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PubMed2006

Molecular physiology of the neural circuit for calcineurin-dependent associative learning in Caenorhabditis elegans.

How learning and memory is controlled at the neural circuit level is a fundamental question in neuroscience. However, molecular and cellular dissection of the neural circuits underlying learning and memory is extremely complicated in higher animals. Here, we report a simple neural circuit for learning behavior in Caenorhabditis elegans, where the calcium-activated phosphatase, calcineurin, acts as an essential modulator. The calcineurin mutant tax-6 showed defective feeding state-dependent learning behavior for temperature and salt. Surprisingly, defective associative learning between temperature and feeding state was caused by malfunctions of two pairs of directly connected interneurons, AIZ and RIA, in the mature nervous system. Monitoring temperature-evoked Ca2+ concentration changes in the AIZ-RIA neural pathway revealed that starvation, a conditioning factor, downregulated AIZ activity through calcineurin during associative learning between temperature and feeding state. Our results demonstrate the molecular and physiological mechanisms of a simple neural circuit for calcineurin-mediated associative learning behavior.

Animals↗

PubMed1994

Associative learning modifies the shortening reflex in the semi-intact leech Hirudo medicinalis: effects of pairing, predictability, and CS preexposure.

Three experiments addressed the importance of the inter-event relationships of contiguity and contingency for associative learning in the semi-intact leech. It was found that both of these relationships are important for the leech to acquire a learned association between a touch (conditional stimulus, CS) and shock (unconditional stimulus, US). The learning can be extinguished if training is followed by explicitly unpaired presentations of the CS and US, which removes the contiguity between the stimuli. Learning is degraded by the introduction of unpredicted USs, as well as by unreinforced presentations of the CS (CS preexposure), both manipulations reduce the contingency between the CS and US. These results suggest that the associative process in both vertebrates and invertebrates share considerable functional similarity in the inter-event relationships important to learning.

Animals↗

PubMed1999

Responses of macaque perirhinal neurons during and after visual stimulus association learning.

Recent lesion studies have implicated the perirhinal cortex in learning that two objects are associated, i.e., visual association learning. In this experiment we tested whether neuronal responses to associated stimuli in perirhinal cortex are altered over the course of learning. Neurons were recorded from monkeys during performance of a visual discrimination task in which a predictor stimulus was followed, after a delay, by a GO or NO-GO choice stimulus. Association learning had two major influences on neuronal responses. First, responses to frequently paired predictor-choice stimuli were more similar to one another than was the case with infrequently paired stimuli. Second, the magnitude of activity during the delay was correlated with the magnitude of responses to both the predictor and choice stimuli. Both of these learning effects were found only for stimulus pairs that had been associated on at least 2 d of training. Early in training, the delay activity was correlated only with the response to the predictor stimuli. Thus, with long-term training, perirhinal neurons tend to link the representations of temporally associated stimuli.

Animals↗

PubMed2004

Antennal movements reveal associative learning in the American cockroach Periplaneta americana.

Using antennal movements as an indicator of learning and retention, an associative learning paradigm has been developed to investigate associative memory between visual and olfactory stimuli. Experiments were performed on the restrained cockroach Periplaneta americana, which normally moves its antennae towards a localized odor source. Such "antennal projection responses" (APRs) are exploited to demonstrate long-term memory, where an APR is elicited by a conditioned stimulus (CS; green light point source) paired with a spatially coincident odor [the unconditioned stimulus (US)]. Association of the CS with the US is established after five trials. Before training, a visual cue alone does not elicit an APR. This behavior is elicited by a visual cue only after pairing it with an odor stimulus. The acquired APR to the green light cue persists for up to 72 h, indicative of long-term memory. This paradigm is thus suitable for future studies of neural correlates of learning and memory on restrained animals.

Animals↗

PubMed2006

Tonic dopaminergic stimulation impairs associative learning in healthy subjects.

Endogenous dopamine plays a central role in salience coding during associative learning. Administration of the dopamine precursor levodopa enhances learning in healthy subjects and stroke patients. Because levodopa increases both phasic and tonic dopaminergic neurotransmission, the critical mechanism mediating the enhancement of learning is unresolved. We here probed how selective tonic dopaminergic stimulation affects associative learning. Forty healthy subjects were trained in a novel vocabulary of 45 concrete nouns over the course of 5 consecutive training days in a prospective, randomized, double-blind, placebo-controlled design. Subjects received the tonically stimulating dopamine-receptor agonist pergolide (0.1 mg) vs placebo 120 min before training on each training day. The dopamine agonist significantly impaired novel word learning compared to placebo. This learning decrement persisted up to the last follow-up 4 weeks post-training. Subjects treated with pergolide also showed restricted emotional responses compared to the PLACEBO group. The extent of 'flattened' affect with pergolide was related to the degree of learning inhibition. These findings suggest that tonic occupation of dopamine receptors impairs learning by competition with phasic dopamine signals. Thus, phasic signaling seems to be the critical mechanism by which dopamine enhances associative learning in healthy subjects and stroke patients.

Adult↗

PubMed2006

Methylphenidate can reduce selectivity in associative learning in an aversive trace conditioning task.

There are good grounds to expect that methylphenidate (MP) should enhance cognitive function. However, experimental evidence on this point is scant. The present study therefore examined the effects of MP on learning the association between a conditioned stimulus (CS, in this case, noise) and an unconditioned stimulus (UCS, in this case, footshock) in an aversive variant of a trace conditioning procedure. Learning was measured off-the-baseline as conditioned suppression of drinking (both latencies to drink, expressed as suppression ratios, and the amount drunk, expressed as the number of licks, in the presence of the CS). In addition to the measures of discrete cue conditioning, MP effects on contextual conditioning were measured as suppression to apparatus cues and an experimental background stimulus. MP was administered at 1 or 5 mg/kg prior to conditioning sessions. As attention deficit hyperactivity disorder (ADHD) has been characterized as involving a ;wide attentional window' (e.g. Shalev and Tsal, 2003), it was predicted that MP, as the treatment of choice for ADHD, should increase selectivity (narrowing the attentional window). This outcome would show as reduced levels of conditioning (compared to control rats) to less informative trace and contextual cues present during conditioning. Contrary to prediction, both 1 and 5 mg/kg MP increased learning about all the available stimuli, including the less informative trace CS and the background stimulus. These findings are consistent with reduced rather than increased selectivity in learning (because of increased rather than decreased conditioning to weak cues) under MP.

Animals↗

PubMed2001

An autoassociative neural network model of paired-associate learning.

Hebbian heteroassociative learning is inherently asymmetric. Storing a forward association, from item A to item B, enables recall of B (given A), but does not permit recall of A (given B). Recurrent networks can solve this problem by associating A to B and B back to A. In these recurrent networks, the forward and backward associations can be differentially weighted to account for asymmetries in recall performance. In the special case of equal strength forward and backward weights, these recurrent networks can be modeled as a single autoassociative network where A and B are two parts of a single, stored pattern. We analyze a general, recurrent neural network model of associative memory and examine its ability to fit a rich set of experimental data on human associative learning. The model fits the data significantly better when the forward and backward storage strengths are highly correlated than when they are less correlated. This network-based analysis of associative learning supports the view that associations between symbolic elements are better conceptualized as a blending of two ideas into a single unit than as separately modifiable forward and backward associations linking representations in memory.

Association Learning↗

PubMed1996

The effects of lesions to the mammillary region and the hippocampus on conditional associative learning by rats.

Rats with extensive lesions to the mammillary body region, the hippocampus, or rats which had received a control operation were trained postoperatively on two visuo-spatial conditional associative learning tasks in which they had to learn to associate spatial cues with particular visual/auditory stimuli. The animals were subsequently trained on a spatial working memory task, the eight-arm radial maze. Rats with lesions to the mammillary body region were able to acquire the conditional associative learning tasks at a rate comparable to that of operated control animals, whereas those with hippocampal lesions were not. By contrast, rats with a lesion of the mammillary body region or the hippocampus were significantly impaired in comparison with the operated control animals in the radial maze. The findings suggest that lesions to the mammillary body region impair spatial working memory without affecting the capacity to associate particular exteroceptive cues with spatial locations.

Animals↗

PubMed2005

Postsynaptic receptor trafficking underlying a form of associative learning.

To elucidate molecular, cellular, and circuit changes that occur in the brain during learning, we investigated the role of a glutamate receptor subtype in fear conditioning. In this form of learning, animals associate two stimuli, such as a tone and a shock. Here we report that fear conditioning drives AMPA-type glutamate receptors into the synapse of a large fraction of postsynaptic neurons in the lateral amygdala, a brain structure essential for this learning process. Furthermore, memory was reduced if AMPA receptor synaptic incorporation was blocked in as few as 10 to 20% of lateral amygdala neurons. Thus, the encoding of memories in the lateral amygdala is mediated by AMPA receptor trafficking, is widely distributed, and displays little redundancy.

Amygdala↗

PubMed2001

Molecular biology and anatomy of Drosophila olfactory associative learning.

Most of our current knowledge of olfactory associative learning in Drosophila comes from the behavioral and molecular analysis of mutants that fail to learn. The identities of the genes affected in these mutants implicate new signaling pathways as mediators of associative learning. The expression patterns of these genes provide insight into the neuroanatomical areas that underlie learning. In recent years, there have been great strides in understanding the molecular and neuroanatomical basis for olfaction in insects. It is now clear that much of the association between the conditioned stimuli and the unconditioned stimuli in olfactory learning occurs within mushroom bodies - third order olfactory neurons within the central brain. In this review, we discuss the nature of the behavioral tasks, the molecules, and the neuronal circuits involved in olfactory learning in Drosophila.

Animals↗

PubMed2001

What is learned in patterning discriminations? Further tests of configural accounts of associative learning in human electrodermal conditioning.

Two Pavlovian SCR conditioning experiments investigated positive and negative patterning discriminations in humans by means of transfer tests. In Experiment 1, positive patterning (A-,B-,AB+) was trained interleaved with non-reinforced presentations of an additional stimulus (C-). Then responding to new compounds consisting of either already trained elements (AC,BC) or new elements (DE) was examined. In Experiment 2, negative patterning (A+,B+,AB-) was trained interleaved with reinforced presentations of an additional stimulus (C+). Again, we examined responding to new compounds consisting of either already trained elements (AC,BC) or new elements (DE). In both experiments the initial patterning discrimination was solved successfully. The response patterns to the test compounds in both experiments were in contradiction to configural accounts of associative learning. In positive patterning human participants seemed to utilize 'number' or some other abstract feature in preference to available concrete stimuli. In negative patterning the abstract dimension of 'separate-versus-together' or 'opposite' was used.

Adult↗

PubMed1981

Associative learning in Aplysia: cellular correlates supporting a conditioned fear hypothesis.

Aversive associative learning in Aplysia california survives restraint of the animal and surgical exposure of the central nervous system. The learning is expressed in the intracellularly recorded activity of identified motor neurons mediating three different defensive behaviors: escape locomotion, inking, and siphon withdrawal. In each case, animals that had previously received paired training showed significant facilitation of synaptic input to motor neurons during test stimulation in the presence of the conditioned stimulus. Animals without such training showed no facilitation of input to the motor neurons. Resting potential and input resistance appeared unaffected by conditioning and were not altered by application of the conditioned stimulus. These results show that the conditioned facilitation of defensive responses cannot be explained by subthreshold actions of the conditioned stimulus on the motor neurons and support the hypothesis that Aplysia learn to associate the conditioned stimulus with a fearlike central state.

Action Potentials↗

PubMed1997

Associative learning in children with perinatal brain injury.

Associate learning for visual nonverbal and auditory verbal items was examined in 21 children with spastic diplegic cerebral palsy (SDCP) and 28 healthy children using four paired associate tasks. SDCP children showed poorer performance than the comparison group for learning pairs that required visual nonverbal responses, regardless of the stimulus modality. Within the SDCP group, lesion severity was assessed in 17 of the children. Lesion severity was related to the level of performance on paired associate tasks requiring visual nonverbal responses; lesion severity did not reach statistical significance for tasks requiring auditory verbal responses. The study suggests: (1) periventricular white matter regions are important for the development of basic learning processes, such as associative learning, and (2) learning of visual nonverbal material is disproportionately affected following white matter injury early in life.

Adolescent↗

PubMed1995

Relational object association learning in rats with hippocampal lesions.

The contribution of the hippocampus to relational association learning was examined using a paired associate task in the rat. Using a go/no-go paradigm, rats were trained to respond quickly on positive pair presentations, and to withhold responding on negative pair presentations. Using four constant three-dimensional objects (A, B, C and D), pairing of either AB or CD was assigned as positive pairs, whereas the other possible object combinations (AC, DB, AD, and CB) were never associated with reinforcement. A further type of negative pair trial was also studied which involved object-pairs composed of one of the four constant objects and another object, which varied from trial, and which was selected from a set of 120 junk objects. Electrolytic lesions of the hippocampus were performed on the rats which had previously acquired the paired associate task. Results indicated that hippocampal lesions did not significantly impair the ability of the animals to solve the paired associate task. To study further the exact nature of the learned associations, a transfer test which involved a reversal of the spatial position within each of the six positive as well as negative pair-types was conducted. Results of the transfer test demonstrated that hippocampal-lesioned as well as control rats performed normally by distinguishing the positive from negative pairs regardless of the relative location of stimuli within the object pairs. These data suggest that the hippocampus might not be critical for relational object associations.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed2003

Learning associations between places and visual cues without learning to navigate: neither fornix nor entorhinal cortex is required.

Rats with fornix transection, or with cytotoxic retrohippocampal lesions that removed entorhinal cortex plus ventral subiculum, performed a task that permits incidental learning about either allocentric (Allo) or egocentric (Ego) spatial cues without the need to navigate by them. Rats learned eight visual discriminations among computer-displayed scenes in a Y-maze, using the constant-negative paradigm. Every discrimination problem included two familiar scenes (constants) and many less familiar scenes (variables). On each trial, the rats chose between a constant and a variable scene, with the choice of the variable rewarded. In six problems, the two constant scenes had correlated spatial properties, either Allo (each constant appeared always in the same maze arm) or Ego (each constant always appeared in a fixed direction from the start arm) or both (Allo + Ego). In two No-Cue (NC) problems, the two constants appeared in randomly determined arms and directions. Intact rats learn problems with an added Allo or Ego cue faster than NC problems; this facilitation provides indirect evidence that they learn the associations between scenes and spatial cues, even though that is not required for problem solution. Fornix and retrohippocampal-lesioned groups learned NC problems at a similar rate to sham-operated controls and showed as much facilitation of learning by added spatial cues as did the controls; therefore, both lesion groups must have encoded the spatial cues and have incidentally learned their associations with particular constant scenes. Similar facilitation was seen in subgroups that had short or long prior experience with the apparatus and task. Therefore, neither major hippocampal input-output system is crucial for learning about allocentric or egocentric cues in this paradigm, which does not require rats to control their choices or navigation directly by spatial cues.

Animals↗

PubMed2001

Impaired associative learning in chronic schizophrenics and their first-degree relatives: a study of latent inhibition and the Kamin blocking effect.

The performance of chronic schizophrenic probands (n=21), their first-degree schizotypal (22) and non-schizotypal (19) relatives, and normal controls (24), was measured in two associative learning paradigms, latent inhibition and the Kamin blocking effect. These paradigms assess the effects on learning of initial exposure to other learning contingencies. The normal subjects showed latent inhibition (retarded learning of an association between a burst of white noise and a visually displayed counter increment, if the subject had first been pre-exposed to the white noise without any other consequence) and Kamin blocking (retarded learning of an association between two visual stimuli, if the conditioned stimulus was presented simultaneously with a second, already conditioned stimulus). The schizophrenic probands and both the schizotypal and non-schizotypal relatives were severely impaired in basic associative learning, performing much worse than the normal subjects in the control conditions (i.e. those lacking stimulus pre-exposure of any kind) of both the latent inhibition and the Kamin paradigms and also showed a loss of the normal latent inhibition and Kamin blocking effects. The performance of the three clinically defined groups was statistically indistinguishable. These findings contrast with previous reports of the performance of normal subjects classified as schizotypal by questionnaire, who are not impaired in basic associative learning, and are particularly fast to learn after stimulus pre-exposure. The results question the assumption that high schizotypy, as assessed by questionnaire, is like schizotypy in schizophrenic kin. The severe impairment in basic associative learning in schizophrenic patients and their kin warrants further investigation.

Aged↗

PubMed1984

Paired-associate learning and priming effects in amnesia: a neuropsychological study.

Despite severe deficits of recall and recognition, amnesic patients can exhibit normal priming effects. Amnesic patients have also been reported to perform well on tests of paired-associate learning that involve related word pairs (e.g., table-chair). The present study investigated the role of priming effects in paired-associate learning. Experiment 1 illustrated the distinction between the memory impairment of amnesic patients and their intact priming ability. Amnesic patients were markedly deficient in learning unrelated word pairs, despite exhibiting normal priming as measured by a word-completion test involving the same words. In Experiment 2A, amnesic patients showed good paired-associate learning for related word pairs, though control subjects still performed significantly better. In addition, the good performance by amnesic patients was short-lived, and performance fell to baseline after a 2-hr delay. Control subjects performed well above baseline at all delay conditions. Experiment 2B showed that the forgetting of related word pairs by amnesic patients followed the same time course as the decay of word priming. Experiment 3 showed that amnesic patients were as good as control subjects at learning related word pairs when incidental learning and test procedures were used (a word-association test). The advantage of control subjects over amnesic patients in Experiments 2A and 2B could therefore be attributed to the explicit learning instructions that are standard in paired-associate tests. Finally, Experiment 4 showed that amnesic patients exhibited normal priming when they were asked to "free associate" to words (e.g., child) that were semantically related to previously presented words (e.g., baby). The results indicate that both priming effects and paired-associate learning of related word pairs depend on activation, a process that is preserved in amnesia. Activation can account for the findings of good performance by amnesic patients on tests of word priming (Experiments 1 and 2B), related paired associates (Experiments 2A and 2B), and word association (Experiments 3 and 4). Activation is a transient phenomenon presumed to operate on and facilitate access to preexisting representations. Control subjects can establish new associations and can strengthen preexisting associations by engaging processes that are impaired in amnesia. As a result, when explicit learning instructions are used to test paired-associate learning of related word pairs, control subjects can learn better and can remember longer than can amnesic patients (Experiments 2A and 2B).(ABSTRACT TRUNCATED AT 400 WORDS)

Alcohol Amnestic Disorder↗

PubMed2005

Developmental regulation of cognitive abilities: modified composition of a molecular switch turns on associative learning.

N-methyl-D-aspartate receptors (NMDARs) act as molecular coincidence detectors and allow for association or dissociation between pre- and postsynaptic neurons. NMDA receptors are central to remodeling of synaptic connections during postnatal development and associative learning abilities in adults. The ability to remodel neural networks is altered during postnatal development, possibly due to a change in the composition of NMDARs. That is, as forebrain systems (and cerebellum) develop, synaptic NR2B-containing NMDARs (NR2B-NMDARs) are replaced by NR2A-containing NMDARs (NR2A-NMDARs) and NR2B-NMDARs move to extrasynaptic sites. During the initial phase of the switch, synapses contain both NR2A- and NR2B-NMDARs and both long-term potentiation and long-term depression are enhanced. As NMDAR subunit expression decreases and NR2A-NMDARs come to predominate in the synapse, channel function and synaptic plasticity are reduced, and remodeling ability dissipates. The end result is a balance of plasticity and stability that is optimal for information processing and storage. Associative learning abilities involving different sensory modalities emerge sequentially, in accordance with synaptic maturation in related cortical and underlying brain structures. Thus, developmental alterations in NMDAR composition that occur at different ages in various brain structures may explain the protracted nature of the maturation of various associative learning abilities.

Animals↗