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

A functional MRI study of human amygdala responses to facial expressions of fear versus anger

Functional magnetic resonance imaging (fMRI) of the human brain was used to compare changes in amygdala activity associated with viewing facial expressions of fear and anger. Pictures of human faces bearing expressions of fear or anger, as well as faces with neutral expressions, were presented to 8 healthy participants. The blood oxygen-level dependent (BOLD) fMRI signal within the dorsal amygdala was significantly greater to Fear versus Anger, in a direct contrast. Significant BOLD signal changes in the ventral amygdala were observed in contrasts of Fear versus Neutral expressions and, in a more spatially circumscribed region, to Anger versus Neutral expressions. Thus, activity in the amygdala is greater to fearful facial expressions when contrasted with either neutral or angry faces. Furthermore, directly contrasting fear with angry faces highlighted involvement of the dorsal amygdaloid region.

Pavlovian conditioning of emotional responses to olfactory and contextual stimuli: a potential model for the development and expression of chemical intolerance

Chemical intolerance (CI) in humans is a poorly understood phenomenon of uncertain etiology, seemingly influenced by multiple factors both within and between affected individuals. Several authors have suggested that the development of CI in some individuals may be due, at least in part, to Pavlovian conditioning processes in which the expression of overt symptoms to certain substances reflects classically conditioned responses to previously neutral olfactory and contextual stimuli. In this paper, we describe the potential relationship between olfactory and contextual conditioning in experimental animals and the development and expression of CI in humans. Furthermore, as significant advances have been made in delineating the brain areas that underlie these learned responses, we also review recent research on the contributions of the amygdala and perirhinal cortical region to olfactory and contextual fear conditioning.

Time courses of left and right amygdalar responses to fearful facial expressions

Despite the many studies highlighting the role of the amygdala in fear perception, few have examined differences between right and left amygdalar responses. Using functional magnetic resonance imaging (fMRI), we examined neural responses in three groups of healthy volunteers (n = 18) to alternating blocks of fearful and neutral faces. Initial observation of extracted time series of both amygdalae to these stimuli indicated more rapid decreases of right than left amygdalar responses to fearful faces, and increasing magnitudes of right amygdalar responses to neutral faces with time. We compared right and left responses statistically by modeling each time series with (1) a stationary fit model (assuming a constant magnitude of amygdalar response to consecutive blocks of fearful faces) and (2) an adaptive model (no assumptions). Areas of significant sustained nonstationarity (time series points with significantly greater adaptive than stationary model fits) were demonstrated for both amygdalae. There was more significant nonstationarity of right than left amygdalar responses to neutral, and left than right amygdalar responses to fearful faces. These findings indicate significant variability over time of both right and left amygdalar responses to fearful and neutral facial expressions and are the first demonstration of specific differences in time courses of right and left amygdalar responses to these stimuli.

Amygdala response to facial expressions in children and adults

BACKGROUND

The amygdala plays a central role in the human response to affective or emotionally charged stimuli, particularly fear-producing stimuli. We examined the specificity of the amygdala response to facial expressions in adults and children.

METHODS

Six adults and 12 children were scanned in a 1.5-T scanner during passive viewing of fearful and neutral faces using an EPI BOLD sequence. All scans were registered to a reference brain, and analyses of variance were conducted on the pooled data to examine interactions with age and gender.

RESULTS

Overall, we observed predominantly left amygdala and substantia innominata activity during the presentation of nonmasked fearful faces relative to fixation, and a decrease in activation in these regions with repeated exposure to the faces. Adults showed increased left amygdala activity for fearful faces relative to neutral faces. This pattern was not observed in the children who showed greater amygdala activity with neutral faces than with fearful faces. For the children, there was an interaction of gender and condition whereby boys but not girls showed less activity with repeated exposure to the fearful faces.

CONCLUSIONS

This is the first study to examine developmental differences in the amygdala response to facial expressions using functional magnetic resonance imaging.

Differential prefrontal cortex and amygdala habituation to repeatedly presented emotional stimuli

Repeated presentations of emotional facial expressions were used to assess habituation in the human brain using fMRI. Significant fMRI signal decrement was present in the left dorsolateral prefrontal and premotor cortex, and right amygdala. Within the left prefrontal cortex greater habituation to happy vs fearful stimuli was evident, suggesting devotion of sustained neural resources for processing of threat vs safety signals. In the amygdala, significantly greater habituation was observed on the right compared to the left. In contrast, the left amygdala was significantly more activated than the right to the contrast of fear vs happy. We speculate that the right amygdala is part of a dynamic emotional stimulus detection system, while the left is specialized for sustained stimulus evaluations.

Neural correlates of maternal separation in rhesus monkeys

BACKGROUND

The neurobiological basis of stress and anxiety in primates remains poorly understood. In this study, we examined the neural response to a naturalistic social stressor: maternal separation. We used rhesus monkeys as an animal model because of their close phylogenetic affinity with humans.

METHODS

Six juvenile rhesus monkeys received [(18)F]-fluorodeoxyglucose positron emission tomography scans following 1) a period together with their mothers and again after separation from their mothers 2) with or 3) without visual contact. Image subtraction revealed brain regions that exhibited altered activity during separation. In addition, plasma cortisol concentrations obtained following each condition were tested for correlations with regional brain activity.

RESULTS

Maternal separation activated the right dorsolateral prefrontal cortex and the right ventral temporal/occipital lobe. There was also decreased activity in left dorsolateral prefrontal cortex associated with separation stress. Correlational analyses demonstrated these activated and deactivated regions to be positively and negatively correlated with cortisol, respectively. Additionally, correlational analyses revealed cortisol-related activation in brainstem areas previously implicated in stress and anxiety.

CONCLUSIONS

In juvenile rhesus monkeys, the stress of maternal separation is associated with activation in the right dorsolateral prefrontal cortex and ventral temporal/occipital lobes and decreased activity in the left dorsolateral prefrontal cortex.

The anxiolytic effect of the CRH(1) receptor antagonist R121919 depends on innate emotionality in rats

Hyperactivity of central corticotropin-releasing hormone (CRH) circuits appears to contribute to the symptomatology of affective and anxiety disorders and therefore CRH receptor antagonists have attracted attention as potential therapeutic agents. R121919, a novel high-affinity nonpeptide CRH(1) receptor antagonist, displaced (125)I-oCRH in rat pituitary, cortex and amygdala, but not in choroid plexus or cerebral blood vessels in vitro and in vivo, which is consistent with CRH(1) receptor antagonism. In vivo, R121919 significantly inhibited stress-induced corticotropin release in rats selectively bred for high- and low-anxiety-related behaviour but displayed anxiolytic effects in high-anxiety rats only. These data, corroborated by ex vivo receptor occupancy studies, suggest that this animal model is appropriate for the evaluation of CRH(1) receptor antagonists and that compounds such as R121919 will be beneficial whenever the central stress hormone system is hyperactive.

The amygdala: vigilance and emotion

Here we provide a review of the animal and human literature concerning the role of the amygdala in fear conditioning, considering its potential influence over autonomic and hormonal changes, motor behavior and attentional processes. A stimulus that predicts an aversive outcome will change neural transmission in the amygdala to produce the somatic, autonomic and endocrine signs of fear, as well as increased attention to that stimulus. It is now clear that the amygdala is also involved in learning about positively valenced stimuli as well as spatial and motor learning and this review strives to integrate this additional information. A review of available studies examining the human amygdala covers both lesion and electrical stimulation studies as well as the most recent functional neuroimaging studies. Where appropriate, we attempt to integrate basic information on normal amygdala function with our current understanding of psychiatric disorders, including pathological anxiety.

Auditory fear conditioning increases CS-elicited spike firing in lateral amygdala neurons even after extensive overtraining

We examined the influence of extensive overtraining (75 trials) of auditory fear conditioning on the expression of conditional stimulus (CS)-elicited spike firing in lateral amygdala (LA) neurons. Single units were recorded from chronic multichannel electrodes implanted in the LA of conscious and freely moving rats. In sequential training sessions, the rats received either five or 70 fear conditioning trials, which consisted of a white-noise CS and a coterminating footshock unconditional stimulus (US). Unpaired (sensitization) controls received the same number of trials, but the CS and US were explicitly unpaired. Paired CS-US presentations yielded robust increases in CS-elicited spike firing in LA neurons after both five and 70 conditioning trials, and the magnitude of the spike firing increases was correlated with the expression of conditional freezing to the CS. After 75 training trials, maximal conditioning-related increases in LA firing were exhibited within 20 ms of CS onset, indicating that this increase is mediated by direct thalamo-amygdala projections. There was no significant increase in CS-elicited spike firing or freezing behaviour in the unpaired group. These results complement amygdala lesion studies [e.g. Maren, S. (1999a) J. Neurosci., 19, 8696-8703] and support the view that the basolateral complex of the amygdala is involved in the encoding and storage of fear memories even after extensive overtraining.

Emotion, plasticity, context, and regulation: perspectives from affective neuroscience

The authors present an overview of the neural bases of emotion. They underscore the role of the prefrontal cortex (PFC) and amygdala in 2 broad approach- and withdrawal-related emotion systems. Components and measures of affective style are identified. Emphasis is given to affective chronometry and a role for the PFC in this process is proposed. Plasticity in the central circuitry of emotion is considered, and implications of data showing experience-induced changes in the hippocampus for understanding psychopathology and stress-related symptoms are discussed. Two key forms of affective plasticity are described--context and regulation. A role for the hippocampus in context-dependent normal and dysfunctional emotional responding is proposed. Finally, implications of these data for understanding the impact on neural circuitry of interventions to promote positive affect and on mechanisms that govern health and disease are considered.

Neuroimaging studies of mood disorders

Neuroimaging studies of major depression have identified neurophysiologic abnormalities in multiple areas of the orbital and medial prefrontal cortex, the amygdala, and related parts of the striatum and thalamus. Some of these abnormalities appear mood state-dependent and are located in regions where cerebral blood flow increases during normal and other pathologic emotional states. These neurophysiologic differences between depressives and control subjects may thus implicate areas where physiologic activity changes to mediate or respond to the emotional, behavioral, and cognitive manifestations of major depressive episodes. Other abnormalities persist following symptom remission, and are found in orbital and medial prefrontal cortex areas where postmortem studies demonstrate reductions in cortex volume and histopathologic changes in primary mood disorders. These areas appear to modulate emotional behavior and stress responses, based upon evidence from brain mapping, lesion analysis, and electrophysiologic studies of humans and/or experimental animals. Dysfunction involving these regions is thus hypothesized to play a role in the pathogenesis of depressive symptoms. Taken together, these findings implicate interconnected neural circuits in which pathologic patterns of neurotransmission may result in the emotional, motivational, cognitive, and behavioral manifestations of primary and secondary affective disorders.

Subcortical and cortical brain activity during the feeling of self-generated emotions

In a series of [15O]PET experiments aimed at investigating the neural basis of emotion and feeling, 41 normal subjects recalled and re-experienced personal life episodes marked by sadness, happiness, anger or fear. We tested the hypothesis that the process of feeling emotions requires the participation of brain regions, such as the somatosensory cortices and the upper brainstem nuclei, that are involved in the mapping and/or regulation of internal organism states. Such areas were indeed engaged, underscoring the close relationship between emotion and homeostasis. The findings also lend support to the idea that the subjective process of feeling emotions is partly grounded in dynamic neural maps, which represent several aspects of the organism's continuously changing internal state.

Emotion circuits in the brain

The field of neuroscience has, after a long period of looking the other way, again embraced emotion as an important research area. Much of the progress has come from studies of fear, and especially fear conditioning. This work has pinpointed the amygdala as an important component of the system involved in the acquisition, storage, and expression of fear memory and has elucidated in detail how stimuli enter, travel through, and exit the amygdala. Some progress has also been made in understanding the cellular and molecular mechanisms that underlie fear conditioning, and recent studies have also shown that the findings from experimental animals apply to the human brain. It is important to remember why this work on emotion succeeded where past efforts failed. It focused on a psychologically well-defined aspect of emotion, avoided vague and poorly defined concepts such as "affect," "hedonic tone," or "emotional feelings," and used a simple and straightforward experimental approach. With so much research being done in this area today, it is important that the mistakes of the past not be made again. It is also time to expand from this foundation into broader aspects of mind and behavior.

Performance on indirect measures of race evaluation predicts amygdala activation

We used fMRI to explore the neural substrates involved in the unconscious evaluation of Black and White social groups. Specifically, we focused on the amygdala, a subcortical structure known to play a role in emotional learning and evaluation. In Experiment 1, White American subjects observed faces of unfamiliar Black and White males. The strength of amygdala activation to Black-versus-White faces was correlated with two indirect (unconscious) measures of race evaluation (Implicit Association Test [IAT] and potentiated startle), but not with the direct (conscious) expression of race attitudes. In Experiment 2, these patterns were not obtained when the stimulus faces belonged to familiar and positively regarded Black and White individuals. Together, these results suggest that amygdala and behavioral responses to Black-versus-White faces in White subjects reflect cultural evaluations of social groups modified by individual experience.

Learning about pain: the neural substrate of the prediction error for aversive events

Associative learning is thought to depend on detecting mismatches between actual and expected experiences. With functional magnetic resonance imaging (FMRI), we studied brain activity during different types of mismatch in a paradigm where contrasting-colored lights signaled the delivery of painful heat, nonpainful warmth, or no stimulation. When painful heat stimulation was unexpected, there was increased FMRI signal intensity in areas of the hippocampus, superior frontal gyrus, cerebellum, and superior parietal gyrus that was not found with mismatch between expectation and delivery of nonpainful warmth stimulation. When painful heat stimulation was unexpectedly omitted, the FMRI signal intensity decreased in the left superior parietal gyrus and increased in the other regions. These contrasting activation patterns correspond to two different mismatch concepts in theories of associative learning (Rescorla-Wagner, temporal difference vs. Pearce-Hall, Mackintosh). Searching for interventions to specifically modulate activation of these brain regions therefore offers an approach to identifying new treatments for chronic pain, which often has a substantial associative learning component.

Fear conditioning and brain activity: a positron emission tomography study in humans

Regional cerebral blood flow (rCBF) was measured with H2 (15)O positron emission tomography in 8 healthy women before and after fear conditioning (i.e., paired shocks) and unpaired shocks to videotape cues. Conditioning was supported by enhanced peripheral nervous system recordings and subjective ratings. Fear conditioning increased rCBF in the central gray of the midbrain; bilaterally in the hypothalamus, the thalamus, and the left striatum; and in the right and left anterior cingulate and right prefrontal cortices. Regional CBF was attenuated bilaterally in the right and left prefrontal, temporal (including the amygdala), parietal, and occipital cortices, and in the left orbitofrontal cortex. When compared with unpaired shock presentations, fear conditioning resulted in elevated rCBF in the left cerebellum. Hence, in the present paradigm, only neural activity in the left cerebellum solely reflected processes associated with true Pavlovian conditioning.

Dysfunction in the neural circuitry of emotion regulation--a possible prelude to violence

Emotion is normally regulated in the human brain by a complex circuit consisting of the orbital frontal cortex, amygdala, anterior cingulate cortex, and several other interconnected regions. There are both genetic and environmental contributions to the structure and function of this circuitry. We posit that impulsive aggression and violence arise as a consequence of faulty emotion regulation. Indeed, the prefrontal cortex receives a major serotonergic projection, which is dysfunctional in individuals who show impulsive violence. Individuals vulnerable to faulty regulation of negative emotion are at risk for violence and aggression. Research on the neural circuitry of emotion regulation suggests new avenues of intervention for such at-risk populations.

Differential limbic--cortical correlates of sadness and anxiety in healthy subjects: implications for affective disorders

BACKGROUND

Affective disorders are associated with comorbidity of depression and anxiety symptoms. Positron emission tomography resting-state studies in affective disorders have generally failed to isolate specific symptom effects. Emotion provocation studies in healthy volunteers have produced variable results, due to differences in experimental paradigm and instructions.

METHODS

To better delineate the neural correlates of sad mood and anxiety, this study used autobiographical memory scripts in eight healthy women to generate sadness, anxiety, or a neutral relaxed state in a within-subject design.

RESULTS

Sadness and anxiety, when contrasted to a neutral emotional state, engaged a set of distinct paralimbic-cortical regions, with a limited number of common effects. Sadness was accompanied by specific activations of the subgenual cingulate area (BA) 25 and dorsal insula, specific deactivation of the right prefrontal cortex BA 9, and more prominent deactivation of the posterior parietal cortex BAs 40/7. Anxiety was associated with specific activations of the ventral insula, the orbitofrontal and anterior temporal cortices, specific deactivation of parahippocampal gyri, and more prominent deactivation of the inferior temporal cortex BAs 20/37.

CONCLUSIONS

These findings are interpreted within a model in which sadness and anxiety are represented by segregated corticolimbic pathways, where a major role is played by selective dorsal cortical deactivations during sadness, and ventral cortical deactivations in anxiety.

Activation of anterior paralimbic structures during guilt-related script-driven imagery

BACKGROUND

Several recent neuroimaging studies have examined the neuroanatomical correlates of normal emotional states, such as happiness, sadness, fear, anger, anxiety, and disgust; however, no previous study has examined the emotional state of guilt.

METHODS

In the current study, we used positron emission tomography and the script-driven imagery paradigm to study regional cerebral blood flow (rCBF) during the transient emotional experience of guilt in eight healthy male participants. In the Guilt condition, participants recalled and imagined participating in a personal event involving the most guilt they had ever experienced. In the Neutral condition, participants recalled and imagined participating in an emotionally neutral personal event.

RESULTS

In the Guilt versus Neutral comparison, rCBF increases occurred in anterior paralimbic regions of the brain: bilateral anterior temporal poles, anterior cingulate gyrus, and left anterior insular cortex/inferior frontal gyrus.

CONCLUSIONS

These results, along with those of previous studies, are consistent with the notion that anterior paralimbic regions of the brain mediate negative emotional states in healthy individuals.

A pilot study of amygdala volumes in pediatric generalized anxiety disorder

BACKGROUND

The neurodevelopment of childhood anxiety disorders is not well understood. Basic research has implicated the amygdala and circuits related to these nuclei as being central to several aspects of fear and fear-related behaviors in animals.

METHODS

Magnetic resonance imaging was used to measure amygdala volumes and comparison brain regions in 12 child and adolescent subjects with generalized anxiety disorder and 24 comparison subjects. Groups were matched on age, sex, height, and handedness and were also similar on measures of weight, socioeconomic status, and full scale IQ.

RESULTS

Right and total amygdala volumes were significantly larger in generalized anxiety disorder subjects. Intracranial, cerebral, cerebral gray and white matter, temporal lobe, hippocampal, and basal ganglia volumes and measures of the midsagittal area of the corpus callosum did not differ between groups.

CONCLUSIONS

Although these data are preliminary and from a small sample, the results are consistent with a line of thinking that alterations in the structure and function of the amygdala may be associated with pediatric generalized anxiety disorder.

FMRI visualization of brain activity during a monetary incentive delay task

Comparative studies have implicated striatal and mesial forebrain circuitry in the generation of autonomic, endocrine, and behavioral responses for incentives. Using blood oxygen level-dependent functional magnetic resonance imaging, we sought to visualize functional activation of these regions in 12 normal volunteers as they anticipated and responded for monetary incentives. Both individual and group analyses of time-series data revealed significant activation of striatal and mesial forebrain structures (including insula, caudate, putamen, and mesial prefrontal cortex) during trials involving both monetary rewards and punishments. In addition to these areas, during trials involving punishment, group analysis revealed activation foci in the anterior cingulate and thalamus. These results corroborate comparative studies which implicate striatal and mesial forebrain circuitry in the elaboration of incentive-driven behavior. This report also introduces a new paradigm for probing the functional integrity of this circuitry in humans.

The distributed human neural system for face perception

Face perception, perhaps the most highly developed visual skill in humans, is mediated by a distributed neural system in humans that is comprised of multiple, bilateral regions. We propose a model for the organization of this system that emphasizes a distinction between the representation of invariant and changeable aspects of faces. The representation of invariant aspects of faces underlies the recognition of individuals, whereas the representation of changeable aspects of faces, such as eye gaze, expression, and lip movement, underlies the perception of information that facilitates social communication. The model is also hierarchical insofar as it is divided into a core system and an extended system. The core system is comprised of occipitotemporal regions in extrastriate visual cortex that mediate the visual analysis of faces. In the core system, the representation of invariant aspects is mediated more by the face-responsive region in the fusiform gyrus, whereas the representation of changeable aspects is mediated more by the face-responsive region in the superior temporal sulcus. The extended system is comprised of regions from neural systems for other cognitive functions that can be recruited to act in concert with the regions in the core system to extract meaning from faces.

Impaired preference conditioning after anterior temporal lobe resection in humans

Research with animals suggests that structures within the amygdaloid nuclear complex (ANC) are critical for acquiring associations between rewarding events and neutral stimuli, a form of conditioning often manifested in a subsequent preference for those (conditioned) stimuli. In this study, we investigated the relationship between the ANC and preference learning in humans. Three abstract monochrome patterns were presented to each subject over 180 trials in the context of a counting task requiring working memory. One pattern was paired with food reward on 90% of the trials in which it was presented and with no food reward on the other 10% of trials. The other patterns were similarly reinforced, but at ratios of 50:50% and 10:90% with reward and nonreward, respectively. Subsequently, a group of 21 normal participants preferred the pattern paired most often with reward to that paired least often with reward, and they did not explicitly relate their preferences to the conditioning procedure, but instead attributed them to the characteristics of the patterns themselves. Unlike the normal controls, a group of patients with unilateral surgical lesions that included the ANC (15 left, 18 right) did not show conditioned preferences, but performed normally on a measure of working memory. In contrast, 13 patients with unilateral damage confined to frontal cortex exhibited normal conditioned preferences but were impaired on the working memory task. This double dissociation provides clear evidence that, in humans as in other animals, reward-related learning (conditioned reward) critically depends on a circuit involving inferotemporal cortex and the ANC.

Differential amygdala responses to winning and losing: a functional magnetic resonance imaging study in humans

The amygdala has been shown to respond to many distinct types of affective stimuli, including reward and punishment feedback in animals. In humans, winning and losing situations can be considered as reward and punishment experiences, respectively. In this study, we used functional magnetic resonance imaging (fMRI) to measure regional brain activity when human subjects were given feedback on their performance during a simple response time task in a fictitious competitive tournament. Lexical stimuli were used to convey positive 'win' or negative 'lose' feedback. The frequency of positive and negative trials was parametrically varied by the experimenters independently from the subjects' actual performance and unbeknownst to them. The results showed that the parametric increase of winning was associated with left amygdala activation whereas the parametric increase of losing was associated with right amygdala activation. These findings provide functional evidence that the human amygdala differentially responds to changes in magnitude of positive or negative reinforcement conveyed by lexical stimuli.

Classical fear conditioning in functional neuroimaging

Classical conditioning, the simplest form of associative learning, is one of the most studied paradigms in behavioural psychology. Since the formal description of classical conditioning by Pavlov, lesion studies in animals have identified a number of anatomical structures involved in, and necessary for, classical conditioning. In the 1980s, with the advent of functional brain imaging techniques, particularly positron emission tomography (PET), it has been possible to study the functional anatomy of classical conditioning in humans. The development of functional magnetic resonance imaging (fMRI)--in particular single-trial or event-related fMRI--has now considerably advanced the potential of neuroimaging for the study of this form of learning. Recent event-related fMRI and PET studies are adding crucial data to the current discussion about the putative role of the amygdala in classical fear conditioning in humans.

Gender differences in regional cerebral activity during sadness

Functional magnetic resonance imaging and echo-planar-imaging were used to investigate affect related gender differences in regional cerebral activity. The experiment was conducted using a standardized mood induction procedure. Blood-oxygen-level-dependent effect was measured in 13 male and 13 female healthy subjects, during both moods of happiness and sadness, respectively. Parallel to earlier neuroimaging findings, our results show brain activity in the amygdala of males during negative affect. Females failed to demonstrate a similar activation pattern despite matched subjective ratings of negative affect to males. Results point to differential regional cerebral correlates of emotional experience in males and females, which is suggestive of a more focal and subcortical processing of sadness in men.

Expression without recognition: contributions of the human amygdala to emotional communication

A growing body of evidence from humans and other animals suggests the amygdala may be a critical neural substrate for emotional processing. In particular, recent studies have shown that damage to the human amygdala impairs the normal appraisal of social signals of emotion, primarily those of fear. However, effective social communication depends on both the ability to receive (emotional appraisal) and the ability to send (emotional expression) signals of emotional state. Although the role of the amygdala in the appraisal of emotion is well established, its importance for the production of emotional expressions is unknown. We report a case study of a patient with bilateral amygdaloid damage who, despite a severe deficit in interpreting facial expressions of emotion including fear, exhibits an intact ability to express this and other basic emotions. This dissociation suggests that a single neural module does not support all aspects of the social communication of emotional state.

Medial temporal atrophy and memory impairment in early stage of Alzheimer's disease: an MRI volumetric and memory assessment study

Memory impairment and medial temporal lobe (MTL) involvement are the earliest and most prominent features of Alzheimer's disease (AD). A psychological assessment of memory function and an evaluation of the morphological changes in MTL structures, as found in the mild form of AD, are important for early diagnosis as well as for understanding the pathophysiology of the disease. In the present study, we aimed to evaluate correlations in these psychoanatomical changes in terms of the stage of AD. We performed MRI-based volumetric measurements of the MTL structure and neuropsychological tests, using MMSE and the Wechsler memory scale-revised (WMS-R), on 27 elderly normal subjects and 46 probable AD patients, and then checked for possible correlations between the volumetric measurements and memory dysfunction. The severity of the AD patients' condition was assessed by CDR scale. Each MTL structure decreased in volume with increasing severity of AD. In very early AD, the reduction in the amygdala volume was pronounced, while the hippocampal volumes were relatively unchanged. Neuropsychological scores also declined with increasing severity of AD. Scores on the main WMS-R subsets examined (verbal memory, visual memory, and delayed recall) decreased significantly in the very mild group, as compared with normal controls. The WMS-R test scores correlated significantly with the amygdala volumes in normal control subjects and very mild AD patients. Our findings suggest that MRI-based amygdaloid volumetric measurement provides a sensitive marker, and that the degeneration of the amygdala may begin very early in the course of AD.

Amygdala-hippocampal involvement in human aversive trace conditioning revealed through event-related functional magnetic resonance imaging

Previous functional neuroimaging studies have characterized brain systems mediating associative learning using classical delay conditioning paradigms. In the present study, we used event-related functional magnetic resonance imaging to characterize neuronal responses mediating aversive trace conditioning. During conditioning, neutral auditory tones were paired with an aversive sound [unconditioned stimulus (US)]. We compared neuronal responses evoked by conditioned (CS+) and nonconditioned (CS-) stimuli in which a 50% pairing of CS+ and the US enabled us to limit our analysis to responses evoked by the CS+ alone. Differential responses (CS+ vs CS-), related to conditioning, were observed in anterior cingulate and anterior insula, regions previously implicated in delay fear conditioning. Differential responses were also observed in the amygdala and hippocampus that were best characterized with a time x stimulus interaction, indicating rapid adaptation of CS+-specific responses in medial temporal lobe. These results are strikingly similar to those obtained with a previous delay conditioning experiment and are in accord with a preferential role for medial temporal lobe structures during the early phase of conditioning. However, an additional activation of anterior hippocampus in the present experiment supports a view that its role in trace conditioning is to maintain a memory trace between the offset of the CS+ and the delayed onset of the US to enable associative learning in trace conditioning.

Intuition: a social cognitive neuroscience approach

This review proposes that implicit learning processes are the cognitive substrate of social intuition. This hypothesis is supported by (a) the conceptual correspondence between implicit learning and social intuition (nonverbal communication) and (b) a review of relevant neuropsychological (Huntington's and Parkinson's disease), neuroimaging, neurophysiological, and neuroanatomical data. It is concluded that the caudate and putamen, in the basal ganglia, are central components of both intuition and implicit learning, supporting the proposed relationship. Parallel, but distinct, processes of judgment and action are demonstrated at each of the social, cognitive, and neural levels of analysis. Additionally, explicit attempts to learn a sequence can interfere with implicit learning. The possible relevance of the computations of the basal ganglia to emotional appraisal, automatic evaluation, script processing, and decision making are discussed.

Pathophysiology of childhood anxiety disorders

Prior reviews on the pathophysiology of anxiety consistently note the need for more research on biological aspects of childhood social phobia, separation anxiety disorder, and generalized anxiety disorder. The current review summarizes biological research that is relevant to these three disorders. In the first part of the review, barriers that have prevented progress in this area are delineated, and recent developments are discussed that set the stage for major advances in research on childhood anxiety disorders. In the second part of the review, studies are discussed that provide insights on the pathophysiology of childhood social phobia, separation anxiety disorder, and generalized anxiety disorder. Research on each specific disorder illustrates the manner in which recent developments in biological research facilitate novel research approaches uniquely suited for answering essential clinical questions in research on both childhood and adult anxiety disorders. For example, in research on social phobia, biological studies might enhance understandings of the longitudinal associations between individual childhood and adult disorders. In research on separation anxiety disorder, biological studies might enhance understanding on family-genetic associations between childhood and adult disorders. Finally, in research on generalized anxiety disorder, biological studies might enhance understandings of comorbidities among distinct childhood and adult disorders, particularly with respect to the relationship between anxiety and depressive disorders.

Amygdala neurons mediate acquisition but not maintenance of instrumental avoidance behavior in rabbits

Whereas the amygdala is generally understood to be involved in aversively motivated learning, the specific associative function of the amygdala remains controversial. This study addressed the amygdalar role in mediation of discriminative instrumental avoidance learning of rabbits. Bilateral microinjection of the GABA receptor agonist muscimol centered in the basolateral nucleus of the amygdala was given to inactivate amygdalar neurons at each of three stages of acquisition. The absence of behavioral learning in rabbits trained immediately after amygdalar inactivation confirmed previous results with electrolytic lesions. The absence of savings during training after muscimol had become ineffective indicated an amygdalar role in the establishment of acquisition-relevant neural plasticity, not simply in the expression of the learned response. A time-limited role of the amygdala in instrumental avoidance learning was indicated by the finding that intra-amygdalar muscimol failed to disrupt performance of the well-established avoidance response. The passage of time alone (with no training trials) was sufficient to reduce amygdalar involvement in response performance. These results and demonstrations that other limbic system areas make time-limited contributions to learning indicate that the amygdala is part of a larger intermediate memory system that supports learning and performance before habit consolidation.

The noradrenergic system in pathological anxiety: a focus on panic with relevance to generalized anxiety and phobias

Over the past three decades of psychiatric research, abnormalities in the noradrenergic system have been identified in particular anxiety disorders such as panic disorder. Simultaneously, neuroscience research on fear pathways and the stress response have delineated central functions for the noradrenergic system. This review focuses on the noradrenergic system in anxiety spectrum disorders such as panic disorder, generalized anxiety disorder, and phobias for the purpose of elucidating current conceptualizations of the pathophysiologies. Neuroanatomic pathways that are theoretically relevant in anxiogenesis are discussed and the implications for treatment reviewed.

Basolateral amygdala noradrenergic influence enables enhancement of memory consolidation induced by hippocampal glucocorticoid receptor activation

Previously, we reported that bilateral excitotoxic lesions of the basolateral nucleus of the amygdala (BLA) block the enhancing effects of posttraining systemic or intrahippocampal glucocorticoid administration on memory for inhibitory avoidance training. The present study further examined the basis of this permissive influence of the BLA on hippocampal memory functioning. Immediate posttraining unilateral infusions of the specific glucocorticoid receptor agonist RU 28362 (11beta,17beta-dihydroxy-6, 21-dimethyl-17alpha-pregna-4,6-trien-20-yn-3-one; 3.0, 10.0, or 30.0 ng in 0.5 microliter) administered into the dorsal hippocampus of male Sprague-Dawley rats induced dose-dependent enhancement of 48-h inhibitory avoidance retention. Infusions of the beta-adrenoceptor antagonist atenolol (0.5 microgram in 0.2 microliter) into the ipsilateral, but not the contralateral, BLA 10 min prior to training blocked the hippocampal glucocorticoid effects on memory consolidation. Infusions of the muscarinic cholinergic antagonist atropine (0.5 microgram in 0.2 microliter) into either the ipsilateral or contralateral BLA before training did not block the hippocampal glucocorticoid effects. These findings provide further evidence that beta-adrenergic activity in the BLA is essential in enabling glucocorticoid-induced modulation of memory consolidation and are consistent with the hypothesis that the BLA regulates the strength of memory consolidation in other brain structures. The ipsilateral nature of the BLA-hippocampus interaction indicates that BLA influences on hippocampal memory processes are mediated through neural pathways rather than by influences by means of the activation of peripheral stress responses.

Saying it with feeling: neural responses to emotional vocalizations

To determine how vocally expressed emotion is processed in the brain, we measured neural activity in healthy volunteers listening to fearful, sad, happy and neutral non-verbal vocalizations. Enhanced responses to emotional vocalizations were seen in the caudate nucleus, as well as anterior insular, temporal and prefrontal cortices. The right amygdala exhibited decreased responses to fearful vocalizations as well as fear-specific inhibitory interactions with left anterior insula. A region of the pons, implicated in acoustic startle responses also showed fear-specific interactions with the amygdala. The data demonstrate: firstly, that processing of vocal emotion involves a bilaterally distributed network of brain regions; and secondly, that processing of fear-related auditory stimuli involves context-specific interactions between the amygdala and other cortical and brainstem regions implicated in fear processing.

Amygdala co-ordinates sudden falls in ear pinna blood flow in response to unconditioned salient stimuli in conscious rabbits

Tetrodotoxin (10 pmol in 300 nl of Ringer), injected bilaterally into the region of the amygdala in conscious rabbits, virtually abolished the sudden falls in ear pinna blood flow that normally occur in response to salient environmental stimuli (touching the animal's fur, slightly moving its cage, or applying or removing a drape covering the cage). Time spent at 0-20% of maximum flow values during a 10 min observation period, commencing 15 min after injection of tetrodotoxin, significantly decreased compared with the pre-injection control period (30+/-14 s compared with 286+/-24 s, P<0.01, n=8 rabbits) and the time spent at 70-100% of maximum flow values significantly increased (521+/-36 s compared with 127+/-29 s, P<0.01). Vehicle was injected on the day before tetrodotoxin injections in four of eight rabbits and on the day after tetrodotoxin injections in the other four rabbits, in a counterbalanced design. Rabbits fully recovered from the effects of tetrodotoxin in one day. Vehicle did not significantly affect the time spent at different flow percentage values. Falls in ear blood flow elicited by noxious stimuli (ear pinch, inhalation of formaldehyde vapor) occurred in a normal pattern after tetrodotoxin. Amygdaloid circuitry is thus necessary for the production of falls in ear pinna blood flow that occur in response to unconditioned non-noxious stimuli, but not for the falls that occur in response to unconditioned noxious stimuli in rabbits. In humans, the amygdaloid region may also be involved in co-ordinating falls in cutaneous blood flow occurring in response to salient or anxiety-evoking stimuli. Thus, discovery of the neural pathways by which amygdaloid circuitry alters ear pinna blood flow in rabbits may elucidate the manner in which similar cardiovascular changes occur in humans during anxiety reactions.

Auditory temporal integration in the rhesus macaque (Macaca mulatta)

Temporal integration for pure tones was examined in two rhesus macaques. The subjects were required to respond to a brief sound (a tone burst) that deviated from a previous series of sounds (noise bursts) on a trial (a deviant-stimulus detection paradigm). Psychometric functions and thresholds were determined from correct detections (hit proportions) alone, and from d' scores. Two models describing the decline in threshold as a function of stimulus duration, one a power function the other an exponential, were tested against the data. When the decline (slope) in threshold per log stimulus duration is used as a rate measure, our results yield a lower estimate of temporal integration rate in rhesus than did a previous study [Clack, J. Acoust. Soc. Am. 40, 1140-1146 (1966)]. Both studies, however, gave slope estimates of integration rate that were higher than in most other species. Comparison of the models using data from several species, revealed that the exponential, but not the power model, could account for two sources of variation in threshold measurement. One source is due to the range across threshold as a function of duration (the linear rate component), and is described by the constant of proportionality Ik in the model. The other source of variation arises from the rate of decline within this range (the nonlinear rate component), and is described by the time constant tau. In terms of this model, differences in rate estimates between Clack's study and ours (and between rhesus and other species) are primarily due to the linear component. The nonlinear rate component was about equal for our study and Clack's (tau = approximately 150 ms): a time constant that is just slightly larger (indicating a rate of temporal integration slightly slower) than for most other species examined.

Different contributions of the human amygdala and ventromedial prefrontal cortex to decision-making

The somatic marker hypothesis proposes that decision-making is a process that depends on emotion. Studies have shown that damage of the ventromedial prefrontal (VMF) cortex precludes the ability to use somatic (emotional) signals that are necessary for guiding decisions in the advantageous direction. However, given the role of the amygdala in emotional processing, we asked whether amygdala damage also would interfere with decision-making. Furthermore, we asked whether there might be a difference between the roles that the amygdala and VMF cortex play in decision-making. To address these two questions, we studied a group of patients with bilateral amygdala, but not VMF, damage and a group of patients with bilateral VMF, but not amygdala, damage. We used the "gambling task" to measure decision-making performance and electrodermal activity (skin conductance responses, SCR) as an index of somatic state activation. All patients, those with amygdala damage as well as those with VMF damage, were (1) impaired on the gambling task and (2) unable to develop anticipatory SCRs while they pondered risky choices. However, VMF patients were able to generate SCRs when they received a reward or a punishment (play money), whereas amygdala patients failed to do so. In a Pavlovian conditioning experiment the VMF patients acquired a conditioned SCR to visual stimuli paired with an aversive loud sound, whereas amygdala patients failed to do so. The results suggest that amygdala damage is associated with impairment in decision-making and that the roles played by the amygdala and VMF in decision-making are different.