Hippocampal lesions alter conditioning to conditional and contextual stimuli

Factors affecting graded and ungraded memory loss following hippocampal lesions

This review evaluates three current theories--Standard Consolidation (Squire & Wixted, 2011), Overshadowing (Sutherland, Sparks, & Lehmann, 2010), and Multiple Trace-Transformation (Winocur, Moscovitch, & Bontempi, 2010)--in terms of their ability to account for the role of the hippocampus in recent and remote memory in animals. Evidence, based on consistent findings from tests of spatial memory and memory for acquired food preferences, favours the transformation account, but this conclusion is undermined by inconsistent results from studies that measured contextual fear memory, probably the most commonly used test of hippocampal involvement in anterograde and retrograde memory. Resolution of this issue may depend on exercising greater control over critical factors (e.g., contextual environment, amount of pre-exposure to the conditioning chamber, the number and distribution of foot-shocks) that can affect the representation of the memory shortly after learning and over the long-term. Research strategies aimed at characterizing the neural basis of long-term consolidation/transformation, as well as other outstanding issues are discussed.

Parental precaution: neurobiological means and adaptive ends

Humans invest precious reproductive resources in just a few offspring, who remain vulnerable for an extended period of their lifetimes relative to other primates. Therefore, it is likely that humans evolved a rich precautionary psychology that assists in the formidable task of protecting offspring. In this review, we integrate precautionary behaviors during pregnancy and postpartum with the adaptive functions they may serve and what is known of their biological mediators, particularly brain systems motivating security and attachment. We highlight the role of reproductive hormones in (i) priming parental affiliation with young to incentivize offspring protection, (ii) focusing parental attention on cues of potential threat, and (iii) facilitating maternal defense against potentially dangerous conspecifics and predators. Throughout, we center discussion on adaptive responses to threats of disease, accident and assault as common causes of child mortality in the ancestral past.

Differential contribution of dorsal and ventral hippocampus to trace and delay fear conditioning

Trace conditioning relies on the maintained representation of a stimulus across a trace interval, and may involve a persistent trace of the conditioned stimulus (CS) and/or a contribution of contextual conditioning. The role of hippocampal structures in these two types of conditioning was studied by means of pretraining lesions and reversible inactivation of the hippocampus in rats. Similar levels of conditioning to a tone CS and to the context were obtained with a trace interval of 30 s. Neurotoxic lesions of the whole hippocampus or reversible muscimol inactivation of the ventral hippocampus impaired both contextual and tone freezing in both trace- and delay-conditioned rats. Dorsal hippocampal injections impaired contextual freezing and trace conditioning, but not delay conditioning. No dissociation between trace and contextual conditioning was observed under any of these conditions. Altogether, these data indicate that the ventral and dorsal parts of the hippocampus compute different aspects of trace conditioning, with the ventral hippocampus being involved in fear and anxiety processes, and the dorsal hippocampus in the temporal and contextual aspects of event representation.

Models and mechanisms of anxiety: evidence from startle studies

RATIONALE

Preclinical data indicates that threat stimuli elicit two classes of defensive behaviors, those that are associated with imminent danger and are characterized by flight or fight (fear), and those that are associated with temporally uncertain danger and are characterized by sustained apprehension and hypervigilance (anxiety).

OBJECTIVE

The objectives of the study are to (1) review evidence for a distinction between fear and anxiety in animal and human experimental models using the startle reflex as an operational measure of aversive states, (2) describe experimental models of anxiety, as opposed to fear, in humans, (3) examine the relevance of these models to clinical anxiety.

RESULTS

The distinction between phasic fear to imminent threat and sustained anxiety to temporally uncertain danger is suggested by psychopharmacological and behavioral evidence from ethological studies and can be traced back to distinct neuroanatomical systems, the amygdala and the bed nucleus of the stria terminalis. Experimental models of anxiety, not fear, are relevant to non-phobic anxiety disorders.

CONCLUSIONS

Progress in our understanding of normal and abnormal anxiety is critically dependent on our ability to model sustained aversive states to temporally uncertain threat.

Configural cue performance in identical twins discordant for posttraumatic stress disorder: theoretical implications for the role of hippocampal function

BACKGROUND

A significant subgroup of individuals with posttraumatic stress disorder (PTSD) exhibits chronic, unremitting symptomatology that has also been associated with smaller hippocampal volume. The hippocampus plays a significant role in configural processing of contextual cues that facilitates context-appropriate extinction of conditioned fear. We test the hypothesis that hippocampus-based configural processing deficits are a pre-existing vulnerability factor for unremitting forms of PTSD.

METHODS

Participants included male monozygotic twin pairs who were discordant for combat trauma. In 18 twin pairs the combat-exposed brother developed unremitting PTSD, whereas in 23 pairs the combat-exposed brother never developed PTSD. Participants were compared in the capacity to solve allocentric spatial processing tasks, and this performance was examined for its relationship to the severity of PTSD symptomatology and hippocampal volume.

RESULTS

Although not completely differentiated from overall IQ, PTSD combat veterans demonstrated significantly impaired performance in configural processing relative to non-PTSD combat veterans. Despite having neither combat-exposure nor PTSD, the unexposed co-twins of combat veterans with PTSD displayed the same decrements as their brothers. Deficits were significantly related to PTSD severity and hippocampal volume.

CONCLUSIONS

The current study provides the first evidence that the relevance of the hippocampus in PTSD might be related to pre-existing configural cue processing deficits that predispose individuals to develop unremitting forms of the disorder.

A neural network model of hippocampal-striatal-prefrontal interactions in contextual conditioning

The hippocampus is thought to be critical for encoding contextually bound memories and setting the context for ongoing behavior. However, the mechanisms by which the hippocampal-cortical system controls behavior are poorly understood. We propose a computational model in which the hippocampus exerts contextual control over motivated behavior by gating prefrontal cortex inputs to the nucleus accumbens. The model integrates the episodic memory functions of the hippocampus, the prefrontal role in representing the motivational stimuli and cognitive control, and the role of striatal regions in conditioned learning within a single theoretical framework. Simulation results are consistent with the hypothesis that hippocampal-prefrontal interactions may act as the neural substrate that allows contextual cues to override conditioned responses at the level of the nucleus accumbens. Prefrontal and hippocampal input overrides the predominant CS-US association if the context is inconsistent, and promotes flexible selection of previously learned associations and behaviors. Simulated transection of the fornix, effectively eliminating hippocampal and prefrontal influence over the nucleus accumbens, abolishes normal contextual modulation of behavior. The model is consistent with a wide range of empirical data.

Environmental complexity affects contextual fear conditioning following hippocampal lesions in rats

Contextual fear conditioning has become a benchmark measure for hippocampal function, even though several studies report successful acquisition in hippocampal-damaged rodents. The current study examined whether environmental complexity may account for these discrepancies. We directly compared single-session contextual fear conditioning in rats in a simple vs. complex environment. Hippocampal lesions led to reduced fear conditioning in both contexts, as measured by freezing, but the effect was significantly greater in the complex context. As well, lesions led to generalized fear when the complex context was paired with shock, but not when the simple context was paired. We suggest that the representation of the simple context formed by rats with hippocampal lesions was adequate to support associative learning, but the representation of the complex context, which depended to a greater extent on relational learning, was not. The results were interpreted as consistent with theories of hippocampal function that emphasize its role in integrating multiple stimulus elements in a memory trace.

A new neurobehavioral model of autism in mice: pre- and postnatal exposure to sodium valproate

Autism symptoms, including impairments in language development, social interactions, and motor skills, have been difficult to model in rodents. Since children exposed in utero to sodium valproate (VPA) demonstrate behavioral and neuroanatomical abnormalities similar to those seen in autism, the neurodevelopmental effects of this antiepileptic agent were examined in mice following its pre- or postnatal administration. Exposed pups were evaluated in a battery of neurodevelopmental procedures designed to assess VPA-induced retardation (wherein a behavior fails to mature on schedule), regression (wherein a behavior does mature on time but then deteriorates), or intrusions (wherein normal behaviors are overshadowed by stereotypic or self-injurious behaviors). The resulting observations were interpreted in the context of this new strategy to model autism.

Substantia nigra role in fear conditioning consolidation

The substantia nigra (SN) is known to be involved in the memorization of several conditioned responses. To investigate the role of the SN in fear conditioning consolidation this neural site was subjected to fully reversible tetrodotoxin (TTX) inactivation during consolidation in adult male Wistar rats which had undergone fear training to acoustic CS and context. TTX was stereotaxically administered to different groups of rats at increasing intervals after the acquisition session. Memory was assessed as conditioned freezing duration measured during retention testing, always performed 72 and 96 h after TTX administration. In this way there was no interference with normal SN function during either acquisition or retrieval phases, so that any amnesic effect could be due only to consolidation disruption. The results show that SN functional integrity is necessary for contextual fear response consolidation up to the 24-h after-acquisition delay. On the contrary SN functional integrity was shown not to be necessary for the consolidation of acoustic CS fear responses. The present findings help to elucidate the role of the SN in memory consolidation and better define the neural circuits involved in fear memories.

Dorsal hippocampal contributions to unimodal contextual conditioning

Although there is general consensus that the hippocampus is not critically involved in the acquisition of fear conditioned to an explicit conditioned stimulus (CS), the extent to which the hippocampus participates in contextual fear conditioning remains unclear. To further characterize the potential role of the hippocampus in contextual fear conditioning, the present experiments examined the effect of excitotoxic lesions of dorsal hippocampus on the acquisition of a novel contextual fear conditioning paradigm in which a unimodal (olfactory) cue served to disambiguate discrete "contexts" within a single behavioral training chamber. Selective lesions of dorsal hippocampus severely attenuated olfactory contextual conditioning without affecting conditioning to an explicit auditory or olfactory CS. Additional experiments indicate that these contextual conditioning deficits cannot be attributed to a lesion-induced decrement in olfactory perception, a preferential impairment of "weak" forms of conditioning, or hyperactivity. Thus, the hippocampus appears to contribute importantly to the acquisition of fear conditioned to explicitly nonspatial, unimodal, temporally, and spatially diffuse contextual stimuli.

Inhibition of neurogenesis interferes with hippocampus-dependent memory function

Rats treated with low dose irradiation, to inhibit adult hippocampal neurogenesis, and control rats were administered a non-matching-to-sample (NMTS) task, which measured conditional rule learning and memory for specific events, and a test of fear conditioning in which a discrete CS was paired with an aversive US in a complex environment. Irradiated rats were impaired on the NMTS task when the intervals between sample and test trials were relatively long, and in associating the shock-induced fear with contextual cues in the fear conditioning task. Irradiated rats were not impaired in learning the basic NMTS rule or in performing that task when the intervals between the sample and test trials were short. Nor were there group differences in conditioning the fear response to the CS in the fear conditioning task. The results, which extend the range of hippocampus-dependent tasks that can be said to be vulnerable to the effects of neurogenesis suppression, support the hypothesis that new hippocampal cells generated in adulthood participate in a broad range of hippocampal functions.

Hippocampal contributions to recollection in retrograde and anterograde amnesia

Lesions restricted to the hippocampal formation and/or extended hippocampal system (hippocampal formation, fornix, mammillary bodies, and anterior thalamic nuclei) can disrupt conscious recollection in anterograde amnesia, while leaving familiarity-based memory relatively intact. Familiarity may be supported by extra-hippocampal medial temporal lobe (MTL) structures. Within-task dissociations in recognition memory best exemplify this distinction in anterograde amnesia. The authors report for the first time comparable dissociations within recognition memory in retrograde amnesia. An amnesic patient (A.D.) with bilateral fornix and septal nuclei lesions failed to recognize details pertaining to personal past events only when recollection was required, during recognition of episodic details. His intact recognition of generic and semantic details pertaining to the same events was ascribed to intact familiarity processes. Recollective processes in the controls were reflected by asymmetrical Receiver's Operating Characteristic curves, whereas the patient's Receiver's Operating Characteristic was symmetrical, suggesting that his inferior recognition performance on episodic details was reliant on familiarity processes. Anterograde and retrograde memories were equally affected, with no temporal gradient for retrograde memories. By comparison, another amnesic person (K.C.) with extensive MTL damage (involving extra-hippocampal MTL structures in addition to hippocampal and fornix lesions) had very poor recognition and no recollection of either episodic or generic/semantic details. These data suggest that the extended hippocampal system is required to support recollection for both anterograde and retrograde memories, regardless of their age.

Peripheral lipopolysaccharide administration impairs two-way active avoidance conditioning in C57BL/6J mice

Peripheral administration of lipopolysaccharide (LPS) or interleukin-1 (IL-1) may lead to alterations of CNS function and behavioral changes designated "sickness behavior." Further, some experiments show evidence of LPS- and cytokine-mediated alterations in learning and memory. The current series of experiments examined the effects of a single or repeated intraperitoneal LPS injections, at a number of doses and time points before or after test sessions, on behavior in a two-way active avoidance conditioning paradigm. Subjects were able to avoid the mild shock stimulus, escape it, or fail to respond to it. Subjects treated with LPS at many, but not all, of the time points sampled showed impaired learning, by exhibiting significantly fewer avoidance responses than controls. Furthermore, an LPS-induced increase in non-cued inter-trial interval crossings was observed during the later days of testing, suggesting that a greater percentage of their avoidance responses was not conditioned and their behavior was less efficient. Taken together, the results suggest that LPS-treated animals showed a diminished association between conditioned stimulus (CS) and unconditioned stimulus (US). These results support the theory that peripheral immune stimuli may induce deleterious effects on learning, and extend the work to a negatively reinforced operant procedure.

Relational memory for object identity and spatial location in rats with lesions of perirhinal cortex, amygdala and hippocampus

Previous studies dissociate medial temporal lobe regions using non-relational object versus relational spatial tasks. We compared a relational object identity task to the commonly used, relational spatial Morris water task. Lesions of perirhinal cortex, amygdala and hippocampus led to impaired performance on only the relational object preference task. Rats with perirhinal cortex and amygdala lesions performed normally on the Morris water task, but showed reduced perseveration in the correct quadrant on the probe trial. Rats with hippocampal damage were impaired on all measures of the Morris water task. Our findings demonstrate that perirhinal and amygdala damage creates impairments for relational tasks that rely on information processed by these structures (object identity and stimulus valence, respectively). In addition, these structures contribute non-essentially to performance of relational spatial tasks. The hippocampus is critical for all tasks that require the use of relational representations, regardless of whether the disambiguating information is provided by object identity or spatial arrangements. The current pattern of results suggests that the previous object-spatial dissociations among medial temporal lobe regions may be due to the relational nature of the spatial tasks versus the non-relational nature of the object tasks. Further, they illustrate that discrete dissociations among different types of processing may be an oversimplification.

The development of a cognitive model of schizophrenia: Placing it in context

This review provides a historical perspective on a model for schizophrenia based on results of experiments derived from learning theory. It was developed by the author in collaboration with Jeffrey Gray and numerous colleagues, (e.g. [Gray, J.A., McNaughton, N., 2000. The Neuropsychology of Anxiety. second ed. Oxford University Press, Oxford; Hemsley, D.R., 1987a An experimental psychological model for schizophrenia. In: Hafner, H., Gattaz, W.F., Janzarik, W. (Eds.), Search for the Causes of Schizophrenia, vol. 1. Springer, New York, pp. 179-188.; Hemsley, D.R., 1993. A simple (or simplistic?) cognitive model for schizophrenia. Behaviour Research and Therapy 31, 633-646]. It contrasts with earlier cognitive formulations [e.g. Hemsley, D.R., 1975. A two stage model of attention in schizophrenia research. British Journal of Social and Clinical Psychology 14, 81-88], which emphasised a weakening of contextually elicited response biases, and lacked a link to potential neural bases of the disorder. The model emphasizes the need to demonstrate patterns of performance that are not interpretable in terms of the well established 'generalized deficit' manifest in schizophrenia. It proposes that the cognitive disturbance is a change in the way stored material is integrated with sensory input and ongoing motor programmes. In particular, spatial and temporal context fail to activate appropriate stored regularities. A number of possible pathways from the cognitive disturbance to the symptoms of schizophrenia are outlined; again the term 'context' is widely employed. Thus, it has been invoked to explain the occurrence of hallucinations, delusions, thought disorder and disruptions in the sense of personal identity. However the term 'context' is ill-defined and the review indicates the variety of ways in which it may exert its influence. These are unlikely to reflect the operation of a unitary mechanism.

Retrograde amnesia in rats with lesions to the hippocampus on a test of spatial memory

The present study examined remote spatial memory in a test that spans several months to determine whether remote memories are spared relative to more recent ones, as predicted by models of memory consolidation. At 3, 6 or 12 months of age, groups of rats received forced-choice training as to the location of food reward in a cross maze. At 12.5 months, rats received bilateral neurotoxic lesions to the hippocampus or a control surgical procedure and 2 weeks later their memory for the spatial location was tested. Their performance was compared to that of rats with hippocampal or control lesions with no prior training on several measures of savings. The hippocampal group with no pre-training, as expected, was severely impaired in learning the location of the food reward. Compared to this group, rats with hippocampal lesions that were pre-trained consistently performed better at the shortest training-surgery interval but not at the longer ones. That is, rats with hippocampal lesions exhibited retrograde amnesia at all training-surgery intervals and a forgetting curve that paralleled that of the control groups. The results were interpreted within a framework that distinguishes between relational and associative context, and as providing evidence that the hippocampus is necessary for the retention and retrieval of memories that are bound to relational context, regardless of the age of the memory.

Avoidance response in goldfish: emotional and temporal involvement of medial and lateral telencephalic pallium

The hippocampus and the amygdala are involved in avoidance learning in mammals. The medial and lateral pallia of actinopterygian fish have been proposed as homologous to the mammalian pallial amygdala and hippocampus, respectively, on the basis of neuroanatomical findings. This work was aimed at studying the effects of ablation of the medial telencephalic pallia (MP) and lateral telencephalic pallia (LP) in goldfish on the retention of a conditioned avoidance response previously acquired in two experimental conditions. In the first experiment, fish were trained in nontrace avoidance conditioning. In the second experiment, fish were trained in trace avoidance conditioning in which temporal cues were crucial for the learning process. An MP lesion affected the retention of the avoidance response in both procedures; in contrast, an LP lesion impaired the retention only in the trace-conditioning procedure. These data support the presence of two different systems of memory in fish, based on discrete telencephalic areas: the MP, involved in an emotional memory system; and the LP, involved in a spatial, relational, or temporal memory system. Moreover, these differential effects were similar to those produced by amygdalar and hippocampal lesions in mammals. We conclude that these specialized systems of memory could have appeared early during phylogenesis and could have been conserved throughout vertebrate evolution.

A proposed architecture for the neural representation of spatial context

The role of context in guiding animal behavior has attracted increasing attention in recent years, but little is known about what constitutes a context, nor how and where in the brain it is represented. Contextual stimuli can take many forms, but of particular importance are those that collectively define a particular place or situation. The representation of place has been linked to the hippocampus, because its principal neurons ('place cells') are spatially responsive; behavioral experiments also implicate this structure in the processing of contextual stimuli. Together, these findings suggest a hippocampal role in representing 'spatial context'. The present article outlines a proposed architecture for the encoding of spatial context in which spatial inputs to place cells are modulated (or 'gated') by non-spatial stimuli. We discuss recent experimental evidence that spatial context is population-coded, a property which could allow both discrimination between overlapping contexts and generalization across them, and thus provide a foundation for animals' capacity for flexible context-linked place learning.

Contextual modulation of spatial discrimination reversal in developing rats

Reversal of discrimination learning is influenced by manipulation of the training context. In adult and developing rats, contextual changes made between acquisition and reversal aid the learning of the new discrimination, possibly by serving to release proactive interference from the originally acquired discrimination (M. E. Bouton & D. C. Brooks, 1993; N. Spear, G. Smith, R. Bryan, & W. Gordon, 1980). The present study sought to examine this effect in an appetitive T-maze task, as a function of different contextual manipulations. Rats of three ages, Postnatal Day (PND) 19, PND23, and PND30, were tested for their ability to acquire and reverse a position habit in a T-maze. Contextual changes were made between acquisition and reversal sessions and consisted of one of three manipulations: (a) texture; the texture of the maze floor was changed via the addition or subtraction of wire mesh; (b) maze; subjects were reversed in a different maze that was identical in construction to the training maze, but differed in spatial location; (c) texture and maze; subjects were shifted to the new maze, the floor of which differed in texture from the training maze but was otherwise identical in construction. Results showed that the texture-maze combination was an effective aid to reversal learning at all ages tested. The texture alone, however, was not effective at any age. The maze alone also was an effective cue for reversal, but proved to have the greatest effect for PND30 subjects. During ontogeny, the contextual modulation of reversal learning is importantly influenced by the nature and the salience of the contextual cue.

Environmental manipulation differentially alters c-Fos expression in amygdaloid nuclei following aversive conditioning

This study analyzes the impact of environmental complexity in adult rats on their emotional behavior and c-Fos expression (a transcription factor protein implicated in neuronal plasticity) in various subdivisions of amygdala, as well as selected parts of the thalamus and hypothalamus. The animals were housed for 60 days in either enriched or impoverished conditions and then one group of rats was tested in an open field test, and a second group of rats was treated to footshock-motivated aversive training. Two and 24 h later, the animals from the second group, along with the appropriate controls, were sacrificed and their brains were used for the immunocytochemical analysis of c-Fos levels. We found that this long-term environmental manipulation exerts significant effects on animals' emotionality and this behavioral difference is accompanied by the differential c-Fos activation (at 2 h after the aversive training) in the amygdala, a brain structure believed to subserve emotional reactions. On the other hand, no difference was found in c-Fos expression between both groups of animals in the thalamus and hypothalamus. At 24 h after the training, c-Fos levels were down to the values observed in naive rats that did not differentiate between enriched versus impoverished breeding conditions. These results may help to explain differential emotional aspects of behavior that arise following differential housing conditions of adult animals.

Attenuation of context-specific inhibition on reversal learning of a stimulus-response task in rats with neurotoxic hippocampal damage

Rats with hippocampal or sham lesions were trained on a stimulus-response task developed for the 8-arm radial maze. After reaching a stringent learning criterion, different context manipulations were performed. In Experiment I, the different groups were transferred to an identical radial maze in a different room to determine the context specificity of the discrimination learning. Experiment I revealed that although rats with hippocampal lesions did not show a normal context detection effect, the expression of the discrimination was not context dependent for either the lesion or sham groups. In Experiment II, animals were trained to criterion on the discrimination task and then both groups were divided into sub-groups based on whether they would experience reversal training in the same or different context from original training. Experiment II indicated that animals with hippocampal lesions and shams reversed in a different context were significantly enhanced in reaching the learning criterion compared to either counterparts that were reversed in the same context. Reversal learning in rats with hippocampal lesions was faster than sham animals in the same context suggesting that the context-specific inhibition effect was hippocampal-based. After learning the reversal task, the groups of animals trained and reversed in different contexts were brought back into the original training context to test for competitive effects. Animals with hippocampal lesions that were reversed in the different context, did not show a competition between the most recently acquired discrimination and a context-specific association acquired during original training whereas sham animals in the same condition did. Taken together these results suggest that rats with hippocampal lesions do not acquire normal context-specific inhibition during discrimination learning.

Fear and power-dominance drive motivation: neural representations and pathways mediating sensory and mnemonic inputs, and outputs to premotor structures

Based on the available literature on activation of brain structures by fear- and anger-inducing stimuli, on the effects of electrical and chemical stimulation and lesions of candidate structures, and on connectional data, we propose that both the fear and power-dominance drives are represented in four distinct locations: the medial hypothalamus, lateral/dorsolateral periaqueductal gray, midline thalamic nuclei, and medial prefrontal cortex. The hypothalamic fear representation is located in the dorsomedial and posterior hypothalamic nuclei, the midbrain representation in the caudal part of the lateral/dorsolateral periaqueductal gray, the thalamic representation primarily in parts of the paraventricular and reuniens thalamic nuclei, and the cortical representation in prelimbic cortex. The hypothalamic power-dominance representation is located in the anterior hypothalamic nucleus, dorsomedial aspect of the ventromedial nucleus, and in adjacent parts of the medial preoptic area. The corresponding midbrain representation occurs in rostral part of the lateral/dorsolateral periaqueductal gray, and the thalamic representation in parts of the paraventricular, parataenial, and reuniens thalamic nuclei. We discuss sensory/mnemonic inputs to these representations, and outputs to premotor structures in the medulla, caudate-putamen, and cortex, and their differential contributions to involuntary, learned sequential, and voluntary motor acts. We examine potential contributions of neuronal activities in these representations to the subjective awareness of fear and anger.

Cerebellar role in fear-conditioning consolidation

Some cerebellar structures are known to be involved in the memorization of several conditioned responses. The role of the interpositus nucleus (IN) and the vermis (VE) in fear-conditioning consolidation was investigated by means of a combined behavioral and neurophysiological technique. The IN and VE were subjected to fully reversible tetrodotoxin (TTX) inactivation during consolidation in adult male Wistar rats that underwent acoustic conditioned stimulus (CS) and context fear training. TTX was injected in different groups of rats at increasing intervals after the acquisition session. Memory was assessed as conditioned freezing duration measured during retention testing, always performed 72 and 96 h after the stereotaxic TTX administration. This schedule ensures that there is no interference with normal cerebellar function during either the acquisition or the retrieval phase so that any amnesic effect may be due only to consolidation disruption. Our results show that IN functional integrity is necessary for acoustic CS fear response memory formation up to the 96-h after-acquisition delay. VE functional integrity was shown to be necessary for memory formation of both context (up to the 96-h after-acquisition delay) and acoustic CS (up to the 192-h after-acquisition delay) fear responses. The present findings help to elucidate the role of the cerebellum in memory consolidation and better define the neural circuits involved in fear memories.

New views on old memories: re-evaluating the role of the hippocampal complex

Evidence of temporally graded retrograde amnesia (RA) following hippocampal damage has fuelled the long-standing belief that memory undergoes a consolidation process, whereby memories are progressively modified in neocortical regions until they are independent of the hippocampal (HPC) complex. Support for this position derives from both the animal and human RA literature, although the results are not consistent. Specifically, consolidation theory does not account for loss of episodic (detail) information in humans and context-dependent information in animals, which often extend back for much of the life span. We discuss an alternative approach, the Multiple Trace Theory, which suggests that the HPC complex contributes to the retrieval of recent and remote episodic and context-dependent memories. According to this view, such memory traces are represented as spatially distributed interactions between the HPC and neocortex that persist for as long as those memories exist. On the other hand, semantic, or context-free, memories can become independent of the HPC as consolidation theory predicts. In support of this view, we report recent accounts of relatively flat RA gradients in autobiographical and spatial detail loss in patients and animal models with extensive bilateral HPC lesions. By comparison, temporally graded RA was observed in tests of semantic and context-free memory. We also report neuroimaging studies in which hippocampal activity, elicited during recollection of autobiographical memories, did not distinguish recent from remote episodes. Our discussion suggests ways to reconcile discrepancies in the literature and guide predictions of the occurrence of flat versus temporally limited gradients of remote episodic and semantic memory loss following lesions to HPC.

The effects of water deprivation on conditioned freezing to contextual cues and to a tone in rats

In two experiments we used an automated system for quantifying freezing responses in rats to replicate and extend Maren et al. (Maren S, DeCola JP, Fanselow MS. Water deprivation enhances fear conditioning to contextual, but not discrete, conditional stimuli in rats. Behav Neurosci 1994;108:645-9; Maren S, DeCola JP, Swain RA, Fanselow MS, Thompson RF. Parallel augmentation of hippocampal long-term potentiation, theta rhythm and contextual fear conditioning in water deprived rats. Behav Neurosci 1994;108:44-57) who found that water deprivation in rats produced a selective enhancement in conditioning to context, as opposed to conditioning to a tone. In experiment 1 we gave water deprived and non-deprived rats either three or ten pairings of a tone and foot shock. During conditioning water deprivation decreased overall freezing only in rats that received ten pairings. On 2 subsequent days we assessed conditioned freezing (1) to the contextual cues of the conditioning chamber and (2) to the tone when presented in a distinctive, novel environment. We found, in direct contrast to Maren et al. (Maren S, DeCola JP, Fanselow MS. Water deprivation enhances fear conditioning to contextual, but not discrete, conditional stimuli in rats. Behav Neurosci 1994;108:645-9), that (a) water deprived rats did not differ from non-deprived rats in levels of conditioned contextual freezing and that (b) water deprived rats did show reduced levels of freezing to the tone stimulus. In the same experiment we found that the number of tone-shock pairings did not affect levels of conditioned contextual freezing but that rats that had received three pairings did show reduced levels of freezing to the tone stimulus compared with rats that had received ten pairings, thereby demonstrating that the behavioural procedure and analysis system that we used was appropriately sensitive to differences in conditioning. In experiment 2, therefore, we sought to replicate Maren et al. (Maren S, DeCola JP, Fanselow MS. Water deprivation enhances fear conditioning to contextual, but not discrete, conditional stimuli in rats. Behav Neurosci 1994;108:645-9) using, as far as possible, exactly the same procedural parameters. Here we found that water deprivation produced no effects on conditioned freezing to the contextual cues or to the tone. We conclude that there is sufficient reason to doubt the generality of the previously reported findings.

Is the hippocampus necessary for contextual fear conditioning?

The hippocampus is widely believed to be essential for learning about the context in which conditioning occurs. This view is based primarily on evidence that lesions of the dorsal hippocampus disrupt freezing to contextual cues after fear conditioning. However, lesions that disrupt freezing produce no effect on fear-potentiated startle, a second measure of contextual fear. Moreover, hippocampal lesions also do not disrupt the contextual 'blocking' phenomenon, which provides an indirect measure of contextual fear. In these paradigms, at least, it appears that hippocampal lesions disrupt the expression of freezing, rather than contextual fear itself. This interpretation is supported by the finding that rats showing preserved contextual blocking after hippocampal lesions show deficits not only in contextual freezing, but also in unconditioned freezing. These findings are consistent with a growing body of data from other conditioning paradigms that contextual learning is spared after lesions of the dorsal hippocampus. Nonetheless, there remain some reports of impaired contextual fear conditioning after hippocampal lesions that cannot be attributed easily to a disruption of freezing. Thus, it is concluded that the hippocampus may be involved in contextual learning under certain--as yet, unspecified--circumstances, but is not critical for contextual learning in general.

Auditory thalamus, dorsal hippocampus, basolateral amygdala, and perirhinal cortex role in the consolidation of conditioned freezing to context and to acoustic conditioned stimulus in the rat

On the basis of previous experimental evidence, it is known that the auditory thalamus (AT), the dorsal hippocampus (DH), the basolateral amygdala (BLA), and the perirhinal cortex (PC) are involved in the mnemonic processing of conditioned freezing. In particular, BLA and PC appear to be involved both in conditioned stimulus (CS) and context conditioned freezing. Through AT, the auditory CS is sent to other sites, whereas DH is involved in context conditioning. Nevertheless, the existing evidence does not make it possible to assess AT, DH, BLA, and PC involvement during the consolidation phase of conditioned freezing. To address this question, fully reversible tetrodotoxin (TTX) inactivation was performed on adult male Wistar rats having undergone CS and context fear training. Anesthetized animals were injected stereotaxically with TTX (either 5 or 10 ng in 0.5 or 1.0 microliter of saline, according to site dimensions) at increasing post-acquisition delays. Context and CS freezing durations were measured during retention testing, always performed 48 and 72 hr after TTX administration. The results showed that AT inactivation does not disrupt consolidation of either contextual or auditory fear memories. In contrast, inactivation of the other three structures disrupted consolidation. For the DH, this disruption was specific to contextual cues and only occurred when inactivation was performed early (up to 1.5 hr) after training. The BLA and PC were shown to be involved in the consolidation of both contextual and auditory fear. Their involvement persisted for longer periods of time (2d for BLA and 8 d for PC). These findings provide information to build a temporal profile for the post-training processing of fear memories in structures known to be important for this form of learning. The results are discussed in relation to previous studies on conditioned freezing and other aversive conditioned response neural correlates.

Role of the hippocampus in contextual memory after classical aversive conditioning in pigeons (C. livia)

We investigated the effects of hippocampal lesions with ibotenic acid (IBO) on the memory of the sound-context-shock association during reexposure to the conditioning context. Twenty-nine adult pigeons were assigned to a non-lesioned control group (CG, N = 7), a sham-lesioned group (SG, N = 7), a hippocampus-lesioned experimental group (EG, N = 7), and to an unpaired nonlesioned group (tone-alone exposure) (NG, N = 8). All pigeons were submitted to a 20-min session in the conditioning chamber with three associations of sound (1000 Hz, 85 dB, 1 s) and shock (10 mA, 1 s). Experimental and sham lesions were performed 24 h later (EG and SG) when EG birds received three bilateral injections (anteroposterior (A), 4.5, 5.25 and 7.0) of IBO (1 microl and 1 microg/microl) and SG received one bilateral injection (A, 5.25) of PBS. The animals were reexposed to the training context 5 days after the lesion. Behavior was videotaped for 20 min and analyzed at 30-s intervals. A significantly higher percent rating of immobility was observed for CG (median, 95.1; range, 79.2 to 100.0) and SG (median, 90.0; range, 69.6 to 95.0) compared to EG (median, 11.62; range, 3.83 to 50.1) and NG (median, 7.33; range, 6.2 to 28.1) (P<0.001) in the training context. These results suggest impairment of contextual fear in birds who received lesions one day after conditioning and a role for the hippocampus in the modulation of emotional aversive memories in pigeons.

Role of the hippocampal system in conditional motor learning: mapping antecedents to action

Macaque monkeys can learn arbitrary mappings between stimuli and spatially directed actions (often termed conditional motor learning), and, after the development of a strong learning set, can do so in just a few trials. Ablation studies have shown that the hippocampus plus subjacent cortex is necessary for this rapid and highly flexible type of learning. We consider evidence that the arbitrary mapping function of the hippocampal system may be more general and fundamental than currently accepted and what limitations there may be, if any, on the information that it can map. Removal of the hippocampal system yields a pattern of deficits and preserved abilities that correlates remarkably closely with that found in human global amnesics, such as patient H.M., on a variety of declarative memory tasks. Thus, the rapid acquisition of arbitrary visuomotor mappings may represent an example of declarative memory in nonhuman primates.

The functional value of sound and exploratory behaviour in detelencephalated pigeons

Exploratory behaviour in response to a sound previously paired to a shock was analyzed before and after massive telencephalic lesions. Six pigeons were submitted to sound shock associations, habituation tests, telencephalic lesion and habituation retesting. Eighteen birds divided into three sham-lesioned groups received sound shock pairing (Control 1), pre-exposure to sound stimulation (Control 2) or pre-exposure to electric shocks (Control 3) before the behavioural tests. The pre-surgery tests consisted of 25 1 s pulses of sound A (1000 Hz, 83 dB) or B (500 Hz, 85 dB), every 30 s. Retests took place 10 days after surgery using the same sequence of procedures. The behaviour exhibited immediately after each sound stimulation was recorded manually and on video during the sessions. Analysis of variance showed an effect of group {F(3, 20)= 13.37, p < 0.0001) suggesting that pre-exposure to tone-shock association affected the exploration in response to the sound presented in another context. The Experimental and Control 1 birds showed no robust decrease in responses during the pre-surgery sessions. Post-surgery exploration data for detelencephalated birds showed a lower level of responding (p < 0.05). These data suggest a retardation in the typical reduction of responses to recurrent stimulation and support other evidences of the potentiating effect of sound shock association on responding. They are also indicative of telencephalic modulation of exploratory behaviour and strengthen the arguments for the subtelencephalic storage of associative information concerning the functional value of acoustic stimulation.

Hippocampus and context in classical conditioning

Recent evidence suggests that contextual learning encompasses a variety of changes in learning and performance processes. Only some of these changes depend on the hippocampus. Specialized functions proposed for the hippocampus in contextual learning include the construction and consolidation of contextual memory representations, incidental contextual learning, and inhibitory contextual learning.