Environmental complexity affects contextual fear conditioning following hippocampal lesions in rats

The impact of fornix lesions in rats on spatial learning tasks sensitive to anterior thalamic and hippocampal damage

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Fornix damage mildly impair spatial biconditional and passive place learning tasks.

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Fornix lesions impair spatial go/no-go and alternation problems.

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Fornix lesions impair tests making flexible demands on spatial memory.

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Fornix connections are not always required for learning fixed spatial responses.

The present study sought to understand how the hippocampus and anterior thalamic nuclei are conjointly required for spatial learning by examining the impact of cutting a major tract (the fornix) that interconnects these two sites. The initial experiments examined the consequences of fornix lesions in rats on spatial biconditional discrimination learning. The rationale arose from previous findings showing that fornix lesions spare the learning of spatial biconditional tasks, despite the same task being highly sensitive to both hippocampal and anterior thalamic nuclei lesions. In the present study, fornix lesions only delayed acquisition of the spatial biconditional task, pointing to additional contributions from non-fornical routes linking the hippocampus with the anterior thalamic nuclei. The same fornix lesions spared the learning of an analogous nonspatial biconditional task that used local contextual cues. Subsequent tests, including T-maze place alternation, place learning in a cross-maze, and a go/no-go place discrimination, highlighted the impact of fornix lesions when distal spatial information is used flexibly to guide behaviour. The final experiment examined the ability to learn incidentally the spatial features of a square water-maze that had differently patterned walls. Fornix lesions disrupted performance but did not stop the rats from distinguishing the various corners of the maze. Overall, the results indicate that interconnections between the hippocampus and anterior thalamus, via the fornix, help to resolve problems with flexible spatial and temporal cues, but the results also signal the importance of additional, non-fornical contributions to hippocampal-anterior thalamic spatial processing, particularly for problems with more stable spatial solutions.

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.

Association rules for rat spatial learning: the importance of the hippocampus for binding item identity with item location

Three cohorts of rats with extensive hippocampal lesions received multiple tests to examine the relationships between particular forms of associative learning and an influential account of hippocampal function (the cognitive map hypothesis). Hippocampal lesions spared both the ability to discriminate two different digging media and to discriminate two different room locations in a go/no-go task when each location was approached from a single direction. Hippocampal lesions had, however, differential effects on a more complex task (biconditional discrimination) where the correct response was signaled by the presence or absence of specific cues. For all biconditional tasks, digging in one medium (A) was rewarded in the presence of cue C, while digging in medium B was rewarded in the presences of cue D. Such biconditional tasks are “configural” as no individual cue or element predicts the solution (AC+, AD−, BD+, and BC−). When proximal context cues signaled the correct digging choice, biconditional learning was seemingly unaffected by hippocampal lesions. Severe deficits occurred, however, when the correct digging choice was signaled by distal room cues. Also, impaired was the ability to discriminate two locations when each location was approached from two directions. A task demand that predicted those tasks impaired by hippocampal damage was the need to combine specific cues with their relative spatial positions (“structural learning”). This ability makes it possible to distinguish the same cues set in different spatial arrays. Thus, the hippocampus appears necessary for configural discriminations involving structure, discriminations that potentially underlie the creation of cognitive maps.

Hippocampal lesions produce both nongraded and temporally graded retrograde amnesia in the same rat

Rats were administered contextual fear conditioning and trained on a water-maze, spatial memory task 28 days or 24 h before undergoing hippocampal lesion or control surgery. When tested postoperatively on both tasks, rats with hippocampal lesions exhibited retrograde amnesia for spatial memory at both delays but temporally graded retrograde amnesia for the contextual fear response. In demonstrating both types of retrograde amnesia in the same animals, the results parallel similar observations in human amnesics with hippocampal damage and provide compelling evidence that the nature of the task and the type of information being accessed are crucial factors in determining the pattern of retrograde memory loss associated with hippocampal damage. The results are interpreted as consistent with our transformation hypothesis (Winocur et al. (2010a) Neuropsychologia 48:2339-2356; Winocur and Moscovitch (2011) J Int Neuropsychol Soc 17:766-780) and at variance with standard consolidation theory and other theoretical models of memory.

The hippocampus supports multiple cognitive processes through relational binding and comparison

It has been well established that the hippocampus plays a pivotal role in explicit long-term recognition memory. However, findings from amnesia, lesion and recording studies with non-human animals, eye-movement recording studies, and functional neuroimaging have recently converged upon a similar message: the functional reach of the hippocampus extends far beyond explicit recognition memory. Damage to the hippocampus affects performance on a number of cognitive tasks including recognition memory after short and long delays and visual discrimination. Additionally, with the advent of neuroimaging techniques that have fine spatial and temporal resolution, findings have emerged that show the elicitation of hippocampal responses within the first few 100 ms of stimulus/task onset. These responses occur for novel and previously viewed information during a time when perceptual processing is traditionally thought to occur, and long before overt recognition responses are made. We propose that the hippocampus is obligatorily involved in the binding of disparate elements across both space and time, and in the comparison of such relational memory representations. Furthermore, the hippocampus supports relational binding and comparison with or without conscious awareness for the relational representations that are formed, retrieved and/or compared. It is by virtue of these basic binding and comparison functions that the reach of the hippocampus extends beyond long-term recognition memory and underlies task performance in multiple cognitive domains.

Motivation to quit smoking and startle modulation in female smokers: context specificity of smoking cue reactivity

RATIONALE

Cue reactivity and startle reflex modulation paradigms have been used in addiction research to determine the affective motivational state of craving induced by viewing drug-related cues. However, recent studies suggest that cue reactivity and startle reflex modulation in people with addictions can be suppressed, or even reversed, depending on context.

OBJECTIVE

The present study looked at the contextual specificity of smoking cue startle modulation by examining individuals with low and high motivation to quit smoking.

MATERIALS AND METHODS

Emotional modulation of the startle reflex was examined in 32 female smokers exposed to affective stimuli and tobacco cues. The sample was divided into high and low motivation to quit smoking groups using the Processes of Change Questionnaire.

RESULTS

The tobacco cues produced a greater startle magnitude in the group with high motivation to quit smoking than the group with low motivation, which was independent of craving level.

CONCLUSION

Motivation to be abstinent is a relevant contextual factor accounting for variance in cue reactivity in individual smokers.

Hippocampus is necessary for spatial discrimination using distal cue-configuration

The role of the hippocampus in processing contextual cues has been well recognized. Contextual manipulation often involves transferring animals between different rooms. Because of vague definition of context in such a paradigm, however, it has been difficult to study the role of the hippocampus parametrically in contextual information processing. We designed a novel task in which a different context can be parametrically defined by the spatial configuration of distal cues. In this task, rats were trained to associate two different configurations of distal cue-sets (standard contexts) with different food-well locations at the end of a radial arm. Experiment 1 tested the role of the dorsal hippocampus in retrieving well-learned associations between standard contexts and rewarding food-well locations by comparing rats with neurotoxic lesions in the dorsal hippocampus with controls. We found that the hippocampal-lesioned rats were unable to retrieve the context-place paired associations learned before surgery. To further test the role of the hippocampus in generalizing altered context, in Experiment 2, rats were trained in a task in which modified versions of the standard contexts (ambiguous contexts) were presented, intermixed with the standard contexts. Rats were able to process the ambiguous contexts immediately by using their similarities to the standard contexts, whereas muscimol inactivation of the dorsal hippocampus in the same animals reversibly deprived such capability. The results suggest that rats can effectively associate discrete spatial locations with spatial configuration of distal cues. More important, rats can generalize or orthogonalize modified contextual environments using learned contextual representation of the environment.

What is an anxiety disorder?

Initiated as part of the ongoing deliberation about the nosological structure of DSM, this review aims to evaluate whether the anxiety disorders share features of responding that define them and make them distinct from depressive disorders, and/or that differentiate fear disorders from anxious-misery disorders. The review covers symptom self-report as well as on-line indices of behavioral, physiological, cognitive, and neural responding in the presence of aversive stimuli. The data indicate that the anxiety disorders share self-reported symptoms of anxiety and fear; heightened anxiety and fear responding to cues that signal threat, cues that signal no threat, cues that formerly signaled threat, and contexts associated with threat; elevated stress reactivity to aversive stimuli; attentional biases to threat-relevant stimuli and threat-based appraisals of ambiguous stimuli; and elevated amygdala responses to threat-relevant stimuli. Some differences exist among anxiety disorders, and between anxiety disorders and depressive disorders. However, the differences are not fully consistent with proposed subdivisions of fear disorders vs. anxious misery disorders, and comparative data in large part are lacking. Given the high rates of co-morbidity, advances in our understanding of the features of responding that are shared across vs. unique to anxiety and depressive disorders will require dimensional approaches. In summary, the extant data help to define the features of responding that are shared across anxiety disorders, but are insufficient to justify revisions to the DSM nosology at this time.

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.

Contextual fear conditioning in humans: cortical-hippocampal and amygdala contributions

Functional imaging studies of cued fear conditioning in humans have mostly confirmed findings in animals, but it is unclear whether the brain mechanisms that underlie contextual fear conditioning in animals are also preserved in humans. We investigated this issue using functional magnetic resonance imaging and virtual reality contexts. Subjects underwent differential context conditioning in which they were repeatedly exposed to two contexts (CXT+ and CXT-) in semirandom order, with contexts counterbalanced across participants. An unsignaled footshock was consistently paired with the CXT+, and no shock was ever delivered in the CXT-. Evidence for context conditioning was established using skin conductance and anxiety ratings. Consistent with animal models centrally implicating the hippocampus and amygdala in a network supporting context conditioning, CXT+ compared with CXT- significantly activated right anterior hippocampus and bilateral amygdala. In addition, context conditioning was associated with activation in posterior orbitofrontal cortex, medial dorsal thalamus, anterior insula, subgenual anterior cingulate, and parahippocampal, inferior frontal, and parietal cortices. Structural equation modeling was used to assess interactions among the core brain regions mediating context conditioning. The derived model indicated that medial amygdala was the source of key efferent and afferent connections including input from orbitofrontal cortex. These results provide evidence that similar brain mechanisms may underlie contextual fear conditioning across species.

Hippocampal lesions in rhesus monkeys disrupt emotional responses but not reinforcer devaluation effects

BACKGROUND

Although the role of the hippocampus in emotional behavior has long been recognized, the extent to which the hippocampus plays a role in the regulation and expression of emotion in rhesus monkeys has not been systematically explored.

METHODS

Rhesus monkeys (Macaca mulatta) with excitotoxic lesions of the hippocampal formation and unoperated control animals were assessed on two different types of emotional processing: defensive reactions to a potential predator (experiment 1) and ability to update the value of positive reinforcers, in this case food (experiment 2). Monkeys with aspiration lesions of the perirhinal cortex were also included in this study as an operated control group.

RESULTS

In experiment 1, whereas both operated groups showed reduced latencies to retrieve food located near an innately fear-provoking stimulus, a fake snake, only monkeys with hippocampal lesions displayed reduced defensive reactions to the snake. In experiment 2, both operated groups performed as well as control animals when choosing objects flexibly based on the current value of a food.

CONCLUSIONS

These findings dissociate the hippocampus and perirhinal cortex in fear expression and specifically implicate the hippocampal formation in generating responses to stimuli that are potentially threatening.

Constitutive activation of the G-protein subunit Galphas within forebrain neurons causes PKA-dependent alterations in fear conditioning and cortical Arc mRNA expression

Memory formation requires cAMP signaling; thus, this cascade has been of great interest in the search for cognitive enhancers. Given that medications are administered long-term, we determined the effects of chronically increasing cAMP synthesis in the brain by expressing a constitutively active isoform of the G-protein subunit Galphas (Galphas*) in postnatal forebrain neurons of mice. Previously, we showed that Galphas* mice exhibit increased adenylyl cyclase activity but decreased cAMP levels in cortex and hippocampus due to a PKA-dependent increase in total cAMP phosphodiesterase (PDE) activity. Here, we extend previous findings by determining if Galphas* mice show increased activity of specific PDE families that are regulated by PKA, if Galphas* mice show PKA-dependent deficits in fear memory, and if these memory deficits are associated with PKA-dependent alterations in neuronal activity as mapped by Arc mRNA expression. Consistent with previous findings, we show here that Galphas* mice exhibit a significant compensatory increase in cAMP PDE1 activity and a trend toward increased cAMP PDE4 activity. Further, inhibiting the presumably elevated PKA activity in Galphas* mice fully rescues short- and long-term memory deficits in a fear-conditioning task, while extending the training session from one to four CS-US pairings partially rescues these deficits. Mapping of Arc mRNA levels suggests these PKA-dependent memory deficits may be related to decreased neuronal activity specifically within the cortex. Galphas* mice show decreased Arc mRNA expression in CA1, orbital cortex, and cortical regions surrounding the hippocampus; however, only the deficits in cortical regions surrounding the hippocampus are PKA dependent. Our results imply that chronically stimulating targets upstream of cAMP may detrimentally affect cognition.

Influence of massed and distributed context preexposure on contextual fear and Egr-1 expression in the basolateral amygdala

Preexposure to the conditioning context can influence the expression of context-conditioned fear. We used behavioral and early growth response gene (egr-1) assays in rats to study the effects of massed and distributed context preexposure on context-conditioned fear. The results demonstrated that massed context preexposure impaired acquisition of contextual fear, an effect here referred to as delayed shock deficit. Spaced context preexposure produced similar inhibitory effects. Significantly, the introduction of a brief change of context prior to conditioning completely reversed the deficit induced by massed, but not by distributed, context preexposure. This reversibility was inversely related to the duration of the context shift. The acquisition of context-conditioned fear was associated with enhanced Egr-1 expression in the basolateral amygdala (BLA). No such increase was evident in animals undergoing distributed context preexposure or in those experiencing massed preexposure without change of context. Remarkably, a brief change of context prior to conditioning not only facilitated learning following massed preexposure but also elicited a significant elevation of Egr-1 protein levels in the BLA. The findings shown demonstrated that the inhibitory effects of massed and distributed context preexposure on conditioning could be dissociable both behaviorally and physiologically. We suggest that the delayed shock deficit associated with massed preexposure derives from perceptual fade-out or inattention and its reversal by a brief change of context from attentional recovery.