the trace interval. There was also an increase in the UCS period that modeled the amplitude-time course of the behavioral unconditioned response. Later in conditioning as CRs emerged, there was no longer neuronal bursting throughout the CS + trace period. Rather, the activity shifted to later in the trace interval and formed a model of the amplitude-time course of the behavioral CR. Activity during the UCS period was similar to that seen earlier in conditioning. Animals in Group UP showed no behavioral conditioning and no increase in neuronal activity. Animals in Group T-2000 showed no long-latency behavioral conditioning and no increase in neuronal activity. The data are discussed in terms of the role of the hippocampus in conditioning during situations in which the CS and UCS are not contiguous in time.     

Hippocampal lesions in rats differentially affect long- and short-trace eyeblink conditioning

Extensive previous research has implicated the hippocampus as an important structure for the acquisition of trace eyeblink conditioning. Evidence from multiple species and various lesioning methods shows that the disruption of conditioned responding (CR) may be partially dependent on the relative lengths of the conditioned stimulus (CS) period and the trace interval. The present study systematically manipulated the length of the CS and the trace interval while matching the interstimulus intervals (ISI) in rats with or without ibotenic acid hippocampal lesions. The long-trace interval condition had a CS duration of 50 ms and a trace interval of 500 ms. The short-trace interval condition had a 500 ms CS and a 50 ms trace interval. We found that control animals in the long-trace interval condition learned at a slower rate than the control animals in the short-trace interval condition. Lesioned animals in both the trace conditions showed deficits in acquisition. Lesioned animals in the short-trace interval condition acquired conditioned responses at a rate almost identical to that of the control animals in the long-trace interval condition. CR onset latencies were impaired for lesioned animals. Peak latencies were not different, indicating no difference in the adaptiveness of the CRs. These results suggest that while the hippocampus is important for acquisition of trace eyeblink conditioning, performance also depends on the parameters used for the task. In particular, the relative lengths of the CS period and the trace interval appear to be important.     

Dorsal hippocampal involvement in conditioned-response timing and maintenance of temporal information in the absence of the CS

Involvement of the dorsal hippocampus (DHPC) in conditioned-response timing and maintaining temporal information across time gaps was examined in an appetitive Pavlovian conditioning task, in which rats with sham and DHPC lesions were first conditioned to a 15-s visual cue. After acquisition, the subjects received a series of non-reinforced test trials, on which the visual cue was extended (45 s) and gaps of different duration, 0.5, 2.5, and 7.5 s, interrupted the early portion of the cue. Dorsal hippocampal-lesioned subjects underestimated the target duration of 15 s and showed broader response distributions than the control subjects on the no-gap trials in the first few blocks of test, but the accuracy and precision of their timing reached the level of that of the control subjects by the last block. On the gap trials, the DHPC-lesioned subjects showed greater rightward shifts in response distributions than the control subjects. We discussed these lesion effects in terms of temporal versus non-temporal processing (response inhibition, generalisation decrement, and inhibitory conditioning).     

Neural activity in the medial geniculate nucleus during auditory trace conditioning

 In classical trace conditioning the acquisition of a conditioned response (CR) is possible even though an interval (the trace interval) elapses between the conditioned stimulus (CS) and unconditioned stimulus (US). This implies that some neural representation of the CS (the stimulus trace) is able to support association between the two stimuli. The medial geniculate nucleus (MGN), particularly the medial division (mMGN), has been identified as one site in the auditory pathway where associative related changes in neural activity occur. If neurons in the MGN are involved in such a sensory trace and in acquisition of a CR, then it is expected that activity following an acoustic CS should be related to both stimulus and response. This study examined the extracellular activity of single units in the MGN during differential auditory trace conditioning of the rabbit nictitating membrane response (NMR). Two 150-ms tones (600 Hz and 1200 Hz) served as CS+ and CS–, and the US was periorbital electrostimulation. Changes in activity during the stimulus and trace interval were largest in the medial and dorsal MGN divisions on CS+ trials and on trials in which a CR was made. Examination of probe stimuli of short (50 ms) and long (600 ms) duration suggested that both CR latency and activity changes in the trace interval were related to stimulus duration and time-locked to stimulus offset. Comparisons of neural activity on the basis of fast or slow CR responses revealed different patterns of response – activity on fast CR trials was generally greater and tended to occur earlier. These results suggest that MGN neurons are involved in the maintenance of a sensory memory trace and possibly play a part in CR generation and timing. Received: 2 June 1995 / Accepted: 30 August 1996     

Hippocampectomy disrupts trace eye-blink conditioning in rabbits

The role of the hippocampus (HPC) in trace eye-blink conditioning was evaluated using a 100-ms tone conditioned stimulus (CS), a 300- or 500-ms trace interval, and a 150-ms air puff unconditioned stimulus (UCS). Rabbits received complete hippocampectomy (dorsal & ventral), sham lesions, or neocortical lesions. Hippocampectomy produced differential effects in relation to the trace interval used. With a 300-ms trace interval, HPC-lesioned Ss showed profound resistance to extinction after acquisition. With a 500-ms trace interval, HPC-lesioned Ss did not learn the task (only 22% conditioned responses (CRs) after 25 sessions, whereas controls showed greater than 80% after 10 sessions), and on the few trials in which a CR occurred, most were "nonadaptive" short-latency CRs (i.e., they started during or just after the CS and always terminated prior to UCS onset). The authors conclude that the HPC encodes a temporal relationship between CS and UCS, and when the trace interval is long enough (e.g., 500 ms), that the HPC is necessary for associative learning of the conditioned eye-blink response.     

Ketaset-Rompun extends the temporal gradient for hypothermia-induced retrograde amnesia.

In studies of experimentally induced retrograde amnesia (RA), as the interval between training and the amnestic treatment is lengthened, amnesia decreases (4). This temporal gradient for RA has been reported with a wide variety of amnestic agents, including RA produced by thermoregulatory disturbances (8). This temporal gradient for RA is not unlike certain characteristics of classical conditioning, where weaker conditioned responding occurs when the interval between the conditioned stimulus (CS) and unconditioned stimulus (US) is lengthened. Furthermore, there is evidence that administration of anesthetics can lengthen the "effective conditioning" interval between the CS and US, as demonstrated in a conditioned taste-aversion (CTA) procedure (10). In that study, little conditioning was observed when a 3-h delay (or more) was incorporated between presentations of the CS (flavor) and the US (toxin). However, if subjects were anesthetized immediately after the CS was delivered and remained anesthetized during the CS-US interval, strong conditioning was observed with CS-US intervals of up to 9 h. The aim of the present experiment was to determine if the temporal gradient for hypothermia-induced RA could also be lengthened. That is, we tested whether the interval between training and hypothermia treatment could be lengthened by anesthetizing subjects with Ketaset-Rompun. The results indicate that the training-to-amnestic agent interval could be lengthened within moderate limits. The implications for hypothermia-induced RA is further discussed.     

Differential effects of 8-OH-DPAT on two forms of appetitive pavlovian conditioning in the rat

Rats were trained on an appetitive Pavlovian conditioning task in which the conditioned stimulus (CS) was either localized (a light in the food tray) or nonlocalized (an increase in the general level of illumination). The conditioned response (CR) of approaching the site of food delivery in the presence of the CS was monitored. Presession treatment with the 5-HT1A agonist 8-OH-DPAT (subcutaneous injections at a dose of 0.15 mg/kg) retarded acquisition of the CR, but only when the localized CS was used. The results confirm the general proposal that serotonergic processes are involved in learning. The selective effect of the drug is not to be explained in terms of its motor effects and is consistent with the specific suggestion that systemic administration of 8-OH-DPAT is especially effective in disrupting learning tasks mediated by hippocampal mechanisms.     

A developmental comparison of trace and delay eyeblink conditioning in rats using matching interstimulus intervals

The effects of conditioning paradigm and interstimulus interval (ISI) were evaluated in this study of the development of associative learning in rats. The acquisition of classical eyeblink conditioning (EBC) was examined in two paradigms (trace vs. delay), with three different ISIs (short, medium, or long) at two ages (postnatal Days 21-23 or 29-31). These data provide the first parametric analysis of ISI in developing animals trained with trace EBC procedures. Further, by comparing trace and delay EBC, it was determined that when ISI is held constant, acquisition is similar for both conditioning paradigms regardless of developmental age. This suggests a similar ontogeny of associative processes for delay and trace EBC that relies on common neural substrates. However, conditioned response timing (onset and peak latencies) was affected by age and paradigm, suggesting that different neural mechanisms may play a role in timing delay versus trace conditioned responses.     

Ontogeny of delay versus trace eyeblink conditioning in the rat

The ontogeny of delay versus trace eyeblink conditioning was examined in 19-, 23-, and 30-day-old rat pups. Pairings of a tone conditioned stimulus (CS) and periocular shock unconditioned stimulus (US; 100-ms) were presented in one of three conditioning paradigms: standard delay [380-ms CS, 280-ms interstimulus interval (ISI)], trace (380-ms CS, 500-ms trace interval), or long-delay (980-ms CS, 880-ms ISI). The results of two experiments indicated that standard delay conditioning emerged between 19 and 23 days of age whereas trace and long-delay eyeblink conditioning emerged more slowly from postnatal Days 19 to 30. Because the acquisition profile for long-delay paralleled that of trace and not standard delay, it appears that the relative deficits in the emergence of trace eyeblink conditioning during development reflect difficulty in forming associations over long ISIs rather than the short-term memory demands of the trace conditioning paradigm.     

Multiple functions of context during conditioning: a developmental analysis

Contextual stimuli may influence conditioned behavior in at least two ways (e.g. Bouton & Bolles, 1985). By becoming associated with the unconditioned stimulus (US), context cues can acquire excitatory strength that facilitates responding to a phasic conditioned stimulus (CS). The context also can function to clarify the meaning of an ambiguous CS. Data obtained with an appetitive Pavlovian conditioning paradigm suggest that the processes mediating these two influences of context are dissociated during development. Evidence of context-US associations was observed in rats that began training on Postnatal Day 17, but no evidence for a disambiguation function was found until pups were 20- to 23-days-old. Evidence for a context-US association was obtained by demonstrating that US alone presentations in the training context restored conditioned responding to an extinguished CS. Evidence for a disambiguation function was obtained by demonstrating that a context shift, concurrent with extinction of responding to a phasic CS, preserved responding to the CS when the subjects were subsequently tested in the training context. These findings were discussed in relation to (a) the development of the rat's ability to use relational representations, and (b) Nadel and Zola-Morgan's (1984) hypothesis linking hippocampal maturation to the role of context during development.     

Conditioned facilitation of the unconditioned reflex after classical eyeblink conditioning.

The present study (N=40) investigated the time-course of conditioned facilitation of the unconditioned eyeblink reflex (UR). In a single-cue delay classical conditioning procedure, a tone conditioned stimulus (CS) signaled an airpuff unconditioned stimulus (airpuff US) to the eye. A paired group received 40 trials of CS/US presentations with an interstimulus interval (ISI) of 200 ms. An unpaired group received an equal number of explicitly unpaired presentations of the CS and US. Thereafter, eyeblink reflex facilitation was assessed by presenting 94 dB white noise (noise US) 10, 30, 50, 100, 150, and 1000 ms after CS onset. In the paired group, URs were significantly increased as early as 100 ms after CS onset compared to the unpaired group. This reflex facilitation was correlated with CR magnitude, indicating that conditioned facilitation of eyeblink URs indexes an early, automatic, preattentive stage in CR formation.     

Amygdala central nucleus function is necessary for learning, but not expression, of conditioned auditory orienting

In Pavlovian appetitive conditioning, rats often acquire 2 classes of conditioned responses: those whose form is determined by the reinforcer, and those whose form is determined by characteristics of the conditioned stimulus (CS). Consistent with the results of previous lesion studies, reversible inactivation of amygdala central nucleus function during pairings of an auditory CS with food prevented the acquisition of conditioned orienting responses specific to auditory CSs, whereas food-related conditioned behaviors were acquired normally. Neither inactivation nor posttraining neurotoxic lesions of the central nucleus affected the expression of previously acquired conditioned orienting. Thus, although the central nucleus is critical to the acquisition of information required for conditioned orienting to auditory cues, it is not needed for maintaining this information for later use.     

Hemispheric Asymmetry and Human Electrodermal Conditioning: The Dichotic Extinction Paradigm

It was hypothesized that classically conditioned responses to an auditory verbal CS initially presented only to the left cerebral hemisphere would be greater than when the same CS was presented to the right hemisphere. The experiment consisted of three different phases in a dichotic extinction paradigm. During the habituation phase the CS+ and CS? were presented binaurally, and separated. All subjects were treated alike. During the acquisition phase the conditioning subjects had the CS+ but not the CS? followed by the UCS. The CS+ and CS? were presented binaurally and separated. The control subjects received no UCS presentations. During the dichotic extinction phase, half of the conditioned subjects had the CS+ presented in the right ear (i.e., with initial left hemisphere input), with the CS? simultaneously presented in the left ear (i.e., with initial right hemisphere input). The other half had the earphones reversed. The control group was similar to the two conditioning groups. During the acquisition phase, significantly larger electrodermal responses were observed to the CS+ compared to the CS? in the conditioning groups, but not in the control group. During the dichotic extinction phase, significantly more resistance to extinction was observed in the CS+ Right-Ear group compared to the CS+ Left-Ear group, with the control group revealing virtually no responding at all. It was concluded that the present results have demonstrated effects of cerebral asymmetry on rate of extinction of the classically conditioned response.     

The effects of amygdala lesions on conditioned stimulus-potentiated eating in rats

Both control rats and rats with neurotoxic lesions of the amygdala central nucleus ate more food during presentations of a conditioned stimulus (CS) previously paired with food than during an unpaired CS. This potentiation occurred regardless of whether the food was presented in its usual place or in a different location. By contrast, rats with neurotoxic lesions of basolateral amygdala showed no evidence for conditioned potentiation of eating. These results are considered in the context of anatomical projections from these amygdalar areas to other brain regions involved in feeding, and the role of amygdala subregions in the acquisition of motivational value in conditioning.     

Temporal patterns of the rabbit's nictitating membrane response to compound and component stimuli under mixed CS-US intervals

The acquisition of the rabbit's nictitating membrane response to a tone and light compound and to its components was examined when compound presentations were reinforced at one conditioned stimulus-unconditioned stimulus (CS-US) interval and individual component presentations were reinforced at another CS-US interval. Examination of the time course of the CRs revealed that (a) despite the mixture of CS-US intervals, conditioned response (CR) timing remained accurate, that is, CRs reached their peaks at the alternative points of US delivery; (b) the momentary magnitude of the CR to the compound was predominantly an additive function of the CR magnitude to the individual components; but (c) there was modest evidence of differentiation between the compound as a unit and the individual components. The results are discussed in terms of their implications for the study of the neural substrates of temporal and sensory integration as they modulate CR acquisition.     

Fear acquisition requires awareness in trace but not delay conditioning

The present study explored fear acquisition in differential delay versus trace conditioning in regard to the potential role of the acquired contingency awareness. One of two neutral pictures (CS+) either coterminated with (delay group; n=32) or was followed by the aversive unconditioned stimulus (UCS) after CS offset (trace group; n=32), while startle blink and skin conductance responses (SCR) were measured. As expected, the acquisition of conditioned startle potentiation in delay conditioning was independent of contingency awareness. In contrast, fear-potentiated startle in trace conditioning was only observed for those participants who were aware of the CS-UCS contingencies. SCR conditioning was generally only obtained for aware participants. The present results suggest a more implicit learning process in delay fear conditioning, whereas the explicit acquisition of contingency awareness might be a prerequisite for trace fear conditioning.     

Generalization of appetitive conditioned responses

A stimulus (conditioned stimulus, CS) associated with an appetitive unconditioned stimulus (US) acquires positive properties and elicits appetitive conditioned responses (CR). Such associative learning has been examined extensively in animals with food as the US, and results are used to explain psychopathologies (e.g., substance‐related disorders or obesity). Human studies on appetitive conditioning exist, too, but we still know little about generalization processes. Understanding these processes may explain why stimuli not associated with a drug, for instance, can elicit craving. Forty‐seven hungry participants underwent an appetitive conditioning protocol during which one of two circles with different diameters (CS+) became associated with an appetitive US (chocolate or salty pretzel, according to participants’ preference) but never the other circle (CS?). During generalization, US were delivered twice and the two CS were presented again plus four circles (generalization stimuli, GS) with gradually increasing diameters from CS? to CS+. We found successful appetitive conditioning as reflected in appetitive subjective ratings (positive valence, higher contingency) and physiological responses (startle attenuation and larger skin conductance responses) to CS+ versus CS?, and, importantly, both measures confirmed generalization as indicated by generalization gradients. Small changes in CS‐US contingency during generalization may have weakened generalization processes on the physiological level. Considering that appetitive conditioned responses can be generalized to non‐US‐associated stimuli, a next important step would be to investigate risk factors that mediate overgeneralization.     

Single neurons in the dentate gyrus and CA1 of the hippocampus exhibit inverse patterns of encoding during trace fear conditioning

Trace fear conditioning is a hippocampus-dependent learning task that requires the association of an auditory conditioned stimulus (CS) and a shock unconditioned stimulus (US) that are separated by a 20-s trace interval. Single-neuron activity was recorded simultaneously from the dentate gyrus (DG) and CA1 of rats during unpaired pseudoconditioning and subsequent trace fear conditioning. Single neurons in DG showed a progressive increase in learning-related activity to the CS and US across trace fear conditioning. Single neurons in CA1 showed an early increase in responding to the CS, which developed into a decrease in firing later in trace conditioning. Correlation analyses showed that DG and CA1 units exhibit inverse patterns of responding to the CS during trace fear conditioning.     

Slow cortical potentials in human aversive trace conditioning

The aim of the present study was to better understand the cortical structures and neuronal processes involved in aversive differential trace conditioning in healthy subjects. According to previous findings in literature, we tested whether the stimulus preceding negativity (SPN) in an emotional trace-conditioning paradigm shows a frontocentral maximum reflecting affective anticipation, e.g. in the anterior cingulate, or a centroparietal maximum reflecting time estimation and sensory anticipation. Two distinct SPN intervals were analyzed, one during the presentation of the CS (comparable to delay conditioning) and another one after CS offset (trace conditioning). In the CS+ condition, where subjects anticipated the onset of an aversive electrical stimulus after CS+ offset, a significantly larger negativity than in the unpaired (CS-) condition was present. SPN revealed a sustained midcentral and posterior parietal negativity during both SPN intervals. Differences between the two analyzed SPN intervals pointed towards occipital activity being found in the first interval (delay), but not in the second (trace). Aversive conditioning paradigms with longer trace intervals seem to rely upon a similar activation pattern as cognitive stimulus anticipation.     

Timing of conditioned eyeblink responses is impaired in children with attention-deficit/hyperactivity disorder

Structural changes of the cerebellum have been reported in several psychiatric diseases like schizophrenia, autism and attention-deficit/hyperactivity disorder (ADHD). Beside behavioral deficits children with ADHD often show slight motor abnormalities. Cerebellar malfunction may contribute. The cerebellum is a structure essential for motor coordination, various forms of motor learning and timing of motor responses. In the present study, eyeblink conditioning was applied to investigate learning and timing of motor responses both in children with ADHD and children with cerebellar lesions. Acquisition, timing and extinction of conditioned eyeblink responses were investigated in children with ADHD, children with chronic surgical cerebellar lesions and controls using a standard delay paradigm with two different interstimulus intervals. Timing of conditioned eyeblink responses was significantly impaired in children with ADHD in the long interstimulus interval condition. Also in children with cerebellar lesions conditioned responses (CR) tended to occur earlier than in controls. Incidences of CRs were significantly reduced in children with cerebellar lesions and tended to be less in children with ADHD than in controls. Extinction of the CRs was impaired in children with cerebellar lesions in both interstimulus interval conditions and in children with ADHD in the long interstimulus interval condition. Cerebellar malfunction may contribute to disordered eyeblink conditioning in ADHD. However, because CR abnormalities differed between ADHD and cerebellar subjects, dysfunction of non-cerebellar structures cannot be excluded.