Human amygdala activation during conditioned fear acquisition and extinction: a mixed-trial fMRI study

Defining the neurocircuitry of borderline personality disorder: Functional neuroimaging approaches

Functional neuroimaging recently has been used to localize brain dysfunction in borderline personality disorder (BPD). Initial studies have examined baseline activity or emotional reactivity, and our group has investigated what we consider to be a crucial interaction between negative emotion and behavioral (dys)control. This research is beginning to identify abnormal frontolimbic circuitry likely underlying core clinical features of this condition. We review the evidence for dysfunction in specific frontolimbic regions, leading to a mechanistic model of symptom formation in BPD. In addition, we offer an integration of these neuroimaging findings with developmental perspectives on the emergence of borderline psychopathology, focusing on the ways in which early psychosocial experience may interact with developing brain systems. We also consider possible mechanisms of psychotherapeutic change at the neural systems level in BPD. Finally, we propose that future neuroimaging studies of BPD should integrate multiple levels of observation (structural, functional, neurochemical, genetic, and clinical) in a model-driven fashion to further understand the dynamic relationship between biological and psychological factors in the development and treatment of this difficult condition.

Dissociable networks for the expectancy and perception of emotional stimuli in the human brain

William James posited that comparable brain regions were implicated in the anticipation and perception of a stimulus; however, dissociable networks (at least in part) may also underlie these processes. Recent functional neuroimaging studies have addressed this issue by comparing brain systems associated with the expectancy and perception of visual, tactile, nociceptive, and reward stimuli. In the present fMRI study, we addressed this issue in the domain of pictorial emotional stimuli (IAPS). Our paradigm involved the experimental conditions emotional expectancy, neutral expectancy, emotional picture perception, and neutral picture perception. Specifically, the emotional expectancy cue was uncertain in that it did not provide additional information regarding the positive or negative valence of the subsequent picture. Neutral expectancy and neutral picture perception served as control conditions, allowing the identification of expectancy and perception effects specific for emotion processing. To avoid contamination of the perception conditions by the preceding expectancy periods, 50% of the pictorial stimuli were presented without preceding expectancy cues. We found that the emotional expectancy cue specifically produced activation in the supracallosal anterior cingulate, cingulate motor area, and parieto-occipital sulcus. These regions were not significantly activated by emotional picture perception which recruited a different neuronal network, including the amygdala, insula, medial and lateral prefrontal cortex, cerebellum, and occipitotemporal areas. This dissociation may reflect a distinction between anticipatory and perceptive components of emotional stimulus processing.

The role of serotonin in impulsive and aggressive behaviors associated with epilepsy-like neuronal hyperexcitability in the amygdala

Neuronal hyperexcitability in limbic areas, especially the amygdala, is a significant underlying mechanism associated with complex partial seizures (CPS). CPS may be comorbid with emotional disturbances, especially major mood disorders, anxiety, and aggression. Anticonvulsant medications such as phenytoin are also mood-stabilizing, and have been used for treatment of behavioral dyscontrol in impulsive aggressive individuals. Because the amygdala has important functional roles in epilepsy, emotion, and behavioral control, there may be common biological mechanisms involving neuronal excitability that contribute to both seizure activity and psychopathology. This review examines physiological mechanisms in the amygdala that regulate neuronal excitability and discusses how this may underlie, in part, disturbances in emotional behavior.

Formal learning theory dissociates brain regions with different temporal integration

Learning can be characterized as the extraction of reliable predictions about stimulus occurrences from past experience. In two experiments, we investigated the interval of temporal integration of previous learning trials in different brain regions using implicit and explicit Pavlovian fear conditioning with a dynamically changing reinforcement regime in an experimental setting. With formal learning theory (the Rescorla-Wagner model), temporal integration is characterized by the learning rate. Using fMRI and this theoretical framework, we are able to distinguish between learning-related brain regions that show long temporal integration (e.g., amygdala) and higher perceptual regions that integrate only over a short period of time (e.g., fusiform face area, parahippocampal place area). This approach allows for the investigation of learning-related changes in brain activation, as it can dissociate brain areas that differ with respect to their integration of past learning experiences by either computing long-term outcome predictions or instantaneous reinforcement expectancies.

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.

Cortical activation during Pavlovian fear conditioning depends on heart rate response patterns: An MEG study

In the present study, we examined stimulus-driven neuromagnetic activity in a delayed Pavlovian aversive conditioning paradigm using steady state visual evoked fields (SSVEF). Subjects showing an accelerative heart rate (HR) component to the CS+ during learning trials exhibited an increased activation in sensory and parietal cortex due to CS+ depiction in the extinction block. This was accompanied by a selective orientation response (OR) to the CS+ during extinction as indexed by HR deceleration. However, they did not show any differential cortical activation patterns during acquisition. In contrast, subjects not showing an accelerative HR component but rather unspecific HR changes during learning were characterized by greater activity in left orbito-frontal brain regions in the acquisition block but did not show differential SSVEF patterns during extinction. The results suggest that participants expressing different HR responses also differ in their stimulus-driven neuromagnetic response pattern to an aversively conditioned stimulus.

Contributions of the Amygdala to Emotion Processing: From Animal Models to Human Behavior

Research on the neural systems underlying emotion in animal models over the past two decades has implicated the amygdala in fear and other emotional processes. This work stimulated interest in pursuing the brain mechanisms of emotion in humans. Here, we review research on the role of the amygdala in emotional processes in both animal models and humans. The review is not exhaustive, but it highlights five major research topics that illustrate parallel roles for the amygdala in humans and other animals, including implicit emotional learning and memory, emotional modulation of memory, emotional influences on attention and perception, emotion and social behavior, and emotion inhibition and regulation.

Temperament and anxiety disorders

Recently, multidisciplinary research teams have come together to assess the emergence, course, and treatment of anxiety disorders in young children and adolescents. A number of researchers have suggested that early temperament traits may play a significant role in the causes and maintenance of early anxiety. This article reviews the current understanding of temperament and anxiety as separate constructs and then attempts to examine the developmental links between the two constructs. The authors examine the outstanding issues that must be addressed before the benefits of bridging these traditionally independent fields of study can be fully exploited.

Emotional learning and glutamate: translational perspectives

Anxiety disorders are a common focus of clinical concern and certain forms of anxiety may be conceptualized as disorders of emotional learning. Behavior therapies effective in the treatment of anxiety are modeled on extinction training as a means of reducing pathological anxiety. The present understanding of human anxiety has been informed by preclinical research using rodent models to study the acquisition and extinction of fear. Glutamate appears to have a central role in both of these processes. The authors review this literature and discuss novel applications of D-cycloserine, a partial N-methyl-D-aspartate agonist, for the treatment of anxiety.

Effects of a 5-HT7 receptor antagonist DR4004 on the exploratory behavior in a novel environment and on brain monoamine dynamics in mice

The present study examined whether serotonin (5-hydroxytryptamine; 5-HT)7 receptors play a role in the modulation of emotionality in mice using the selective 5-HT7 receptor antagonist 2a-[4-(4-phenyl-1,2,3,6-tetrahydropyridyl)butyl]-2a,3,4,5-tetrahydrobenzo (c,d)indol-2-(1H)-one (DR4004). The emotionality of mice was evaluated in terms of exploratory activity in the hole-board test. The mice treated with DR4004 (2.5-10 mg/kg, i.p.) displayed a dose-dependent decrease in locomotor activity by moving less distance in the hole-board, and statistically significant decreases were observed at 5 and 10 mg/kg. On the other hand, DR4004 (10 mg/kg, i.p.) did not affect spontaneous motor activity. In a neurochemical study, decreases in amygdaloid dopamine and 5-HT turnover were observed in mice in which locomotor activity in the hole-board test was attenuated following the administration of DR4004 (10 mg/kg, i.p.). Also, a simple linear regression analysis revealed that locomotor activity on the hole-board was significantly correlated with dopamine and 5-HT turnover in amygdala. Furthermore, co-injection of the selective dopamine reuptake inhibitor 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)piperazine (GBR12909; 1.25-5 mg/kg, i.p.) or the selective 5-HT reuptake inhibitor fluvoxamine (20 mg/kg, i.p.) significantly reversed the DR4004 (10 mg/kg, i.p.)-induced decrease in locomotor activity in the hole-board test. These findings constitute the behavioral evidence that 5-HT7 receptors may play a role in the modulation of emotionality. Furthermore, it is also suggested that amygdaloid dopamine and 5-HT neuronal systems may be involved in this modulation.

Responding to the emotions of others: dissociating forms of empathy through the study of typical and psychiatric populations

Empathy is a lay term that is becoming increasingly viewed as a unitary function within the field of cognitive neuroscience. In this paper, a selective review of the empathy literature is provided. It is argued from this literature that empathy is not a unitary system but rather a loose collection of partially dissociable neurocognitive systems. In particular, three main divisions can be made: cognitive empathy (or Theory of Mind), motor empathy, and emotional empathy. The two main psychiatric disorders associated with empathic dysfunction are considered: autism and psychopathy. It is argued that individuals with autism show difficulties with cognitive and motor empathy but less clear difficulties with respect to emotional empathy. In contrast, individuals with psychopathy show clear difficulties with a specific form of emotional empathy but no indications of impairment with cognitive and motor empathy.

Neural-cardiac coupling in threat-evoked anxiety

Anxiety is a debilitating symptom of many psychiatric disorders including generalized anxiety disorder, mood disorders, schizophrenia, and autism. Anxiety involves changes in both central and peripheral biology, yet extant functional imaging studies have focused exclusively on the brain. Here we show, using functional brain and cardiac imaging in sequential brain and cardiac magnetic resonance imaging (MRI) sessions in response to cues that predict either threat (a possible shock) or safety (no possibility of shock), that MR signal change in the amygdala and the prefrontal and insula cortices predicts cardiac contractility to the threat of shock. Participants with greater MR signal change in these regions show increased cardiac contractility to the threat versus safety condition, a measure of the sympathetic nervous system contribution to the myocardium. These findings demonstrate robust neural-cardiac coupling during induced anxiety and indicate that individuals with greater activation in brain regions identified with aversive emotion show larger magnitude cardiac contractility increases to threat.

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.

Evaluative conditioning is intact after unilateral resection of the anterior temporal lobe in humans

Several lesion and functional imaging studies conducted in animals and humans suggest that structures within the amygdaloid nuclear complex (ANC) are important for the occurrence of fear conditioning. Whether this brain structure is also critical for evaluative conditioning, has been investigated less frequently. In the current experiment, a group of participants with unilateral resection of the anterior temporal lobe and a control group received a differential evaluative flavor-taste conditioning task. In the pre-acquisition phase, two fruit flavors (the conditioned stimuli (CSs)) were presented and participants were instructed to evaluate both. In the subsequent acquisition phase, one of these fruit flavors (CS+) was presented together with a bad tasting substance Tween20 (polysorbate 20, the US), while the other flavor (CS-) was never paired with Tween20. Finally, in the post-acquisition phase, the two flavors were presented again without Tween20 and participants were asked to evaluate both of them for a last time. The control group as well as the lesion group rated the CS+ in the post-acquisition phase less favorable than in the pre-acquisition phase, while the ratings of the CS- remained the same in both phases. We clearly demonstrated evaluative conditioning in both test groups. Because the lesion group had still one intact ANC it would be premature, however, to conclude that the ANC is not involved in evaluative conditioning. We conclude that despite evidence for impaired fear conditioning, unilateral damage to the ANC does not impair evaluative conditioning.

The role of the human amygdala in the production of conditioned fear responses

The amygdala plays a central role in the acquisition and expression of fear memories. Laboratory animal studies indicate that the amygdala both receives sensory information and produces learned behavioral and autonomic fear responses. However, prior functional imaging research in humans has largely focused on amygdala activity elicited by fearful stimuli, giving less attention to this region's role in the production of fear responses. In contrast, the present study used functional magnetic resonance imaging to investigate the amygdala's influence on the generation of conditional fear responses. Significant increases in amygdala activity were observed during the production of conditioned (learning-related), but not orienting, nonspecific, and unconditioned (nonlearning-related) skin conductance responses. Further, greater amygdala activity was demonstrated during conditioned response production than during conditioned stimulus presentation. These results suggest the amygdala not only responds to fearful stimuli, but also generates learning-related changes in human autonomic fear expression.

Variation of human amygdala response during threatening stimuli as a function of 5'HTTLPR genotype and personality style

BACKGROUND

In the brain, processing of fearful stimuli engages the amygdala, and the variability of its activity is associated with genetic factors as well as with emotional salience. The objective of this study was to explore the relevance of personality style for variability of amygdala response.

METHODS

We studied two groups (n=14 in each group) of healthy subjects categorized by contrasting cognitive styles with which they attribute salience to fearful stimuli: so-called phobic prone subjects who exaggerate potential environmental threat versus so-called eating disorders prone subjects who tend to be much less centered around fear. The two groups underwent functional magnetic resonance imaging (fMRI) at 3T during performance of a perceptual task of threatening stimuli and they were also matched for the genotype of the 5' variable number tandem repeat (VNTR) polymorphism in the serotonin transporter.

RESULTS

The fMRI results indicated that phobic prone subjects selectively recruit the amygdala to a larger extent than eating disorders prone subjects. Activity in the amygdala was also independently predicted by personality style and genotype of the serotonin transporter. Moreover, brain activity during a working memory task did not differentiate the two groups.

CONCLUSIONS

The results of the present study suggest that aspects of personality style are rooted in biological responses of the fear circuitry associated with processing of environmental information.

Reinstatement of conditioned fear in humans is context dependent and impaired in amnesia

A contextual reinstatement procedure was developed to assess the contributions of environmental cues and hippocampal function in the recovery of conditioned fear following extinction in humans. Experiment 1 showed context specificity in the recovery of extinguished skin conductance responses after presentations of an auditory unconditioned stimulus. Experiment 2 demonstrated that fear recovery did not generalize to an explicitly unpaired conditioned stimulus. Experiment 3 replicated the context dependency of fear recovery with a shock as an unconditioned stimulus. Two amnesic patients failed to recover fear responses following reinstatement in the same context, despite showing initial fear acquisition. These results extend the known functions of the human hippocampus and highlight the importance of environmental contexts in regulating the expression of latent fear associations.

Positron emission tomographic imaging of neural correlates of a fear acquisition and extinction paradigm in women with childhood sexual-abuse-related post-traumatic stress disorder

BACKGROUND

In the conditioned fear paradigm, repeated pairing of an aversive unconditioned stimulus (US) (e.g. electric shock) with a neutral conditioned stimulus (CS) (e.g. bright light) results in a conditioned fear response to the light alone. Animal studies have shown that the amygdala plays a critical role in acquisition of conditioned fear responses, while the medial prefrontal cortex (including anterior cingulate), through inhibition of amygdala responsiveness, has been hypothesized to play a role in extinction of fear responses. No studies have examined neural correlates of fear conditioning and extinction in patients with post-traumatic stress disorder (PTSD).

METHOD

Women with early childhood sexual-abuse-related PTSD (n = 8) and women without abuse or PTSD (n = 11) underwent measurement of psychophysiological (skin conductance) responding as well as positron emission tomographic (PET) measurement of cerebral blood flow during habituation, acquisition and extinction conditions. During habituation subjects were repeatedly exposed to a blue square on a screen. During acquisition, exposure to the blue square (CS) was paired with an electric shock to the forearm (US). With extinction, subjects were again exposed to the blue squares without shock. On a different day subjects went through the same procedure with electric shocks administered randomly in the absence of the blue square.

RESULTS

Skin conductance responding to the CS was consistent with the development of conditioned responses with this paradigm. PTSD patients had increased left amygdala activation with fear acquisition, and decreased anterior cingulate function during extinction, relative to controls.

CONCLUSIONS

These findings implicate amygdala and anterior cingulate in the acquisition and extinction of fear responses, respectively, in PTSD.

Sex, stress, and fear: individual differences in conditioned learning

It has long been recognized that humans vary in their conditionability, yet the factors that contribute to individual variation in emotional learning remain to be delineated. The goal of the present study was to investigate the relationship among sex, stress hormones, and fear conditioning in humans. Forty-five healthy adults (22 females) underwent differential delay conditioning, using fear-relevant conditioned stimuli and a shock unconditioned stimulus. Salivary cortisol samples were taken at baseline and after acquisition training and a 24-h-delayed retention test. The results showed that acquisition of conditioning significantly correlated with postacquisition cortisol levels in males, but not in females. This sex-specific relationship was found despite similar overall levels of conditioning, unconditioned responding, and cortisol. There was no effect of postacquisition cortisol on consolidation of fear learning in either sex. These findings have implications for the understanding of individual differences in fear acquisition and risk factors for the development of affective disorders.

Amygdala activation to sad pictures during high-field (4 tesla) functional magnetic resonance imaging

Fear-related processing in the amygdala has been well documented, but its role in signaling other emotions remains controversial. The authors recovered signal loss in the amygdala at high-field strength using an inward spiral pulse sequence and probed its response to pictures varying in their degree of portrayed sadness. These pictures were presented as intermittent task-irrelevant distractors during a concurrent visual oddball task. Relative to neutral distractors, sad distractors elicited greater activation along ventral brain regions, including the amygdala, fusiform gyrus, and inferior frontal gyrus. In contrast, oddball targets engaged dorsal sectors of frontal, parietal, and cingulate cortices. The amygdala's role in emotional evaluation thus extends to images of grief and despair as well as to those depicting violence and threat.

The psychobiology of neglect

Child neglect, the most prevalent form of child maltreatment, is associated with adverse psychological and educational outcomes. It is hypothesized that these outcomes may be caused by adverse brain development. However, there are very few published cross-sectional studies and no prospective studies that examine the neurodevelopmental consequences of neglect. In this article, the author comprehensively outlines the issues involved in the psychobiological research of child neglect. Pre-clinical and clinical studies will be reviewed. Throughout the article, suggestions for future research opportunities and novel ways to address methodological difficulties inherent in this field of study are offered. The results of recent neuroimaging studies of maltreated children may provide a basis for understanding the early effects of neglect on childhood brain development. The author is comprehensively examining these issues as part of the Federal Child Neglect Consortium.

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.

Neural basis of fear conditioning induced by video clip: positron emission tomography study

In patients with post-traumatic stress disorder (PTSD), re-experiencing the trauma is often induced by external cues in the environment. The cues, which were emotionally neutral for the patients before the traumatic event, become fearful ones after the event. This phenomenon is considered to be associated with fear conditioning. The paradigm was set up so that the emotionality changes in the patients with PTSD would be reproduced, and the regional cerebral blood flow (rCBF) measured with positron emission tomography (PET) was compared during exposure to the same stimuli before and after acquisition of fear conditioning. Ten healthy male subjects were asked to look at some emotionally neutral photos, then to watch a video with fearful content that also contained images similar to that presented in the photos, and afterwards to look at the photos again. Five of the 10 subjects felt that the object in the photos was more fearful after watching the video than before, and they were considered to have acquired fear conditioning. In those five subjects, the rCBF in the right amygdala and the left posterior cingulate gyrus after acquisition of fear conditioning significantly increased relative to the rCBF before conditioning. Thus, these regions seem to have a critical role in fear conditioning.

Differential time courses and specificity of amygdala activity in posttraumatic stress disorder subjects and normal control subjects

BACKGROUND

Previous neuroimaging studies have demonstrated exaggerated amygdala responses to negative stimuli in posttraumatic stress disorder (PTSD). The time course of this amygdala response is largely unstudied and is relevant to questions of habituation and sensitization in PTSD exposure therapy.

METHODS

We applied blood oxygen level dependent functional magnetic resonance imaging and statistical parametric mapping to study amygdala responses to trauma-related and nontrauma-related emotional words in sexual/physical abuse PTSD and normal control subjects. We examined the time course of this response by separate analysis of early and late epochs.

RESULTS

PTSD versus normal control subjects have a relatively increased initial amygdala response to trauma-related negative, but not nontrauma-related negative, versus neutral stimuli. Patients also fail to show the normal patterns of sensitization and habituation to different categories of negative stimuli. These findings correlate with measured PTSD symptom severity.

CONCLUSIONS

Our results demonstrate differential time courses and specificity of amygdala response to emotional and control stimuli in PTSD and normal control subjects. This has implications for pathophysiologic models of PTSD and treatment response. The results also extend previous neuroimaging studies demonstrating relatively increased amygdala response in PTSD and expand these results to a largely female patient population probed with emotionally valenced words.

The neuroscience of mammalian associative learning

Mammalian associative learning is organized into separate anatomically defined functional systems. We illustrate the organization of two of these systems, Pavlovian fear conditioning and Pavlovian eyeblink conditioning, by describing studies using mutant mice, brain stimulation and recording, brain lesions and direct pharmacological manipulations of specific brain regions. The amygdala serves as the neuroanatomical hub of the former, whereas the cerebellum is the hub of the latter. Pathways that carry information about signals for biologically important events arrive at these hubs by circuitry that depends on stimulus modality and complexity. Within the amygdala and cerebellum, neural plasticity occurs because of convergence of these stimuli and the biologically important information they predict. This neural plasticity is the physical basis of associative memory formation, and although the intracellular mechanisms of plasticity within these structures share some similarities, they differ significantly. The last Annual Review of Psychology article to specifically tackle the question of mammalian associative learning ( Lavond et al. 1993 ) persuasively argued that identifiable "essential" circuits encode memories formed during associative learning. The next dozen years saw breathtaking progress not only in detailing those essential circuits but also in identifying the essential processes occurring at the synapses (e.g., Bi & Poo 2001, Martinez & Derrick 1996 ) and within the neurons (e.g., Malinow & Malenka 2002, Murthy & De Camilli 2003 ) that make up those circuits. In this chapter, we describe the orientation that the neuroscience of learning has taken and review some of the progress made within that orientation.

Amygdala and hippocampal activity during acquisition and extinction of human fear conditioning

Previous functional magnetic resonance imaging (fMRI) studies have characterized brain systems involved in conditional response acquisition during Pavlovian fear conditioning. However, the functional neuroanatomy underlying the extinction of human conditional fear remains largely undetermined. The present study used fMRI to examine brain activity during acquisition and extinction of fear conditioning. During the acquisition phase, participants were either exposed to light (CS) presentations that signaled a brief electrical stimulation (paired group) or received light presentations that did not serve as a warning signal (control group). During the extinction phase, half of the paired group subjects continued to receive the same treatment, whereas the remainder received light alone. Control subjects also received light alone during the extinction phase. Changes in metabolic activity within the amygdala and hippocampus support the involvement of these regions in each of the procedural phases of fear conditioning. Hippocampal activity developed during acquisition of the fear response. Amygdala activity increased whenever experimental contingencies were altered, suggesting that this region is involved in processing changes in environmental relationships. The present data show learning-related amygdala and hippocampal activity during human Pavlovian fear conditioning and suggest that the amygdala is particularly important for forming new associations as relationships between stimuli change.

Sex-related developmental differences in the lateralized activation of the prefrontal cortex and amygdala during perception of facial affect

The lateralization of cognitive abilities is influenced by a number of factors, including handedness, sex, and developmental maturation. To date, a small number of studies have examined sex differences in the lateralization of cognitive and affective functions, and in only few of these have the developmental trajectories of these lateralized differences been mapped from childhood through early adulthood. In the present study, a cross-sectional design was used with healthy children (n=7), adolescents (n= 12), and adults (n= 10) who underwent functional magnetic resonance imaging (fMRI) during a task that required perceiving fearful faces. Males and females differed in the asymmetry of activation of the amygdala and prefrontal cortex across the three age groups. For males, activation within the dorsolateral prefrontal cortex was bilateral in children, right lateralized in adolescents, and bilateral in adults, whereas females showed a monotonic relationship with age, with older females showing more bilateral activation than younger ones. In contrast, amygdala activation was similar for both sexes, with bilateral activation in children, right-lateralized activation in adolescents, and bilateral activation in adults. These results suggest that males and females show different patterns of lateralized cortical and subcortical brain activation across the period of development from childhood through early adulthood.

Abnormal neural responses to emotional visual stimuli in adolescents with conduct disorder

BACKGROUND

It is widely held that aggression and antisocial behavior arise as a consequence of a deficiency in responding to emotional cues in the social environment. We asked whether neural responses evoked by affect-laden pictures would be abnormal in adolescents with conduct disorder (CD).

METHODS

Functional magnetic resonance imaging during passive viewing of pictures with neutral or strong negative affective valence was performed in 13 male adolescents with severe CD aged 9 to 15 years and in 14 healthy age-matched control subjects.

RESULTS

Main effects for negative-neutral affective valence included activations in the amygdala and hippocampus, ventral extrastriate visual cortex, and intraparietal sulcus bilaterally. There was a significant group-by-condition interaction in the right dorsal anterior cingulate cortex that was due to a pronounced deactivation in the patient group during viewing of negative pictures. When correcting for anxiety and depressive symptoms, we additionally found a reduced responsiveness of the left amygdala to negative pictures in patients compared with control subjects.

CONCLUSIONS

We suggest that these findings reflect an impairment of both the recognition of emotional stimuli and the cognitive control of emotional behavior in patients with CD, resulting in a propensity for aggressive behavior.

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.

Implicit and Explicit Evaluation: fMRI Correlates of Valence, Emotional Intensity, and Control in the Processing of Attitudes

Previous work suggests that explicit and implicit evaluations (good–bad) involve somewhat different neural circuits that process different dimensions such as valence, emotional intensity, and complexity. To better understand these differences, we used functional magnetic resonance imaging to identify brain regions that respond differentially to such dimensions depending on whether or not an explicit evaluation is required. Participants made either good–bad judgments (evaluative) or abstract–concrete judgments (not explicitly evaluative) about socially relevant concepts (e.g., “murder,” “happiness,” “abortion,” “welfare”). After scanning, participants rated the concepts for goodness, badness, emotional intensity, and how much they tried to control their evaluation of the concept. Amygdala activation correlated with emotional intensity and right insula activation correlated with valence in both tasks, indicating that these aspects of stimuli were processed by these areas regardless of intention. In contrast, for the explicitly evaluative good–bad task only, activity in the anterior cingulate, frontal pole, and lateral areas of the orbital frontal cortex correlated with ratings of control, which in turn were correlated with a measure of ambivalence. These results highlight that evaluations are the consequence of complex circuits that vary depending on task demands.

Reduced temporal lobe volume in early onset conduct disorder

Regional brain volumes derived from magnetic resonance imaging (MRI) scans from 10 youths with early onset conduct disorder and 10 healthy controls matched for age, sex and handedness were compared to determine whether prefrontal or temporal lobe brain volumes differed in the two groups. Right temporal lobe and right temporal gray matter volumes were significantly reduced in subjects with conduct disorder compared with controls. Prefrontal volumes in subjects with conduct disorder were 16% smaller than in controls, but the difference did not reach statistical significance. Early onset conduct disorder without substance abuse comorbidity was also significantly associated with smaller right temporal gray volumes. Further investigation of both the temporal and frontal localizations of the pathophysiology of early onset conduct disorder is warranted in larger samples.

Well-being and affective style: neural substrates and biobehavioural correlates

One of the most salient features of emotion is the pronounced variability among individuals in their reactions to emotional incentives and in their dispositional mood. Collectively, these individual differences have been described as affective style. Recent research has begun to dissect the constituents of affective style. The search for these components is guided by the neural systems that instantiate emotion and emotion regulation. In this article, this body of research and theory is applied specifically to positive affect and well-being. The central substrates and peripheral biological correlates of well-being are described. A resilient affective style is associated with high levels of left prefrontal activation, effective modulation of activation in the amygdala and fast recovery in response to negative and stressful events. In peripheral biology, these central patterns are associated with lower levels of basal cortisol and with higher levels of antibody titres to influenza vaccine. The article concludes with a consideration of whether these patterns of central and peripheral biology can be modified by training and shifted toward a more salubrious direction.

Resistance to extinction is associated with impaired immediate early gene induction in medial prefrontal cortex and amygdala

Extinction of classical fear conditioning is thought to involve activity-dependent potentiation of synaptic transmission in the medial prefrontal cortex (mPFC), resulting in the inhibition of amygdala-dependent fear responses. While many studies have addressed the mechanisms underlying extinction learning, it is unclear what determines whether extinction memory is consolidated or whether spontaneous recovery of the fear response occurs. Here we show, using a combined electrophysiological and immunocytochemical approach, that spontaneous recovery of conditioned fear in mice is associated with a prolonged expression of long-term depression of synaptic transmission in the mPFC and the failure of induction of the immediate-early genesc-Fos and zif268 in the mPFC and the basolateral nucleus of the amygdala. This suggests that coordinated activity-dependent changes in gene expression in the mPFC and the amygdala may underlie the formation of long-term fear extinction memory.

Human orbitofrontal cortex mediates extinction learning while accessing conditioned representations of value

In extinction, an animal learns that a previously conditioned stimulus (CS+) no longer predicts delivery of a salient reinforcer (unconditioned stimulus, UCS). Rodent studies indicate that extinction relies on amygdala-prefrontal interactions and involves formation of memories that inhibit, without actually erasing, the original conditioning trace. Whether extinction learning in humans follows similar neurobiological principles is unknown. We used functional magnetic resonance imaging to measure human brain activity evoked during olfactory aversive conditioning and extinction learning. Neural responses in orbitofrontal cortex and amygdala were preferentially enhanced during extinction, suggesting potential cross-species preservation of learning mechanisms that oppose conditioning. Moreover, by manipulating UCS aversiveness via reinforcer inflation, we showed that a CS+ retains access to representations of UCS value in distinct regions of ventral prefrontal cortex, even as extinction proceeds.

The neural correlates of mate competition in dominant male rhesus macaques

BACKGROUND

Male sexual jealousy provoked by threatened exclusive access to a female mate is a frequently reported motive in cases involving spousal abuse. Dominant male rhesus macaques also respond aggressively to threats to mating exclusivity.

METHODS

Nine dominant male monkeys were injected with [(18)F]-fluorodeoxyglucose ([(18)F]-FDG) and then exposed to one of two conditions: a "challenge" condition in which they witnessed a potential sexual interaction between their female consort and a rival male, and a control condition in which the consort was present without the rival male. After the brain uptake period for [(18)F]-FDG, dominant males were sedated, blood samples were drawn, and regional cerebral glucose metabolism was measured with positron emission tomographic imaging.

RESULTS

Males that showed larger increases in plasma testosterone in the challenge condition showed more aggression and greater activation in the central gray matter of the midbrain, a brain area rich in androgen receptors. The challenge condition was associated with activation in both right superior temporal sulcus and right amygdala, which might relate to increased social vigilance and anxiety, respectively.

CONCLUSIONS

Sexual jealousy in male humans is also often accompanied by vigilance behavior and anxiety and might recruit a similar neural network to that described here.

Extinction Learning in Humans

Understanding how fears are acquired is an important step in translating basic research to the treatment of fear-related disorders. However, understanding how learned fears are diminished may be even more valuable. We explored the neural mechanisms of fear extinction in humans. Studies of extinction in nonhuman animals have focused on two interconnected brain regions: the amygdala and the ventral medial prefrontal cortex (vmPFC). Consistent with animal models suggesting that the amygdala is important for both the acquisition and extinction of conditioned fear, amygdala activation was correlated across subjects with the conditioned response in both acquisition and early extinction. Activation in the vmPFC (subgenual anterior cingulate) was primarily linked to the expression of fear learning during a delayed test of extinction, as might have been expected from studies demonstrating this region is critical for the retention of extinction. These results provide evidence that the mechanisms of extinction learning may be preserved across species.

Context-dependent Deactivation of the Amygdala during Pain

The amygdala has been implicated in fundamental functions for the survival of the organism, such as fear and pain. In accord with this, several studies have shown increased amygdala activity during fear conditioning and the processing of fear-relevant material in human subjects. In contrast, functional neuroimaging studies of pain have shown a decreased amygdala activity. It has previously been proposed that the observed deactivations of the amygdala in these studies indicate a cognitive strategy to adapt to a distressful but in the experimental setting unavoidable painful event. In this positron emission tomography study, we show that a simple contextual manipulation, immediately preceding a painful stimulation, that increases the anticipated duration of the painful event leads to a decrease in amygdala activity and modulates the autonomic response during the noxious stimulation. On a behavioral level, 7 of the 10 subjects reported that they used coping strategies more intensely in this context. We suggest that the altered activity in the amygdala may be part of a mechanism to attenuate pain-related stress responses in a context that is perceived as being more aversive. The study also showed an increased activity in the rostral part of anterior cingulate cortex in the same context in which the amygdala activity decreased, further supporting the idea that this part of the cingulate cortex is involved in the modulation of emotional and pain networks.

New vistas on amygdala networks in conditioned fear

It is currently believed that the acquisition of classically conditioned fear involves potentiation of conditioned thalamic inputs in the lateral amygdala (LA). In turn, LA cells would excite more neurons in the central nucleus (CE) that, via their projections to the brain stem and hypothalamus, evoke fear responses. However, LA neurons do not directly contact brain stem-projecting CE neurons. This is problematic because CE projections to the periaqueductal gray and pontine reticular formation are believed to generate conditioned freezing and fear-potentiated startle, respectively. Moreover, like LA, CE may receive direct thalamic inputs communicating information about the conditioned and unconditioned stimuli. Finally, recent evidence suggests that the CE itself may be a critical site of plasticity. This review attempts to reconcile the current model with these observations. We suggest that potentiated LA outputs disinhibit CE projection neurons via GABAergic intercalated neurons, thereby permitting associative plasticity in CE. Thus plasticity in both LA and CE would be necessary for acquisition of conditioned fear. This revised model also accounts for inhibition of conditioned fear after extinction.

Acceleration of visually cued conditioned fear through the auditory pathway

Defensive responses elicited by sensory experiences are critical for survival. Mice acquire a conditioned fear response rapidly to an auditory cue but slowly to a visual cue, a difference in learned behavior that is likely to be mediated by direct projections to the lateral amygdala from the auditory thalamus but mainly indirect ones from the visual thalamus. Here, we show that acquisition of visually cued conditioned fear is accelerated in 'rewired' mice that have retinal projections routed to the auditory thalamus. Visual stimuli induce expression of the immediate early gene Fos (also known as c-fos) in the auditory thalamus and the lateral amygdala in rewired mice, similar to the way auditory stimuli do in control mice. Thus, the rewired auditory pathway conveys visual information and mediates rapid activity-dependent plasticity in central structures that influence learned behavior.

Lateralization of amygdala activation: a systematic review of functional neuroimaging studies

Functional neuroimaging studies of emotion processing consistently report amygdala activation. Most of these studies observed lateralized amygdala activity, indicative of a clear hemisphere-specific processing difference between the left and right amygdalae. Because individual studies use varying paradigms and are limited by statistical power and sensitivity, it has remained unclear whether the left or the right amygdala is more consistently involved in emotional processing. By combining results across 54 fMRI and PET studies in a metaanalysis, we sought to establish if a common pattern of lateralized amygdala activation exists. Our findings indicate that across studies, the left amygdala is more often activated than the right amygdala, suggesting different roles for the left and right amygdalae in emotional processing. Further analysis showed that this predominant left amygdala activation is not significantly related to stimulus type, task instructions, differential habituation rates of the left and right amygdalae or elaborate processing. The results are discussed in relation to methodological and theoretical issues regarding functional brain asymmetry.

Early amygdala reaction to fear spreading in occipital, temporal, and frontal cortex: a depth electrode ERP study in human

The amygdala involvement in fear processing has been reported in behavioral, electrophysiological, and functional imaging studies. However, the literature does not provide precise data on the temporal course of facial emotional processing. Intracranial event-related potentials to facial expressions were recorded in epileptic patients implanted with depth electrodes during a presurgical evaluation. Specific potentials to fear beginning 200 ms poststimulus were observed in amygdala, both individually in two patients and in a ten patient population study. These potentials occurred 100 ms earlier than potentials to disgust recorded in insula in a previous study. Potentials to fear were confined in amygdala during a first transient period and then, during a second period of sustained activity, spread to occipito-temporal, anterior temporal, and orbitofrontal cortex in two patients. This study clarifies the temporal course of the involvement of these structures known to be part of a neural network recruited to process emotional information.

Repetition suppression of faces is modulated by emotion

Single-unit recordings and functional brain imaging studies have shown reduced neural responses to repeated stimuli in the visual cortex. By using event-related functional MRI, we compared the activation evoked by repetitions of neutral and fearful faces, which were either task relevant (targets) or irrelevant (distracters). We found that within the inferior occipital gyri, lateral fusiform gyri, superior temporal sulci, amygdala, and the inferior frontal gyri/insula, targets evoked stronger responses than distracters and their repetition was associated with significantly reduced responses. Repetition suppression, as manifested by the difference in response amplitude between the first and third repetitions of a target, was stronger for fearful than neutral faces. Distracter faces, regardless of their repetition or valence, evoked negligible activation, indicating top-down attenuation of behaviorally irrelevant stimuli. Our findings demonstrate a three-way interaction between emotional valence, repetition, and task relevance and suggest that repetition suppression is influenced by high-level cognitive processes in the human brain.

Trajectories of Alcohol Use and Electrophysiological and Morphological Indices of Brain Development: Distinguishing Causes from Consequences

Alcoholism is a major public health problem. Patterns of drinking during adolescence can influence the likelihood of this outcome. Both environmental variation and familial/genetic susceptibility play important roles in this process. While there is some evidence to suggest that metabolic factors play a role in whether some individuals are protected from developing alcohol problems, there is substantial reason to look for cognitive factors that are associated with increased susceptibility. Developmental trajectories for information processing that can be reflected in P300 amplitude changes over time, as well as trajectories describing acquisition of postural control when compared in offspring from families with multiple cases of alcoholism or those with none or few, suggest that brain development provides a clue to why some individuals are more susceptible to becoming alcoholic. Finally, differences seen in amygdala volume between high- and low-risk adolescents suggest that functional differences seen in electrophysiological responding or neuropsychological test performance may have anatomical correlates.

Adaptive and maladaptive psychobiological responses to severe psychological stress: implications for the discovery of novel pharmacotherapy

Post-traumatic stress disorder (PTSD) is one of the few DSM-IV diagnoses contingent upon a psychosocial stressor. In this context, there is an urgent need to acquire a better understanding of both the adaptive and maladaptive psychobiological responses to traumatic stress. Preclinical investigators have utilized a variety of animal models to identify the behavioral and neurobiological features of the organism's response to stress. However, given the complexity of the healthy and pathological human response to physiological and psychological stress, the extent to which the animal data is immediately transferable to human remains to be fully determined. This review draws upon preclinical and clinical literature to examine the transformation of an adaptive human stress response into a maladaptive and debilitating mental disorder. An integrative psychobiological model for PTSD is presented, linking psychological processes and behavioral patterns with current findings in neurocircuitry, neurochemistry and psychophysiology. The implications of this model for the discovery of novel pharmacological approaches to the treatment of severe psychological distress are discussed.

Dissociable amygdala and orbitofrontal responses during reversal fear conditioning

The neural mechanisms underlying the persistence and plasticity of human emotional learning are unknown. Here we describe dissociable neural responses in amygdala and orbitofrontal cortex during acquisition and reversal of discriminatory fear conditioning. During acquisition, increased responses in bilateral amygdala were elicited by a face stimulus (A = CS+) predictive of an aversive noise compared to another nonpredictive face (B = CS-). With subsequent reversal of the conditioning contingency, face B (new CS+) elicited enhanced responses in right orbitofrontal cortex, while face A (old CS+) continued to evoke increased responses in right ventral amygdala. Thus, while orbitofrontal cortex exhibited rapid reversal of acquired fear responses, ventral amygdala showed a persistent, nonreversing "memory" for previous fear-related stimulus associations.

Cerebellar activation during leg withdrawal reflex conditioning: an fMRI study

OBJECTIVE

The aim of the present study was to examine cerebellar areas related to conditioning of the nociceptive leg withdrawal reflex using event-related functional magnetic resonance imaging (fMRI). Because of the aversive nature of the unconditioned stimulus effects of accompanying fear conditioning were expected.

METHODS

In 20 healthy adult subjects leg withdrawal reflex conditioning was performed using a standard delay protocol during MR-scanning. Electromyographic recordings from the anterior tibial and biceps femoris muscles were used to quantify conditioned responses. Fear-related changes of heart rate were assessed.

RESULTS

In the group of all subjects a significant increase of cerebellar activation was found in the anterior and posterior vermis. In the group of subjects (n=9) who showed conditioned leg withdrawal responses cerebellar activation was more pronounced in parts of the anterior vermis, which correspond to the known leg representation. In the group of subjects (n=11) who did not develop conditioned responses cerebellar activation was more pronounced in the posterolateral hemispheres. Changes of heart rate, however, did not significantly differ between groups.

CONCLUSIONS

Results suggest that areas within the anterior vermis are involved in conditioning of the leg withdrawal response. The present results, however, do not allow to differentiate between motor performance, learning or timing-related processes. Areas in the posterior vermis and cerebellar hemispheres may be related to concomitant fear conditioning.

SIGNIFICANCE

Results of the present event-related fMRI study suggest involvement of the human cerebellum in conditioning of the nociceptive leg withdrawal response.