Stimulation of medial prefrontal cortex decreases the responsiveness of central amygdala output neurons

Prefrontal mechanisms in extinction of conditioned fear

Interest in the medial prefrontal cortex (mPFC) as a source of behavioral inhibition has increased with the mounting evidence for a functional role of the mPFC in extinction of conditioned fear. In fear extinction, a tone-conditioned stimulus (CS) previously paired with a footshock is presented repeatedly in the absence of footshock, causing fear responses to diminish. Here, we review converging evidence from different laboratories implicating the mPFC in memory circuits for fear extinction: (1) lesions of mPFC impair recall of extinction under various conditions, (2) extinction potentiates mPFC physiological responses to the CS, (3) mPFC potentiation is correlated with extinction behavior, and (4) stimulation of mPFC strengthens extinction memory. These findings support Pavlov's original notion that extinction is new learning, rather than erasure of conditioning. In people suffering from posttraumatic stress disorder (PTSD), homologous areas of ventral mPFC show morphological and functional abnormalities, suggesting that extinction circuits are compromised in PTSD. Strategies for augmenting prefrontal function for clinical benefit are discussed.

Dissociation of neural responses and skin conductance reactions during fear conditioning with and without awareness of stimulus contingencies

This study investigates the effect of awareness of stimulus contingencies on BOLD responses within the amygdala, the orbitofrontal, and the occipital cortex, and on differential skin conductance responses (SCRs) during fear conditioning. Of two geometric figures, the paired conditioned stimulus (CS+) predicted an electrical stimulus (unconditioned stimulus = UCS), whereas the unpaired conditioned stimulus (CS-) was not followed by the UCS. Awareness of stimulus contingencies was manipulated experimentally, creating an aware and an unaware group: a distracter figure and a working memory task were introduced to conceal the stimulus contingencies of the conditioning paradigm, hence preventing contingency detection in the unaware group. The aware group was informed beforehand about the relation between CS+, CS-, and UCS. Differential SCRs were only obtained in the aware but not in the unaware group. Conversely, we observed enhanced responses of the amygdala, the orbitofrontal, and the occipital cortex to the CS+ in the unaware group only. Thus, we found a dissociation of SCR differentiation and the activation of a neural fear network depending on the presence or absence of awareness. These results support a model of fear conditioning that distinguishes between a more cognitive level of learning, reflected in contingency awareness and differential SCRs, and the awareness independent activation of a fear network.

Prior cocaine exposure disrupts extinction of fear conditioning

Psychostimulant exposure has been shown to cause molecular and cellular changes in prefrontal cortex. It has been hypothesized that these drug-induced changes might affect the operation of prefrontal-limbic circuits, disrupting their normal role in controlling behavior and thereby leading to compulsive drug-seeking. To test this hypothesis, we tested cocaine-treated rats in a fear conditioning, inflation, and extinction task, known to depend on medial prefrontal cortex and amygdala. Cocaine-treated rats conditioned and inflated similar to saline controls but displayed slower extinction learning. These results support the hypothesis that control processes in the medial prefrontal cortex are impaired by cocaine exposure.

Fos immunolabelling evidence for brain regions involved in the Pavlovian degraded contingency effect and in its disruption by atropine

Using a fear conditioning preparation, [Carnicella, S., Pain, L., Oberling, P., 2005a. Cholinergic effects on fear conditioning I: The degraded contingency effect is disrupted by atropine but reinstated by physostigmine. Psychopharmacology 178, 524-532] showed that the muscarinic receptor antagonist atropine disrupted the degraded contingency effect (DCE) in the rat, that is, the processes by which contextual memory competes with cued memory for the control over conditioned responding. Here, we investigated neural substrates involved in the expression of normal and atropine-disrupted DCE, using the protein Fos as a marker of neuronal activity. Compared to contingent conditioning, the DCE was associated with a decrease of the amount of Fos immunoreactive neurons within the auditory system and the amygdala and an increase within the medial prefrontal cortex (mPFC). Compared to the normal DCE, atropine-induced disruption of the DCE was associated with an increase of the amount of Fos immunoreactive neurons within the central nucleus of the amygdala. When atropine-induced suppression of the DCE, Fos pattern was modified in the mPFC with a change in Fos immunoreactivity, but no longer associated with the DCE. However, the mPFC was the unique structure studied in which the amount of Fos immunoreactive neurons was differentially affected according to both the conditioning procedure and the pharmacological treatment. These results are discussed in the framework of the cholinergic modulation of context processing in the rat and are put in parallel with an emerging set of studies in humans regarding the role of the PFC in such processing.

Fear extinction in rats: Implications for human brain imaging and anxiety disorders

Fear extinction is the decrease in conditioned fear responses that normally occurs when a conditioned stimulus (CS) is repeatedly presented in the absence of the aversive unconditioned stimulus (US). Extinction does not erase the initial CS-US association, but is thought to form a new memory. After extinction training, extinction memory competes with conditioning memory for control of fear expression. Deficits in fear extinction are thought to contribute to post-traumatic stress disorder (PTSD). Herein, we review studies performed in rats showing that the medial prefrontal cortex plays a critical role in the retention and expression of extinction memory. We also review human studies indicating that prefrontal areas homologous to those critical for extinction in rats are structurally and functionally deficient in patients with PTSD. We then discuss how findings from rat studies may allow us to: (1) develop new fear extinction paradigms in humans, (2) make specific predictions as to the location of extinction-related areas in humans, and (3) improve current extinction-based behavioral therapies for anxiety disorders.

The anxiogenic drug FG-7142 increases serotonin metabolism in the rat medial prefrontal cortex

The neural mechanisms underlying anxiety states are believed to involve interactions among forebrain limbic circuits and brainstem serotonergic systems. Consistent with this hypothesis, FG-7142, a partial inverse agonist at the benzodiazepine allosteric site of the GABAA receptor, increases c-Fos expression within a subpopulation of brainstem serotonergic neurons. Paradoxically, FG-7142 has no effect on extracellular serotonin concentrations, as measured using in vivo microdialysis, in certain anxiety-related brain structures. This study tested the hypothesis that FG-7142 alters serotonin metabolism within one or more nodes of a defined anxiety-related forebrain circuit. Rats received one of four treatments (vehicle, 1.9, 3.8, or 7.5 mg/kg FG-7142, i.p.) and brains were collected 1 h following treatment. Thirteen forebrain regions were microdissected and analyzed for l-tryptophan, serotonin, and 5-hydroxyindoleacetic acid concentrations using high pressure liquid chromatography with electrochemical detection. FG-7142 (7.5 mg/kg) increased l-tryptophan, serotonin, and 5-hydroxyindoleacetic acid concentrations in the prelimbic cortex but not in several other regions studied including subdivisions of the amygdala and bed nucleus of the stria terminalis. These data demonstrate that FG-7142 alters brain tryptophan concentrations and serotonin metabolism in specific components of an anxiety-related forebrain circuit including the medial prefrontal cortex, an important structure involved in executive function and the regulation of emotional behavior.

Common and distinct neural correlates of obsessive-compulsive and related disorders

Obsessive-compulsive disorder (OCD) often co-occurs with other anxiety disorders and a number of other disorders of similar phenomenology known as the "OCD spectrum" disorders. Neurobiologically, it is unclear how all these disorders relate to each other. The picture is further complicated by the clinical heterogeneity of OCD itself. This article reviews the literature on the common and distinct neural correlates of OCD, its symptom dimensions, and other anxiety and OCD spectrum disorders with the hope of providing a conceptual and heuristic framework to help understand the relationship between these phenomena.

Opposing influence of basolateral amygdala and footshock stimulation on neurons of the central amygdala

BACKGROUND

The basolateral complex (BLA) and the central nucleus of the amygdala (CeA) are believed to mediate the expression of affective responses. After affective learning, conditioned stimulus-related information is thought to be conveyed from the BLA to the CeA; the medial CeA (Cem), in turn, projects to hypothalamic and brainstem structures involved with induction of affective responses. Although the conditioned stimulus and unconditioned stimulus both evoke affective responses, the precise response often differs. It is unknown whether this difference is represented by distinct activity patterns of single Cem neurons. Furthermore, the nature of the interaction between the BLA and Cem is unknown.

METHODS

Using in vivo extracellular and intracellular recordings, we examined how the BLA affects the Cem and compared this with effects induced by footshock (unconditioned stimulus) in the same neurons.

RESULTS

Our results demonstrate that, contrary to conventional views, BLA stimulation primarily inhibits Cem neurons by a polysynaptic circuit, and show that single Cem neurons respond to both BLA input and footshock in an opposite manner.

CONCLUSIONS

These results demonstrate the predominantly inhibitory nature of the BLA-Cem interaction. These data further demonstrate the distinct cellular events that might lead to differential modulation of conditioned and unconditioned affective responses.

Increased sensitivity to restraint stress and novelty-induced emotionality following long-term, high dose cannabinoid exposure

Cannabinoids have long been known to affect anxiety, emotionality and stress responsivity; however, the direction of this effect has been controversial. This research aimed to compare the effects of chronic administration of both a very low dose (5 microg/kg) and a very high dose (100 microg/kg) of the potent cannabinoid CB(1) receptor agonist HU-210 on emotionality and stress responsivity. Twelve day administration of the high dose of HU-210 increased anxiety-like behavior as indicated by a significant reduction in time spent in the central quadrant and an increase in fecal boli in the open field test. However, neither dose of HU-210 elicited any significant behavioral effect in the elevated plus maze. Stress responsivity appeared to be sensitized in animals that had received high dose HU-210 treatment, as 30 min of restraint resulted in significantly higher plasma corticosterone levels in these subjects than in stressed controls or those receiving the low dose treatment. Furthermore, restraint stress elicited a non-significant increase in c-fos induction in the central amygdala, which was significantly potentiated following high-dose treatment with HU-210. This interaction was not seen in the basolateral amygdala. Together, these findings suggest that chronic high dose, but not low dose, cannabinoid administration may result in increased emotionality and sensitization of the stress axis.

Single neurons in the medial prefrontal cortex of the rat exhibit tonic and phasic coding during trace fear conditioning

Trace fear conditioning is a learning task that requires the association of an auditory conditioned stimulus (CS) and a shock unconditioned stimulus (US) that are separated by a 20-s trace interval. Single neuron activity was recorded from the prelimbic and infralimbic areas of the medial prefrontal cortex in rats during trace fear conditioning or nonassociative unpaired training. Prelimbic neurons showed learning-related increases in activity to the CS and US, whereas infralimbic neurons showed learning-related decreases in activity to these stimuli. A subset of prelimbic neurons exhibited sustained increases in activity during the trace interval. These sustained prelimbic responses may provide a bridging code that allows for overlapping representations of CS and US information within the trace fear conditioning circuit.

Different mechanisms of fear extinction dependent on length of time since fear acquisition

Fear extinction is defined as a decline in conditioned fear responses (CRs) following nonreinforced exposure to a feared conditioned stimulus (CS). Behavioral evidence indicates that extinction is a form of inhibitory learning: Extinguished fear responses reappear with the passage of time (spontaneous recovery), a shift of context (renewal), and unsignaled presentations of the unconditioned stimulus (reinstatement). However, there also is evidence to suggest that extinction is an "unlearning" process corresponding to depotentiation of potentiated synapses within the amygdala. Because depotentiation is induced more readily at short intervals following LTP induction and is not inducible at all at a sufficient delay, it may be that extinction initiated shortly following fear acquisition preferentially engages depotentiation/"unlearning," whereas extinction initiated at longer delays recruits a different mechanism. We investigated this possibility through a series of behavioral experiments examining the recoverability of conditioned fear following extinction. Consistent with an inhibitory learning mechanism of extinction, rats extinguished 24-72 h following acquisition exhibited moderate to strong reinstatement, renewal, and spontaneous recovery. In contrast, and consistent with an erasure mechanism, rats extinguished 10 min to 1 h after acquisition exhibited little or no reinstatement, renewal, or spontaneous recovery. These data support a model in which different neural mechanisms are recruited depending on the temporal delay of fear extinction.

Brain mechanisms of expectation associated with insula and amygdala response to aversive taste: implications for placebo

The experience of aversion is shaped by multiple physiological and psychological factors including one's expectations. Recent work has shown that expectancy manipulation can alter perceptions of aversive events and concomitant brain activation. Accruing evidence indicates a primary role of altered expectancies in the placebo effect. Here, we probed the mechanism by which expectation attenuates sensory taste transmission by examining how brain areas activated by misleading information during an expectancy period modulate insula and amygdala activation to a highly aversive bitter taste. In a rapid event-related fMRI design, we showed that activations in the rostral anterior cingulate cortex (rACC), orbitofrontal cortex (OFC), and dorsolateral prefrontal cortex to a misleading cue that the taste would be mildly aversive predicted decreases in insula and amygdala activation to the highly aversive taste. OFC and rACC activation to the misleading cue were also associated with less aversive ratings of that taste. Additional analyses revealed consistent results demonstrating functional connectivity among the OFC, rACC, and insula. Altering expectancies of upcoming aversive events are shown here to depend on robust functional associations among brain regions implicated in prior work on the placebo effect.

GABAergic neurons: gate masters of the amygdala, mastered by dopamine

A hyperdopaminergic state, such as stress, is associated with an increase in affective behavior. In this issue of Neuron, Marowsky and colleagues identify two clusters of paracapsular intercalated GABA neurons in amygdala slice preparations of GAD67-GFP mice. These GABA neurons mediate inhibition from cortical afferents to both the major input and output station of the amygdala, are inhibited during action of dopamine via D1 receptors, and are thus likely to represent important cellular players during dopaminergic disinhibition related to increased affective behavior.

A specialized subclass of interneurons mediates dopaminergic facilitation of amygdala function

The amygdala is under inhibitory control from the cortex through the activation of local GABAergic interneurons. This inhibition is greatly diminished during heightened emotional states due to dopamine release. However, dopamine excites most amygdala interneurons, suggesting that this dopaminergic gate may be mediated by an unknown subpopulation of interneurons. We hypothesized that this gate is mediated by paracapsular intercalated cells, a subset of interneurons that are innervated by both cortical and mesolimbic dopaminergic afferents. Using transgenic mice that express GFP in GABAergic interneurons, we show that paracapsular cells form a network surrounding the basolateral complex of the amygdala. We found that they provide feedforward inhibition into the basolateral and the central amygdala. Dopamine hyperpolarized paracapsular cells through D1 receptors and substantially suppressed their excitability, resulting in a disinhibition of the basolateral and central nuclei. Suppression of the paracapsular system by dopamine provides a compelling neural mechanism for the increased affective behavior observed during stress or other hyperdopaminergic states.

Animal studies of amygdala function in fear and uncertainty: relevance to human research

This article reviews research in both animals and humans on the considerable progress made in elucidating a brain circuitry of fear, particularly the importance of the amygdala in fear conditioning. While there is considerable agreement about the participation of the amygdala in fear in both animals and humans, there are several issues about the function of the amygdala raised by the human research that have not been addressed by or may be answered by animal research. Three of these are addressed in this article: (1) is the amygdala involved in or necessary for both fear learning and unconditioned fear? (2) Does the amygdala code for intensity of fear? (3) Is the amygdala preferentially involved in fear, or is it also activated when there are no overt fear or aversive stimuli, but where the situation can be described as uncertain? We present evidence indicating that the rodent amygdala is involved in some types of fear (conditioned fear), but not all types (unconditioned fear), and may therefore have significance for a differential neurobiology of certain anxiety disorders in humans. Further, similar to the human amygdala, the rodent amygdala responds to varying intensities of aversive stimulation. Finally, it is suggested that, similar to humans, the rodent amygdala is involved in the evaluation of uncertainty. We conclude that progress on elucidating the role of the amygdala in fear is facilitated by corroboration of findings from both animal and human research.

Facial expressions and complex IAPS pictures: common and differential networks

Neuroimaging studies investigating emotion have commonly used two different visual stimulus formats, facial expressions of emotion or emotionally evocative scenes. However, it remains an important unanswered question whether or not these different stimulus formats entail the same processes. Facial expressions of emotion may elicit more emotion recognition/perception, and evocative pictures may elicit more direct experience of emotion. In spite of these differences, common areas of activation have been reported across different studies, but little work has investigated activations in response to the two stimulus formats in the same subjects. In this fMRI study, we compared BOLD activation patterns to facial expression of emotions and to complex emotional pictures from the International Affective Picture System (IAPS) to determine if these stimuli would activate similar or distinct brain regions. Healthy volunteers passively viewed blocks of expressive faces and IAPS pictures balanced for specific emotion (happy, sad, anger, fear, neutral), interleaved with blocks of fixation. Eye movement, reaction times, and off-line subjective ratings including discrete emotion, valence, and arousal were also recorded. Both faces and IAPS pictures activated similar structures, including the amygdala, posterior hippocampus, ventromedial prefrontal cortex, and visual cortex. In addition, expressive faces uniquely activated the superior temporal gyrus, insula, and anterior cingulate more than IAPS pictures, despite the faces being less arousing. For the most part, these regions were activated in response to all specific emotions; however, some regions responded only to a subset.

Extending animal models of fear conditioning to humans

A goal of fear and anxiety research is to understand how to treat the potentially devastating effects of anxiety disorders in humans. Much of this research utilizes classical fear conditioning, a simple paradigm that has been extensively investigated in animals, helping outline a brain circuitry thought to be responsible for the acquisition, expression and extinction of fear. The findings from non-human animal research have more recently been substantiated and extended in humans, using neuropsychological and neuroimaging methodologies. Research across species concur that the neural correlates of fear conditioning include involvement of the amygdala during all stages of fear learning, and prefrontal areas during the extinction phase. This manuscript reviews how animal models of fear are translated to human behavior, and how some fears are more easily acquired in humans (i.e., social-cultural). Finally, using the knowledge provided by a rich animal literature, we attempt to extend these findings to human models targeted to helping facilitate extinction or abolishment of fears, a trademark of anxiety disorders, by discussing efficacy in modulating the brain circuitry involved in fear conditioning via pharmacological treatments or emotion regulation cognitive strategies.

Enhancement of conditioned fear extinction by infusion of the GABAAagonist muscimol into the rat prefrontal cortex and amygdala

In auditory fear conditioning, repeated presentation of the tone in the absence of the shock leads to extinction of the acquired fear response. Both the infra limbic prefrontal cortex (IL) and the basolateral amygdala (BLA) are involved in extinction. In this study, we examine the involvement of these two regions in extinction by manipulating the gamma-aminobutyric acid (GABA)ergic system, in the Sprague-Dawley rat. We microinfused a low dose of the GABA(A) agonist muscimol into the IL or BLA. Muscimol infused to IL before extinction training, but not after either a short (five-trials) or long (15-trials) extinction training, resulted in long-term facilitation of extinction. Infusion of muscimol to the BLA following a short (five-trial) extinction session facilitated extinction at least 48-h post-drug infusion. The differences in the temporal parameters of the effects of muscimol in the IL or BLA, suggest differential involvement of these structures in long-term extinction of fear memory. We propose a facilitating role for GABA(A) neurotransmission in the IL in triggering the onset of fear extinction and its maintenance, whereas in the BLA, GABA(A) neurotransmission facilitates extinction consolidation. The involvement of GABA(A) receptors in fear extinction in the prefrontal cortex and amygdala is of particular interest, because of the role of these areas in emotional processes, and the role of the GABA(A) receptors in anxiety states.

Association of neural and physiological responses during voluntary emotion suppression

Recent neuroimaging studies have shown that several prefrontal regions play critical roles in inhibiting activation of limbic regions during voluntary emotion regulation. The present study aimed to confirm prior findings and to extend them by identifying the frontal neural circuitry associated with regulation of peripheral physiological responses during voluntary emotion suppression. Ten healthy female subjects were presented with affectively positive, neutral, and negative pictures in each of an Attending and Suppression task. Regional cerebral blood-flow changes were measured using 15O-water positron emission tomography, and autonomic (heart rate: HR, skin conductance response: SCR) and endocrine (adrenocorticotropic hormone: ACTH) indices were measured during both tasks. The left amygdala and the right anterior temporal pole were activated during the Attending task, whereas activation was observed in the left lateral prefrontal cortex (LPFC), including the adjacent medial prefrontal cortex (MPFC), and medial orbitofrontal cortex (MOFC) during the Suppression task. In the Attending task, activation in the amygdala and MOFC positively correlated with magnitudes of the SCR and ACTH responses. Emotion suppression elicited enhancement of SCR and the strength of the effect positively correlated with activation in the MOFC. These results suggest that the MOFC plays a pivotal role in top-down regulation of peripheral physiological responses accompanying emotional experiences.

Neurophysiological theory of Kamin blocking in fear conditioning

Kamin blocking in fear conditioning is thought to reflect diminished processing of the unconditional stimulus (US) in the presence of a conditional stimulus (CS-super(+)) that was previously paired with this US. According to Fanselow's (1998) hypothesis, the CS-super(+) drives output from the amygdala that ultimately produces analgesia by causing opiate release onto afferent pain circuits. This hypothesis was explored quantitatively through neurophysiological simulations. The results suggest that opiate-mediated, negative-feedback control of US processing is too slow for efficient blocking of cue conditioning. The reason is that conditioning-produced synaptic modifications can be induced before the opiate-mediated inhibition has any substantial effect on US processing. The results suggest the existence of an additional, faster-acting, inhibitory neurotransmitter in the blocking circuit.

Behavioral control, the medial prefrontal cortex, and resilience

The degree of control that an organism has over a stressor potently modulates the impact of the stressor, with uncontrollable stressors producing a constellation of outcomes that do not occur if the stressor is behaviorally controllable. It has generally been assumed that this occurs because uncontrollability actively potentiates the effects of stressors. Here it will be suggested that in addition, or instead, the presence of control actively inhibits the impact of stressors. At least in part, this occurs because (i) the presence of control is detected by regions of the ventral medial prefrontal cortex (mPFCv); and (ii) detection of control activates mPFCv output to stress-responsive brain stem and limbic structures that actively inhibit stress-induced activation of these structures. Furthermore, an initial experience with control over stress alters the mPFCv response to subsequent stressors so that mPFCv output is activated even if the subsequent stressor is uncontrollable, thereby making the organism resilient. The general implications of these results for understanding resilience in the face of adversity are discussed.

Fear extinction in rodents: basic insight to clinical promise

Fear extinction, the reduction of fear by repeated exposure to the object of fear, is a crucial paradigm of inhibitory learning and the acknowledged preclinical model for behavior therapy of human anxiety. Recent insights have clarified roles for infralimbic prefrontal cortex, hippocampus and periaqueductal gray in extinction learning, while maintaining a central role for the basolateral amygdaloid nucleus in the acquisition and storage of this learning. Simultaneously, molecular insights have implicated several neurotransmitter and second messenger systems in extinction learning, and revealed that extinction is surprisingly easy to improve, yielding the promise of a novel approach to improved psychiatric treatments for a variety of human anxiety disorders.

Cortico-amygdala circuits: role in the conditioned stress response

The amygdala plays a crucial role in the orchestration and modulation of the organism response to aversive, stressful events. This response could be conceived as the result of two interdependent components. The first is represented by sets of visceral and motor responses aimed at helping the organism to cope with the present event. The second is the acquisition and modulation of memories relative to the stressful stimulus and its context. This latter component contributes to the instatement of conditioned stress responses that are essential to the capability of the organism to predict future exposures to similar stimuli in order to avoid them or counteract them effectively. In the amygdala, these two components become fully integrated. Massive networks link the amygdala to the hypothalamus, midbrain and brainstem. These networks convey visceral, humoral and nociceptive information to the amygdala and mediate its effects on the hypothalamic-pituitary-adrenal axis as well on autonomic and motor centers. On the other hand, interactions between the amygdala and interconnected cortical networks play a crucial role in acquisition, consolidation and extinction of learning relative to the stressful stimulus. Within the scope of this review, current evidence relative to the interaction between the amygdala and cortical networks will be considered in relationship to the integration of the conditioned response to stress.

Orbitofrontal thickness, retention of fear extinction, and extraversion

People differ in their personality traits and in their ability to modulate fear. Does our personality determine how well we extinguish conditioned fear responses? Or is the opposite true? Herein, we examine the relationships between personality traits, memory for fear extinction, and cortical thickness as a measure of brain structure. We found that in healthy humans, extinction retention and thickness of the medial orbitofrontal cortex are positively correlated with extraversion. Path analysis indicates that extinction retention mediates the relationship between the medial orbitofrontal cortex thickness and extraversion, thereby illustrating one path through which brain structure influences personality.

Peptides of love and fear: vasopressin and oxytocin modulate the integration of information in the amygdala

Neuropeptides vasopressin and oxytocin regulate a variety of behaviors ranging from maternal and pair bonding to aggression and fear. Their role in modulating fear responses has been widely recognized, but not yet well understood. Animal and human studies indicate the major role of the amygdala in controlling fear and anxiety. The amygdala is involved in detecting threat stimuli and linking them to defensive behaviors. This is accomplished by projections connecting the central nucleus of the amygdala (CeA) to the brain stem and to hypothalamic structures, which organize fear responses. A recent study by Huber et al demonstrates that vasopressin and oxytocin modulate the excitatory inputs into the CeA in opposite manners. Therefore this finding elucidates the mechanisms through which these neuropeptides may control the expression of fear.

Contingency awareness in human aversive conditioning involves the middle frontal gyrus

In contrast to the wealth of data describing the neural mechanisms underlying classical conditioning, we know remarkably little about the mechanisms involved in acquisition of explicit contingency awareness. Subjects variably acquire contingency awareness in classical conditioning paradigms, in which they are able to describe the temporal relationship between a conditioned cue and its outcome. Previous studies have implicated the hippocampus and prefrontal cortex in the acquisition of explicit knowledge, although their specific roles remain unclear. We used functional magnetic resonance imaging to track the trial-by-trial acquisition of explicit knowledge in a concurrent trace and delay conditioning paradigm. We show that activity in bilateral middle frontal gyrus and parahippocampal gyrus correlates with the accuracy of explicit contingency awareness on each trial. In contrast, amygdala activation correlates with conditioned responses indexed by skin conductance responses (SCRs). These results demonstrate that brain regions known to be involved in other aspects of learning and memory also play a specific role, reflecting on each trial the acquisition and representation of contingency awareness.

Mapping cerebral blood flow changes during auditory-cued conditioned fear in the nontethered, nonrestrained rat

Conditioned fear (CF) is one of the most frequently used behavioral paradigms; however, little work has mapped changes in cerebral perfusion during CF in the rat-the species which has dominated CF research. Adult rats carrying an implanted minipump were exposed to a tone (controls, n = 8) or a tone conditioned in association with footshocks (CS group, n = 9). During reexposure to the tone 24 h later, animals were injected intravenously by remote activation with [14C]-iodoantipyrine using the pump. Significant group differences in regional CBF-related tissue radioactivity (CBF-TR) were determined by region-of-interest analysis of brain autoradiographs, as well as in the reconstructed, three-dimensional brain by statistical parametric mapping (SPM). CS animals demonstrated significantly greater, fear-enhanced increases in CBF-TR in auditory cortex than controls. The lateral amygdala was activated, whereas the basolateral/basomedial and central amygdala were deactivated. In the hippocampus and medial prefrontal cortex, CBF-TR increased significantly ventrally but not dorsally. Significant activations were noted in medial striatum and the thalamic midline and intralaminar nuclei. However, the ventrolateral/dorsolateral striatum and its afferents from motor and somatosensory cortex were deactivated, consistent with the behavioral immobility seen during CF. Significant activations were also noted in the lateral septum, periaqueductal gray, and deep mesencephalic nucleus/tegmental tract. Our results show that auditory stimuli endowed with aversive properties through conditioning result in significant redistribution of cerebral perfusion. SPM is a useful tool in the brain mapping of complex rodent behaviors, in particular the changes in activation patterns in limbic, thalamic, motor, and cortical circuits during CF.

A neuroendocrine mechanism for sustaining fear

Fear is an adaptive response to recognition of a potentially dangerous event. Glucocorticoids are essential for maintaining a wide variety of behavioral events by their regulation of numerous genes; one such gene encodes corticotrophin-releasing hormone (CRH). CRH is involved in diverse behavioral responses to changing environmental demands. In this review, we focus on one aspect of glucocorticoid regulation of CRH--namely, fear-related responses to diverse classes of adverse events, such as those represented by contextual and cue-specific stimuli. Three extra-hypothalamic forebrain sites appear crucial for fear-related behavioral responses: the amygdala and the bed nucleus of the stria terminalis for sustaining adaptive fear-related behaviors, and the medial prefrontal cortex for modulating fear-related behaviors. Central regulation of CRH by glucocorticoids is important for adaptive and sustained fear-related behaviors, and its aberration is associated with anxiety and depressive disorders.

Neurohypophysial peptides: gatekeepers in the amygdala

A recent paper by Huber, Veinante and Stoop reports electrophysiological studies in slices of the amygdala in which the authors are able to demonstrate a cellular and spatial dissociation between the sites of action of oxytocin and vasopressin. These studies are important for determining how these brain peptides might gate autonomic responses to fear and other emotional stimuli.

Chronic cold stress alters prefrontal cortical modulation of amygdala neuronal activity in rats

BACKGROUND

Recent studies suggest that long-term exposure to stress can sensitize animals to subsequent novel or acute stressors. Stressors affect amygdala activity, and the prefrontal cortex has been implicated in the regulation of responses to stress. Little is known, however, about how the physiology of amygdala neurons is altered by chronic stressors or the role of the prefrontal cortex in these changes.

METHODS

We used in vivo extracellular recordings from neurons in the rat central and basolateral amygdala nuclei to examine the effects of chronic stress on the basal firing and responses of amygdala neurons to a novel stressor. Additionally, prefrontal cortical afferents were severed to examine its role in the modulation of the response to stressors.

RESULTS

Chronic exposure to cold enhanced the sensitivity of central amygdala neurons to footshock. A portion of this may be due to enhanced basolateral amygdala output. Furthermore, prefrontal cortical regulation of this response is weakened by chronic stress.

CONCLUSIONS

The physiology of the amygdala is altered by chronic stress. Furthermore, the prefrontal cortical regulation of these responses may be weakened after chronic stress. This is a potential biological substrate for abnormal affect upon chronic stress and its effect on affective disorders.

Neural circuits and mechanisms involved in Pavlovian fear conditioning: a critical review

Pavlovian or classical fear conditioning is recognized as a model system to investigate the neurobiological mechanisms of learning and memory in the mammalian brain and to understand the root of fear-related disorders in humans. In recent decades, important progress has been made in delineating the essential neural circuitry and cellular-molecular mechanisms of fear conditioning. Converging lines of evidence indicate that the amygdala is necessarily involved in the acquisition, storage and expression of conditioned fear memory, and long-term potentiation (LTP) in the lateral nucleus of the amygdala is often proposed as the underlying synaptic mechanism of associative fear memory. Recent studies further implicate the prefrontal cortex-amygdala interaction in the extinction (or inhibition) of conditioned fear. Despite these advances, there are unresolved issues and findings that challenge the validity and sufficiency of the current amygdalar LTP hypothesis of fear conditioning. The purpose of this review is to critically evaluate the strengths and weaknesses of evidence indicating that fear conditioning depend crucially upon the amygdalar circuit and plasticity.

Neural circuit changes mediating lasting brain and behavioral response to predator stress

This paper reviews recent work which points to critical neural circuitry involved in lasting changes in anxiety like behavior following unprotected exposure of rats to cats (predator stress). Predator stress may increase anxiety like behavior in a variety of behavioral tests including: elevated plus maze, light dark box, acoustic startle, and social interaction. Studies of neural transmission in two limbic pathways, combined with path and covariance analysis relating physiology to behavior, suggest long term potentiation like changes in one or both of these pathways in the right hemisphere accounts for stress induced changes in all behaviors changed by predator stress except light dark box and social interaction. Findings will be discussed within the context of what is known about neural substrates activated by predator odor.

Effects of cognitive-behavioral therapy on brain activation in specific phobia

Little is known about the effects of successful psychotherapy on brain function in subjects with anxiety disorders. The present study aimed to identify changes in brain activation following cognitive-behavioral therapy (CBT) in subjects suffering from specific phobia. Using functional magnetic resonance imaging (fMRI), brain activation to spider videos was measured in 28 spider phobic and 14 healthy control subjects. Phobics were randomly assigned to a therapy-group (TG) and a waiting-list control group (WG). Both groups of phobics were scanned twice. Between scanning sessions, CBT was given to the TG. Before therapy, brain activation did not differ between both groups of phobics. As compared to control subjects, phobics showed greater responses to spider vs. control videos in the insula and anterior cingulate cortex (ACC). CBT strongly reduced phobic symptoms in the TG while the WG remained behaviorally unchanged. In the second scanning session, a significant reduction of hyperactivity in the insula and ACC was found in the TG compared to the WG. These results propose that increased activation in the insula and ACC is associated with specific phobia, whereas an attenuation of these brain responses correlates with successful therapeutic intervention.

Neural correlates of genetically abnormal social cognition in Williams syndrome

Williams-Beuren syndrome (WBS), caused by a microdeletion of approximately 21 genes on chromosome 7q11.23, is characterized by unique hypersociability combined with increased non-social anxiety. Using functional neuroimaging, we found reduced amygdala activation in individuals with WBS for threatening faces but increased activation for threatening scenes, relative to matched normal controls. Activation and interactions of prefrontal regions linked to amygdala, especially orbitofrontal cortex, were abnormal, suggesting a genetically controlled neural circuitry for regulating human social behavior.

Opposite effects of tetanic stimulation of the auditory thalamus or auditory cortex on the acoustic startle reflex in awake rats

The amygdala mediates both emotional learning and fear potentiation of startle. The lateral amygdala nucleus (LA) receives auditory inputs from both the auditory thalamus (medial geniculate nucleus; MGN) and auditory association cortex (AAC), and is critical for auditory fear conditioning. The central amygdala nucleus, which has intra-amygdaloid connections with LA, enhances startle magnitude via midbrain connections to the startle circuits. Tetanic stimulation of either MGN or AAC in vitro or in vivo can induce long-term potentiation in LA. In the present study, behavioural consequences of tetanization of these auditory afferents were investigated in awake rats. The acoustic startle reflex of rats was enhanced by tetanic stimulation of MGN, but suppressed by that of AAC. All the tetanization-induced changes of startle diminished within 24 h. Blockade of GABAB receptors in the LA area reversed the suppressive effect of tetanic stimulation of AAC on startle but did not change the enhancing effect of tetanic stimulation of MGN. Moreover, transient electrical stimulation of MGN enhanced the acoustic startle reflex when it lagged behind acoustic stimulation, but inhibited the acoustic startle reflex when it preceded acoustic stimulation. The results of the present study indicate that MGN and AAC afferents to LA play different roles in emotional modulation of startle, and AAC afferents are more influenced by inhibitory GABAB transmission in LA.

Repeated stress induces dendritic spine loss in the rat medial prefrontal cortex

The prefrontal cortex (PFC) plays an important role in higher cognitive processes, and in the regulation of stress-induced hypothalamic-pituitary-adrenal (HPA) activity. Here we examined the effect of repeated restraint stress on dendritic spine number in the medial PFC. Rats were perfused after receiving 21 days of daily restraint stress, and intracellular iontophoretic injections of Lucifer Yellow were carried out in layer II/III pyramidal neurons in the anterior cingulate and prelimbic cortices. We found that stress results in a significant (16%) decrease in apical dendritic spine density in medial PFC pyramidal neurons, and confirmed a previous observation that total apical dendritic length is reduced by 20% in the same neurons. We estimate that nearly one-third of all axospinous synapses on apical dendrites of pyramidal neurons in medial PFC are lost following repeated stress. A decrease in medial PFC dendritic spines may not only be indicative of a decrease in the total population of axospinous synapses, but may impair these neurons' capacity for biochemical compartmentalization and plasticity in which dendritic spines play a major role. Dendritic atrophy and spine loss may be important cellular features of stress-related psychiatric disorders where the PFC is functionally impaired.

Repeated stress and structural plasticity in the brain

Although adrenal steroid receptors are distributed widely throughout the central nervous system, specific limbic and cortical regions targeted by stress hormones play a key role in integrating behavioral and physiological responses during stress and adaptation to subsequent stressors. When the stressor is of a sufficient magnitude or prolonged, it may result in abnormal changes in brain plasticity that, paradoxically, may impair the ability of the brain to appropriately regulate and respond to subsequent stressors. Here we review how repeated stress produces alterations in brain plasticity in animal models, and discuss its relevance to behavioral changes associated with these regions. Interestingly, prolonged stress produces opposing effects on structural plasticity, notably dendritic atrophy and excitatory synapse loss in the hippocampus and prefrontal cortex, and growth of dendrites and spines in the amygdala. The granule cells of the dentate gyrus are also significantly affected through a decrease in the rate neurogenesis following prolonged stress. How functional impairments in these brain regions play a role in stress-related mental illnesses is discussed in this context. Finally, we discuss the cumulative impact of stress-induced structural plasticity in aging.

Building and burying fear memories in the brain

The world is a dangerous place. Whether this danger takes the form of an automobile careening toward you or a verbal threat from a stranger, your brain is highly adapted to perceive such threats, organize appropriate defensive behaviors, and record the circumstances surrounding the experience. Indeed, memories of fearful events serve a critical biological function by allowing humans and other animals to anticipate future dangers. But these memories can also feed pathological fear, yielding crippling clinical conditions such as panic disorder. In this review, the author will examine how the brain builds fear memories and how these memories come to be suppressed when they no longer predict danger. The review will focus on the fundamental role for synapses in the amygdala in acquiring fear memories and the function of neural circuits interconnecting the amygdala, hippocampus, and prefrontal cortex in modulating the expression of such memories once learned. The discovery of the neural architecture for fear memory highlights the powerful interplay between animal and human research and the promise for understanding the neurobiological mechanisms of other complex cognitive phenomena.

Hemodynamic responses of the amygdala, the orbitofrontal cortex and the visual cortex during a fear conditioning paradigm

Functional magnetic resonance imaging (fMRI) studies consistently demonstrate an enhanced activation of the visual cortex in reaction to emotionally salient visual stimuli. This increase of activation is probably modulated by top-down processes, that are initiated in emotion processing structures, specifically the amygdala and the orbitofrontal cortex. In the present fMRI study, a differential fear conditioning paradigm was applied to investigate this assumed modulation. Hemodynamic responses towards a neutral visual stimulus (CS+) predicting an electrical stimulation (UCS) were compared with responses towards a neutral and unpaired stimulus (CS-). Thereby, particularly the time courses of neural responses were considered. Skin conductance measures were concurrently recorded. Our results show that the differentiation between CS+ and CS- within the amygdala and the extended visual cortex was accomplished during a late acquisition phase. In the orbitofrontal cortex the differentiation occurred at an earlier stage and was then sustained throughout acquisition. It is suggested that these altering activation patterns are reflecting different phases of learning, integrating the analyzed regions to varying degrees. Additionally, the results indicate that statistical analyses comprising a temporal variation of hemodynamic responses are more likely to detect amygdala activation.

Vasopressin and oxytocin excite distinct neuronal populations in the central amygdala

Vasopressin and oxytocin strongly modulate autonomic fear responses, through mechanisms that are still unclear. We describe how these neuropeptides excite distinct neuronal populations in the central amygdala, which provides the major output of the amygdaloid complex to the autonomic nervous system. We identified these two neuronal populations as part of an inhibitory network, through which vasopressin and oxytocin modulate the integration of excitatory information from the basolateral amygdala and cerebral cortex in opposite manners. Through this network, the expression and endogenous activation of vasopressin and oxytocin receptors may regulate the autonomic expression of fear.

Distinct Neural Signatures for Safety and Danger in the Amygdala and Striatum of the Mouse

The ability to identify, develop, and exploit conditions of safety and security is central to survival and mental health, but little is known of the neurobiology of these processes or associated positive modulations of affective state. We studied electrophysiological and affective correlates of learned safety by negatively correlating an auditory conditioned stimulus (CS) with aversive events (US). This CS came to signify a period of protection, reducing fear responses to predictors of the US and increasing adventurous exploration of a novel environment. In nonaversive conditions, mice turn on the CS when given the opportunity. Thus, conditioned safety involves a reduction of learned and instinctive fear, as well as positive affective responses. Concurrent electrophysiological measurements identified a safety learning-induced long-lasting depression of CS-evoked activity in the lateral nucleus of the amygdala, consistent with fear reduction, and an increase of CS-evoked activity in a region of the striatum involved in positive affect, euphoric responses, and reward.

Synergistic interactions between cannabinoids and environmental stress in the activation of the central amygdala

Anxiety and panic are the most common adverse effects of cannabis intoxication; reactions potentiated by stress. Data suggest that cannabinoid (CB1) receptor modulation of amygdalar activity contributes to these phenomena. Using Fos as a marker, we tested the hypothesis that environmental stress and CB1 cannabinoid receptor activity interact in the regulation of amygdalar activation in male mice. Both 30 min of restraint and CB1 receptor agonist treatment (Delta9-tetrahydrocannabinol (2.5 mg/kg) or CP55940 (0.3 mg/kg); by i.p. injection) produced barely detectable increases in Fos expression within the central amygdala (CeA). However, the combination of restraint and CB1 agonist administration produced robust Fos induction within the CeA, indicating a synergistic interaction between environmental stress and CB1 receptor activation. An inhibitor of endocannabinoid transport, AM404 (10 mg/kg), produced an additive interaction with restraint within the CeA. In contrast, fatty acid amide hydrolase (FAAH) inhibitor-treated mice (URB597, 1 mg/kg) and FAAH-/- mice did not exhibit any differences in amygdalar activation in response to restraint compared to control mice. In the basolateral (BLA) and medial amygdala, restraint stress produced a low level of Fos induction, which was unaffected by cannabinoid treatment. Interestingly, the CB1 receptor antagonist SR141716 dose-dependently increased Fos expression in the BLA and CeA. These data suggest the CeA is an important neural substrate subserving the interactions between cannabinoids and environmental stress, and could be relevant to understanding the context-dependent emotional and affective changes induced by marijuana intoxication and the role of endocannabinoid signaling in the modulation of amygdalar activity.

Brief social defeat stress: long lasting effects on cocaine taking during a binge and zif268 mRNA expression in the amygdala and prefrontal cortex

Social stress can engender behavioral and neural sensitization and this process appears to enhance the transition to compulsive drug abuse. Exposures to brief social defeat stress in rats have significant consequences on cocaine-reinforced behavior and on the level of functional activation within regions of the mesocorticolimbic dopamine system. The objectives of the current study were to examine the enduring consequences of brief episodes of social defeat stress on cocaine bingeing (during 24 h of continuous access) and on the emergence of neural adaptations as revealed by zif268 immediate early gene expression. Adult, male Long-Evans rats were subjected to four 25 min episodes of social defeat (once every 72 h). After 2 months, cocaine binges or zif268 mRNA gene expression were studied after confirming behavioral cross-sensitization to stimulant challenge. Sensitization to social defeat increased cocaine intake during a 24 h binge, effectively abolishing the typical circadian pattern of intake. Furthermore, 60 days after exposure to the sensitizing regimen of social defeat, levels of functional activation, measured by zif268 mRNA expression, in the central and medial amygdala were increased, while levels of activation in the medial prefrontal cortex were decreased. Persistent stress-induced levels of zif268 in the central and medial amygdala were attenuated by an injection of amphetamine (1.0 mg/kg). Divergent changes in zif268 within the amygdala and cortex 2 months after social defeat stress indicate the vulnerability of distinct cellular populations in networks that modulate the behavioral actions of psychomotor stimulants.

Neural substrates for voluntary suppression of negative affect: a functional magnetic resonance imaging study

BACKGROUND

Successful control of affect partly depends on the capacity to modulate negative emotional responses through the use of cognitive strategies. Although the capacity to regulate emotions is critical to mental well-being, its neural substrates remain unclear.

METHODS

We used functional magnetic resonance imaging to ascertain brain regions involved in the voluntary regulation of emotion and whether dynamic changes in negative emotional experience can modulate their activation. Fourteen healthy subjects were scanned while they either maintained the negative affect evoked by highly arousing and aversive pictures (e.g., experience naturally) or suppressed their affect using cognitive reappraisal. In addition to a condition-based analysis, online subjective ratings of intensity of negative affect were used as covariates of brain activity.

RESULTS

Inhibition of negative affect was associated with activation of dorsal anterior cingulate, dorsal medial prefrontal, and lateral prefrontal cortices, and attenuation of brain activity within limbic regions (e.g., nucleus accumbens/extended amygdala). Furthermore, activity within dorsal anterior cingulate was inversely related to intensity of negative affect, whereas activation of the amygdala was positively covaried with increasing negative affect.

CONCLUSIONS

These findings highlight a functional dissociation of corticolimbic brain responses, involving enhanced activation of prefrontal cortex and attenuation of limbic areas, during volitional suppression of negative emotion.

CB1 cannabinoid receptors modulate kinase and phosphatase activity during extinction of conditioned fear in mice

Cannabinoid receptors type 1 (CB1) play a central role in both short-term and long-term extinction of auditory-cued fear memory. The molecular mechanisms underlying this function remain to be clarified. Several studies indicated extracellular signal-regulated kinases (ERKs), the phosphatidylinositol 3-kinase with its downstream effector AKT, and the phosphatase calcineurin as potential molecular substrates of extinction behavior. To test the involvement of these kinase and phosphatase activities in CB1-dependent extinction of conditioned fear behavior, conditioned CB1-deficient mice (CB1(-/-)) and wild-type littermates (CB1(+/+)) were sacrificed 30 min after recall of fear memory, and activation of ERKs, AKT, and calcineurin was examined by Western blot analysis in different brain regions. As compared with CB1(+/+), the nonreinforced tone presentation 24 h after auditory-cued fear conditioning led to lower levels of phosphorylated ERKs and/or calcineurin in the basolateral amygdala complex, ventromedial prefrontal cortex, dorsal hippocampus, and ventral hippocampus of CB1(-/-). In contrast, higher levels of phosphorylated p44 ERK and calcineurin were observed in the central nucleus of the amygdala of CB1(-/-). Phosphorylation of AKT was more pronounced in the basolateral amygdala complex and the dorsal hippocampus of CB1(-/-). We propose that the endogenous cannabinoid system modulates extinction of aversive memories, at least in part via regulation of the activity of kinases and phosphatases in a brain structure-dependent manner.

Hippocampal and amygdalar involvement in discriminatory place learning

A conflict task was developed that simultaneously examines place aversion learning and fear-motivated context discrimination. The task superimposed Pavlovian discriminative fear conditioning on an appetitively motivated instrumental response (alternation). Rats were trained to alternate along a high-walled, diamond-shaped runway between two chambers for food. On half of the trials, a tone CS signaled the fact that a fixed section at the apex of the runway was electrified. Both the tone and the shock were turned on at the beginning of, and remained on for the duration of, each tone trial. A new trial was initiated at the time the animal entered the subsequent food chamber. Therefore, during a tone trial, in order to attain additional food reinforcement, the animal had to cross over the electrified region at the runway apex. Behavioral performance of rats with small lesions of the amygdala or dorsal hippocampus (DH) was compared with that of sham-operated controls. All groups displayed significant discriminative responding, hesitating more on tone trials while in areas of the runway adjacent to the shock region. Animals with lesions of the DH were similar to controls with respect to the tone-mediated discrimination, yet were delayed in the initial expression of a location-specific fear response. Conversely, amygdala lesions did not affect place learning; however, these animals were impaired in their suppression of the fear response following repeated unpaired trials.

Infralimbic cortex activation increases c-Fos expression in intercalated neurons of the amygdala

Recently, it was reported that stimulation of the infralimbic cortex produces a feedforward inhibition of central amygdala neurons. The interest of this observation comes from the fact that the central nucleus is the main output station of the amygdala for conditioned fear responses and evidence that the infralimbic cortex plays a critical role in the extinction of conditioned fear. However, the identity of the neurons mediating this infralimbic-evoked inhibition of the central nucleus remains unknown. Likely candidates are intercalated amygdala neurons. Indeed, these cells receive glutamatergic afferents from the infralimbic cortex, use GABA as a transmitter, and project to the central amygdala. Thus, the present study was undertaken to test whether, in adult rats, the infralimbic cortex can affect the activity of intercalated neurons. To this end, disinhibition of the infralimbic cortex was induced by local infusion of the non-competitive GABA-A receptor antagonist picrotoxin. Subsequently, neuronal activation was determined bilaterally within the amygdala using induction of the immediate early gene Fos. Infralimbic disinhibition produced a significant increase in the number of Fos-immunoreactive intercalated cells bilaterally whereas no change was detected in the central nucleus. In the basolateral amygdaloid complex, increases in the number of Fos-immunoreactive cells only reached significance in the contralateral lateral nucleus. These results suggest that glutamatergic inputs from the infralimbic cortex directly activate intercalated neurons. Thus, our findings raise the possibility that the infralimbic cortex inhibits conditioned fear via the excitation of intercalated cells and the consequent inhibition of central amygdala neurons.

Individual Differences in the Modulation of Fear-Related Brain Activation by Attentional Control

In this article, we consider the extent to which variations in the neural activation associated with fear-related stimuli are obligatory or optional. More specifically, we investigated modulation of activation according to type of encoding operation, and how this relates to individual differences in fearfulness and attentional control. In an fMRI study, fear-related (relative to neutral) pictures preferentially activated many of the regions involved in a hierarchical system responsible for organizing defensive behavior, and differential activation in some of these areas was related to self-reported individual variations in fearfulness. Preferential activation according to type of stimulus persisted to a limited extent even when attention was diverted from its emotional aspects. Importantly, however, encoding tasks involving attention to emotional versus nonemotional attributes of the same pictures revealed a pattern of greater activation during emotional encoding, similar to that differentiating fear-related from neutral stimuli. Again, the degree of modulation varied according to individual differences. We conclude that fear-related pictures can recruit activation in the defensive system even when attention is directed elsewhere, but that the extent of this activation is modulated by attentional control mechanisms. More critically, both differential activation and its modulation by attentional control are related to individual variations in emotional vulnerability, in a manner that conforms to predictions derived from existing theoretical accounts.