The amygdala: vigilance and emotion

Glucocorticoid regulation of diverse cognitive functions in normal and pathological emotional states

The glucocorticoid hormone cortisol is essential for many forms of regulatory physiology and for cognitive appraisal. Cortisol, while associated with fear and stress response, is also the hormone of energy metabolism and it coordinates behavioral adaptation to the environmental and internal conditions through the regulation of many neurotransmitters and neural circuits. Cortisol has diverse effects on many neuropeptide and neurotransmitter systems thus affecting functional brain systems. As a result, cortisol affects numerous cognitive domains including attention, perception, memory, and emotional processing. When certain pathological emotional states are present, cortisol may have a role in differential activation of brain regions, particularly suppression of hippocampal activation, enhancement of amygdala activity, and dendritic reshaping in these regions as well as in the ventral prefrontal cortex. The coordinated actions of glucocorticoid regulation on various brain systems such as those implicated in emotional processing can lead to perceptual and cognitive adaptations and distortions of events that may be relevant for understanding mood disorders.

Progressive striatal and cortical dopamine receptor dysfunction in Huntington's disease: a PET study

We have studied the progression of striatal and extrastriatal post-synaptic dopaminergic changes in a group of 12 patients with Huntington's disease using serial (11)C-raclopride PET, a specific marker of D2 dopamine receptor binding. All patients had two (11)C-raclopride PET scans 29.2 +/- 12.8 months apart, and six of them had a third scan 13.2 +/- 3.9 months later. We found a mean annual 4.8% loss of striatal (11)C-raclopride binding potential (BP) between the first and second scans, and a 5.2% loss between the second and third scans. Statistical Parametric Mapping (SPM) localized significant baseline reductions in (11)C-raclopride BP in both striatal and extrastriatal areas, including amygdala, temporal and frontal cortex in Huntington's disease compared with normal subjects matched for age and sex. When the (11)C-raclopride scans performed 29 months after the baseline scans were considered, SPM revealed further significant striatal, frontal and temporal reductions in (11)C-raclopride BP in Huntington's disease. Cross-sectional Unified Huntington's Disease Rating Scale (UHDRS) scores correlated with (11)C-raclopride binding, but there was no correlation between individual changes in UHDRS motor scores and changes in striatal binding. Performance on all neuropsychological measures deteriorated with time but only the accuracy score of the one-touch Tower of London test correlated significantly with striatal and putamen D2 binding. In summary, serial (11)C-raclopride PET demonstrates a linear progression of striatal loss of D2 receptors in early clinically affected Huntington's disease patients over 3 years. SPM also revealed a progressive loss of temporal and frontal D2 binding. Changes over time in clinical scores and in neuropsychological assessments, except for measures of planning, did not correlate with striatal D2 binding. This probably reflects both contributions from other affected brain structures and high variance in these measures.

Developmental psychobiology and response to threats: relevance to trauma in children and adolescents

Interest in developmental and psychobiological aspects of trauma has grown with recent research in adults with mood and anxiety disorders reporting histories of trauma during childhood. Studies conducted directly in children and adolescents could add much to ongoing research in this area. This review summarizes data in three areas of developmental science that might inform future studies. First, the review briefly summarizes current data on clinical aspects of trauma in juveniles, focusing on associations with psychopathology and moderators of outcome. Second, the review summarizes data from the basic sciences delineating experiential and developmental changes in brain systems involved in threat perception and response. This review incorporates knowledge gained from research examining the effects of rearing manipulations on regulation of the stress response in rodents and primates. Third, the review summarizes data from cognitive neuroscience studies among both adults and children, again focusing on studies examining aspects of the threat response. This summary includes a review from studies in patients with posttraumatic stress disorder.

Sensitization associated with stressors and cytokine treatments

Like stressors, interleukin-1beta and tumor necrosis factor-alpha (TNF-alpha) increase hypothalamic-pituitary-adrenal (HPA) activity and monoamine turnover at hypothalamic and extrahypothalamic sites. These effects can be re-elicited more readily upon reintroduction of these challenges (sensitization), depending on their time of re-exposure and the particular system being assessed. Following TNF-alpha administration, the co-expression of corticotropin releasing hormone (CRH) and arginine vasopressin increased within the median eminence, peaking 7-14 days after treatment, and was associated with an early corticosterone sensitization. However, the re-elicitation of sickness symptoms and corticosterone release was most pronounced at lengthy re-exposure intervals (28 days), possibly reflecting histamine release from mast cells. In addition, the cytokine engendered the sensitization of norepinephrine and serotonin utilization, and CRH immunoreactivity at mesocorticolimbic sites, but these effects were most prominent at brief re-exposure intervals (1-7 days). Cytokines may independently prime multiple regulatory systems, and by virtue of the neurochemical changes imparted, have both immediate and proactive influences on the evolution of psychopathology.

Neurotrophic effects of electroconvulsive therapy: a proton magnetic resonance study of the left amygdalar region in patients with treatment-resistant depression

Negatively balanced neurotrophic factors may be important in precipitating clinical depression. Recently, it has been reported that antidepressant therapy may exert positive neurotrophic effects. The aim of this study was to detect probable neurotrophic changes during electroconvulsive therapy (ECT). For this purpose, N-acetylaspartate (NAA), an amino acid exclusively located in neurons, and other brain metabolites such as glutamine/glutamate (Glx), choline (Cho), and creatine (Cr) were measured in patients by localized proton magnetic resonance spectroscopy. A total of 28 severely depressed patients (DSM-IV) were enrolled, and the left amygdalar region was investigated by proton STEAM spectroscopy before and after unilateral ECT. The results were compared with 28 age- and gender-matched controls using nonparametric paired and unpaired tests. A significant increase in NAA was observed only in ECT responders (n=14; p=0.019). Five out of 14 nonresponders to ECT monotherapy were remeasured following a clinical improvement after continued ECT combined with antidepressants and were then found also to present a significant increase in NAA. In all successfully treated patients, parallel observations, that is, increased levels, were made for Glx, whereas Cho and Cr were unchanged. In conclusion, our preliminary finding of increased NAA concentrations after successful ECT may indicate a probable neurotrophic effect of ECT.

Extended amygdala and emotional salience: a PET activation study of positive and negative affect

Functional neuroimaging studies have implicated amygdaloid and basal forebrain regions, including sublenticular extended amygdala (SLEA), in the mediation of aversive emotional responses. However, it is not clear whether SLEA responds to 'aversiveness' or to general stimulus salience. We predicted that both pleasant and aversive stimuli would activate this region. Using [(15)O] water PET, we studied 10 healthy subjects while viewing pleasant, aversive, neutral, and blank images. Each subject underwent eight scans, which were processed and averaged with standard statistical methods. Both positive and negative stimuli activated regions in SLEA. Both positive and negative content activated the visual cortex, relative to neutral content. Aversive stimuli deactivated the left frontal pole, relative to positive and neutral stimuli. These findings demonstrate that both positive and negative emotional content evokes processing in the sublenticular/extended amygdala region, suggesting that this region is involved in general emotional processing, such as detection or attribution of salience.

Amygdaloid regional cerebral blood flow and subjective fear during symptom provocation in anxiety disorders

Whether the amygdala is involved predominantly in emotional perception or in the generation of emotional states has been debated. We reviewed and reanalyzed data from our laboratory, indicating that subjective feelings of fear and distress are correlated with regional cerebral blood flow (rCBF) in the right but not the left amygdala during anxiety provocation in individuals with social anxiety disorder, specific phobias. and posttraumatic stress disorder. Positron emission tomography is a correlative technique, and casual inferences cannot be drawn. However, because studies demonstrate that treatment of social anxiety disorder with cognitive behavior therapy and selective serotonin reuptake inhibitors results in reduced rCBF in the amygdaloid complex and prospective studies reveal that treatment-induced alterations in amygdala rCBF can predict 1 year follow-up status in social anxiety disorder data support the notion that the amygdala, at least in part, seem casually involved in generating the subjective experience of fear.

Spatio-temporal dynamics of brain mechanisms in aversive classical conditioning: high-density event-related potential and brain electrical tomography analyses

Social cognition, including complex social judgments and attitudes, is shaped by individual learning experiences, where affect often plays a critical role. Aversive classical conditioning-a form of associative learning involving a relationship between a neutral event (conditioned stimulus, CS) and an aversive event (unconditioned stimulus, US)-represents a well-controlled paradigm to study how the acquisition of socially relevant knowledge influences behavior and the brain. Unraveling the temporal unfolding of brain mechanisms involved appears critical for an initial understanding about how social cognition operates. Here, 128-channel ERPs were recorded in 50 subjects during the acquisition phase of a differential aversive classical conditioning paradigm. The CS+ (two fearful faces) were paired 50% of the time with an aversive noise (CS upward arrow + /Paired), whereas in the remaining 50% they were not (CS upward arrow + /Unpaired); the CS- (two different fearful faces) were never paired with the noise. Scalp ERP analyses revealed differences between CS upward arrow + /Unpaired and CS- as early as approximately 120 ms post-stimulus. Tomographic source localization analyses revealed early activation modulated by the CS+ in the ventral visual pathway (e.g. fusiform gyrus, approximately 120 ms), right middle frontal gyrus (approximately 176 ms), and precuneus (approximately 240 ms). At approximately 120 ms, the CS- elicited increased activation in the left insula and left middle frontal gyrus. These findings not only confirm a critical role of prefrontal, insular, and precuneus regions in aversive conditioning, but they also suggest that biologically and socially salient information modulates activation at early stages of the information processing flow, and thus furnish initial insight about how affect and social judgments operate.

Neocortical modulation of the amygdala response to fearful stimuli

BACKGROUND

The cortical circuitry involved in conscious cognitive processes and the subcortical circuitry involved in fear responses have been extensively studied with neuroimaging, but their interactions remain largely unexplored. A recent functional magnetic resonance imaging (fMRI) study demonstrated that the engagement of the right prefrontal cortex during the cognitive evaluation of angry and fearful facial expressions is associated with an attenuation of the response of the amygdala to these same stimuli, providing evidence for a functional neural network for emotional regulation.

METHODS

In the current study, we have explored the generalizability of this functional network by using threatening and fearful non-face stimuli derived from the International Affective Picture System (IAPS), as well as the influence of this network on peripheral autonomic responses.

RESULTS

Similar to the earlier findings with facial expressions, blood oxygen level dependent fMRI revealed that whereas perceptual processing of IAPS stimuli was associated with a bilateral amygdala response, cognitive evaluation of these same stimuli was associated with attenuation of this amygdala response and a correlated increase in response of the right prefrontal cortex and the anterior cingulate cortex. Moreover, this pattern was reflected in changes in skin conductance.

CONCLUSIONS

The current results further implicate the importance of neocortical regions, including the prefrontal and anterior cingulate cortices, in regulating emotional responses mediated by the amygdala through conscious evaluation and appraisal.

Real-time fMRI of temporolimbic regions detects amygdala activation during single-trial self-induced sadness

Temporolimbic circuits play a crucial role in the regulation of human emotion. A highly sensitive single-shot multiecho functional magnetic resonance imaging (fMRI) technique with gradient compensation of local magnetic field inhomogeneities and real-time data analysis were used to measure increases in amygdala activation during single 60-s trials of self-induced sadness. Six healthy male and female subjects performed a validated mood induction paradigm with randomized presentation of sad or neutral faces in 10 trials per scan. Subjects reported the intensity of experienced sadness after each trial. Immediate feedback of amygdala activation was given to the subjects during the ongoing scan to reinforce mood induction. Correspondence between increased intensity of predominantly left sided amygdala activation and self-rating of sadness was found in 78% of 120 sad trials, in contrast to only 14% of neutral trials. Amygdala activation was reproducible during repeated scanning sessions and displayed the strongest correlation with self-rating among all regions. These results suggest that amygdala activation may be closely associated with self-induced sadness. This novel real-time fMRI technology is applicable to a wide range of neuroscience studies, particularly those of the limbic system, and to neuropsychiatric conditions, such as depression, in which pathology of the amygdala has been implicated.

The rhythm of the heart in the blink of an eye: emotion-modulated startle magnitude covaries with heart rate variability

Emotion-modulated startle is a robust phenomenon that has been demonstrated in a wide range of experimental situations. Similarly, heart rate variability (HRV) has been associated with a diverse range of processes including affective and attentional regulation. The present study sought to examine the relationship between these two important measures of affective behavior. Ninety female participants viewed pleasant, neutral, and unpleasant pictures while exposed to acoustic startle stimuli. The eyeblink startle was recorded both during the affective foregrounds and during intertrial intervals. HRV was assessed during a resting baseline and relationships between HRV and startle magnitudes examined. Results indicated that resting HRV was inversely related to startle magnitude during both intertrial intervals and affective foregrounds. In addition, the participants with the highest HRV showed the most differentiated emotion-modulated startle effects, whereas those with the lowest HRV, compared to those with the highest HRV, showed significantly potentiated startle to neutral foregrounds and marginally potentiated startle to pleasant foregrounds. The findings are consistent with models that posit that prefrontal cortical activity modulates subcortical motivation circuits. These results have important implications for the use of startle probe methodology and for HRV in the study of emotional regulation and dysregulation.

Individual differences in the responses of naïve rhesus monkeys to snakes

The authors demonstrated individual differences in inhibited behavior and withdrawal responses of laboratory-born rhesus monkeys when initially exposed to a snake. Most monkeys displayed a small significant increase in their behavioral inhibition in the presence of a snake. A few monkeys had marked responses, and some actively withdrew. Although the responses of the most extreme laboratory-born monkeys were comparable to feral-born monkeys, the responses of the laboratory-born monkeys rapidly habituated. The individual differences in the responses of naïve monkeys likely reflect a continuum from orienting to wariness to fear. A neurobiological model is presented that addresses potential mechanisms underlying these individual differences, their relation to fear, and how they may predispose to phobia development.

A computational study into the evolution of dual-route dynamics for affective processing

The evolutionary justification by LeDoux (1996) for his dual-route model of fear processing was analyzed computationally by applying genetic algorithms to artificial neural networks. The evolution was simulated of a neural network controlling an agent that gathered food in an artificial world and that was occasionally menaced by a predator. Connections could not change in the agent's "lifetime," so there was no learning in the simulations. Only if the smells of food and predator were hard to distinguish and the fitness reflected time pressures in escaping from the predator did the type of dual processing postulated by LeDoux emerge in the surviving agents. Processing in the "quick and dirty" pathway of the fear system ensured avoidance of both predators and food, but a distinction between food and predator was made only in the long pathway. Elaborate processing inhibited the avoidance reaction and reversed it into an approach reaction to food, but strengthened the avoidance reaction to predators (and more finely tuned the direction of escape). It is suggested that "computational neuroethology" (Beer, 1990) may help constrain reasoning in evolutionary psychology, particularly when applied to specific neurobiological models, and in the future may even generate new hypotheses for cognitive neuroscience.

Quetiapine produces a prolonged reversal of the sensorimotor gating-disruptive effects of basolateral amygdala lesions in rats

Prepulse inhibition (PPI) of startle is impaired in schizophrenia and in rats after manipulations of limbic cortical and subcortical regions. The atypical antipsychotic quetiapine was used to reverse PPI deficits after basolateral amygdala (BLA) lesions in rats. BLA quinolinic acid lesions significantly disrupted PPI 1 week postsurgery. Tests with quetiapine (0 vs. 7.5 mg/kg) in a within-subject design 2-3 weeks postsurgery revealed a normalization of PPI. Carry-over effects lasted up to 3 weeks, with a return of lesion-induced deficits by Week 5 postsurgery. This dose of quetiapine also blocked the PPI-disruptive effects of phencyclidine. PPI deficits after BLA lesions are reversed by quetiapine, in a manner that is sustained beyond its acute pharmacological effects and which may be mediated downstream from the BLA.

Single-shot T(2)* mapping with 3D compensation of local susceptibility gradients in multiple regions

Macroscopic magnetic field inhomogeneities severely limit sensitivity of blood oxygenation level-dependent (BOLD) functional MRI (fMRI) in frontal and central brain regions close to brain stem. A single-shot multiecho echo-planar imaging method (TurboPEPSI) was developed that combines quantitative T(2)* mapping with gradient compensation of local susceptibility inhomogeneities in multiple volumes of interest (VOIs). Gradient compensation was optimized in individual subjects based on magnetic field mapping and applied at selected echo times, interleaved with acquisition of uncompensated echoes. Intrinsic T(2)* values from uncompensated echoes were obtained in real-time simultaneously with effective T(2)* values from gradient compensated echoes. It is demonstrated that up to three VOIs can be compensated in a single excitation, in addition to collecting uncompensated data, using 8-echo acquisition on a clinical 1.5 Tesla scanner. A theory was developed to optimize the sequence of uncompensated and compensated echoes to achieve maximum BOLD sensitivity. Gradient compensation increased effective T(2)* values in left and right amygdala on average by 18.8 +/- 7.5 ms, while maintaining sensitivity in uncompensated brain areas. In orbitofrontal cortex effective T(2)* values increased by 22.2 +/- 5.3 ms. A CO(2) challenge paradigm was used to demonstrate that this gradient compensation method significantly enhances BOLD signal changes in amygdala as compared to conventional echo-planar imaging (EPI) and uncompensated TurboPEPSI.

The contribution of the amygdala to conditioned thalamic arousal

Previous research has demonstrated that thalamocortical neurons within the dorsal lateral geniculate nucleus (dLGN) are affected by an acoustic, fear-arousing, conditioned stimulus (Cain et al., 2000). This effect is reflected in an increase in activity and a tonic firing pattern, a pattern that assures the most accurate relay of information from the retina to the visual neocortex. Such an effect is considered to be indicative of a heightened state of arousal. The present research was designed to determine the extent to which the central nucleus of the amygdala (ACe) contributes to this effect. To this end, in experiment 1 extracellular recordings were made from single dLGN neurons in the awake rabbit during electrical stimulation of the ACe. Increased neuronal activity was observed in response to stimulation in the majority of neurons. Neurons that were in a burst firing pattern immediately before stimulation assumed a tonic firing pattern in response to stimulation. Experiment 2 was designed to determine whether inactivation of the ACe with muscimol would attenuate the response of dLGN neurons in the awake rabbit to the presentation of acoustic, fear-arousing, conditioned stimuli. Compared with vehicle injections, infusions of muscimol attenuated both the spontaneous activity and the response of dLGN neurons to the presentations of these stimuli. The results provide support for the hypothesis that the amygdala, and in particular the ACe, contributes to a heightened state of arousal during conditioned fear.

The emerging dialogue between psychoanalysis and neuroscience: neuroimaging perspectives

Current progress in the cognitive neurosciences is highly relevant to the development of psychoanalytic theory and practice. Neuroscience is today becoming mature enough to provide empirical biological approaches for the investigation of psychoanalytic models and observations. The current state of functional neuroimaging techniques is reviewed, selected paradigms and findings relevant to psychotherapy research are presented, and ways to pursue the dialogue between psychoanalysts and neuroscientists are discussed, as are some related obstacles and pitfalls. The emerging dialogue between psychoanalysts and neuroscientists may help not only to reestablish a solid position of psychodynamic theory and treatment in contemporary medicine, but also to bridge the division between "psychological" and "somatic" treatments, and gain important insights into the mind-brain relationship.

Imaging genomics

The recent completion of a working draft of the human genome sequence promises to provide unprecedented opportunities to explore the genetic basis of individual differences in complex behaviours and vulnerability to neuropsychiatric illness. Functional neuroimaging, because of its unique ability to assay information processing at the level of brain within individuals, provides a powerful approach to such functional genomics. Recent fMRI studies have established important physiological links between functional genetic polymorphisms and robust differences in information processing within distinct brain regions and circuits that have been linked to the manifestation of various disease states such as Alzheimer's disease, schizophrenia and anxiety disorders. Importantly, all of these biological relationships have been revealed in relatively small samples of healthy volunteers and in the absence of observable differences at the level of behaviour, underscoring the power of a direct assay of brain physiology like fMRI in exploring the functional impact of genetic variation.

Acute SSRI administration affects the processing of social cues in healthy volunteers

Enhancement of serotonin neurotransmission plays an important role in the antidepressant response to agents presently available to treat depression. This response forms the major evidence for the role of serotonin in affective and social behaviour in humans. The present study investigated the effects of acute administration of the selective serotonin reuptake inhibitor (SSR1), citalopram (10 mg, i.v.) upon a measure of emotional processing in healthy female volunteers. Subjects completed a facial expression recognition task following infusion of citalopram or saline (between-subjects design, double-blind). Facial expressions associated with five basic emotions--happiness, sadness, fearfulness, anger and disgust--were displayed. Each face had been 'morphed' between neutral (0%) and each emotional standard (100%) in 10% steps, leading to a range of emotional intensities. Mood and subjective experience were also monitored throughout the testing session. Volunteers receiving citalopram detected a higher number of facial expressions of fear and happiness, with reduced response times, relative to those given the placebo. By contrast, changes in the recognition of other basic emotions were not observed following citalopram. Notable differences in mood were also not apparent in these volunteers. These results suggest that acute administration of antidepressant drugs may affect neural processes involved in the processing of social information. This effect may represent an early acute effect of SSRIs on social and emotional processing that is relevant to their therapeutic actions.

The graded anxiety test: a novel test of murine unconditioned anxiety based on the principles of the elevated plus-maze and light-dark test

Standard tests of murine unconditioned anxiety such as the elevated plus-maze and light-dark test are based on a dichotomy of avoidance behaviour (walled vs. open arms and dark vs. light compartments). We combined the principles of both tests by modifying the elevated plus-maze as follows: one walled arm was made transparent and had a white floor (WTW), whereas the other walled arm was opaque-gray having a black floor (WOB). Furthermore, one open arm had a white floor (OW), while the other had a black one (OB). These modifications allow the distinction between more than two sub-compartments that elicit different degrees of avoidance behaviour, thus having a higher discriminative potency. Additionally, the paradigm was thought to permit the within-task detection of pharmacological side effects on the perception of the anxiogenic stimuli provided. The degree of avoidance of the sub-compartments exhibited by saline-treated mice for the distal parts of the four arms was distributed as follows: WOB<WTW=OB<OW. This pattern demonstrates that the sub-compartments elicited at least three differed degrees of fear in control mice, and that these were able to discriminate between bright/dark compartments WOB vs. WTW and OB vs. OW. Diazepam given at 1, 2, and 3 mg/kg doses increased the number of entries into the distal part of the most aversive open white arm and increased the total time spent on the undivided open white arm. Both the 2 and 3 mg/kg dose increased motor activity and impaired bright/dark discrimination for the open but not for the walled arms. Thus, the graded anxiety test might be useful to screen for substances that retain the normal perception of anxiogenic stimuli, but prevent the transact of fear into undue panic reactions.

Alpha-2A-adrenergic receptors are present on neurons in the central nucleus of the amygdala that project to the dorsal vagal complex in the rat

The descending pathway between the central nucleus of the amygdala (CeA) and the dorsal vagal complex (DVC) is an important substrate for autonomic functions associated with emotion. Activity in this circuit is crucially modulated by catecholamines and agonists of the alpha-2A-adrenergic receptor (alpha(2A)-AR), which relieve cardiovascular and gastrointestinal symptoms associated with experience of aversive stimuli. The subcellular distribution of alpha(2A)-AR within the CeA, however, has not been characterized. It is also not known if any alpha(2A)-AR-expressing neurons in the CeA project to the dorsal vagal complex. In order to address these questions, we examined the immunocytochemical labeling of alpha(2A)-AR in the CeA of rats receiving microinjection of the retrograde tracer fluorogold (FG) into the dorsal vagal complex at the level of the area postrema, an area involved in cardiorespiratory and gastrointestinal functions. Of all alpha(2A)-AR-labeled profiles in the CeA, the majority were either dendrites (42%) or somata (24%). alpha(2A)-AR labeling was often present on the plasmalemma in dendrites and was mainly found in endosome-like organelles in somata. Of all alpha(2A)-AR immunoreactive somata, 62% also contained immunolabeling for FG and 23% of all dendrites also showed labeling for the retrograde tracer. The intracellular distribution of alpha(2A)-AR did not differ in somata or dendrites with or without detectable FG. The remaining singly labeled alpha(2A)-AR profiles consisted of axons (11%), axon terminals (12%), and glial processes (13%). In numerous instances, alpha(2A)-AR-labeled glia or axon terminals were apposed to DVC projecting neurons. Together, this evidence suggests that the principal site for alpha(2A)-AR activation is at extrasynaptic sites on dendrites of CeA neurons, many of which project to the DVC and also show endosomal receptor labeling. In addition, these results indicate that activation of alpha(2A)-AR in the CeA may influence the activity of DVC projecting neurons through indirect mechanisms, including changes in presynaptic transmitter release or glial function. These results suggest that alpha(2A)-AR agonists in the CeA may modulate numerous processes including stress-evoked autonomic reactions and feeding behavior.

Identification of a signaling network in lateral nucleus of amygdala important for inhibiting memory specifically related to learned fear

We identified the Grp gene, encoding gastrin-releasing peptide, as being highly expressed both in the lateral nucleus of the amygdala, the nucleus where associations for Pavlovian learned fear are formed, and in the regions that convey fearful auditory information to the lateral nucleus. Moreover, we found that GRP receptor (GRPR) is expressed in GABAergic interneurons of the lateral nucleus. GRP excites these interneurons and increases their inhibition of principal neurons. GRPR-deficient mice showed decreased inhibition of principal neurons by the interneurons, enhanced long-term potentiation (LTP), and greater and more persistent long-term fear memory. By contrast, these mice performed normally in hippocampus-dependent Morris maze. These experiments provide genetic evidence that GRP and its neural circuitry operate as a negative feedback regulating fear and establish a causal relationship between Grpr gene expression, LTP, and amygdala-dependent memory for fear.

Galanin peptide levels in hippocampus and cortex of galanin-overexpressing transgenic mice evaluated for cognitive performance

Galanin-overexpressing transgenic mice (GAL-tg) generated on a dopamine beta-hydroxylase promoter were previously shown to express high levels of galanin mRNA in the locus coeruleus, and to perform poorly on challenging cognitive tasks. The present study employed radioimmunoassay to quantitate the level of galanin peptide overexpression in two brain regions relevant to learning and memory, the hippocampus and cerebral cortex. Approximately 4-fold higher levels of galanin were detected in the hippocampus of GAL-tg as compared to WT. Approximately 10-fold higher levels of galanin were detected in the frontal cortex of GAL-tg as compared to WT. A second cohort of GAL-tg and WT again showed high levels of galanin overexpression in GAL-tg as compared to WT in both brain regions. Correlation analyses were conducted between galanin peptide concentrations and behavioral scores on four learning and memory tasks: the Morris water maze, social transmission of food preference, standard delay fear conditioning, and trace fear conditioning. While some significant correlations were detected, neither hippocampal nor cortical galanin levels in the two cohorts of GAL-tg consistently correlated with performance across these diverse cognitive tasks. Several interpretations of these findings are discussed, including the possibility that a threshold level of galanin overexpression is sufficient to impair performance on learning and memory tasks in mice.

Cerebral blood flow during anticipation of public speaking in social phobia: a PET study

BACKGROUND

The aim was to examine the neural correlates of anxiety elicited by the anticipation of public speaking in individuals with social phobia. Positron emission tomography and (15)O-water was used to measure regional cerebral blood flow in subjects with DSM-IV defined social phobia during anxiety anticipation. Heart rate and subjective anxiety were also recorded. While being scanned, subjects were speaking alone either before or after speaking in public. To evaluate anticipatory anxiety we compared individuals speaking alone before they were speaking in front of an audience with those who did the reverse.

RESULTS

Heart rate and subjective anxiety measures confirmed anticipatory anxiety in social phobics who performed their private speech before their public. This was accompanied by enhanced cerebral blood flow in the right dorsolateral prefrontal cortex, left inferior temporal cortex, and in the left amygdaloid-hippocampal region. Brain blood flow was lower in the left temporal pole and bilaterally in the cerebellum in the anticipation group.

CONCLUSIONS

Brain regions with altered perfusion presumably reflect changes in neural activity associated with worry about anticipated public performance. We speculate that anticipatory anxiety in social phobics originates in an affect sensitive fear network encompassing the amygdaloid-hippocampal region, prefrontal, and temporal areas.

Magnesium and affective disorders

There are several findings on the action of magnesium ions supporting their possible therapeutic potential in affective disorders. Examinations of the sleep-electroencephalogram (EEG) and of endocrine systems point to the involvement of the limbic-hypothalamus-pituitary-adrenocortical axis as magnesium affects all elements of this system. Magnesium has the property to suppress hippocampal kindling, to reduce the release of adrenocorticotrophic hormone (ACTH) and to affect adrenocortical sensitivity to ACTH. The role of magnesium in the central nervous system could be mediated via the N-methyl-D-aspartate-antagonistic, gamma-aminobutyric acidA-agonistic or a angiotensin II-antagonistic property of this ion. A direct impact of magnesium on the function of the transport protein p-glycoprotein at the level of the blood-brain barrier has also been demonstrated, possibly influencing the access of corticosteroids to the brain. Furthermore, magnesium dampens the calciumion-proteinkinase C related neurotransmission and stimulates the Na-K-ATPase. All these systems have been reported to be involved in the pathophysiology of depression. Despite the antagonism of lithium to magnesium in some cell-based experimental systems, similarities exist on the functional level, i.e. with respect to kindling, sleep-EEG and endocrine effects. Controlled clinical trials examining the effect of Mg in affective disorder are warranted.

Neuroimaging of emotion and personality: scientific evidence and ethical considerations

Affective neuroscience has seen an explosion of research efforts using modern neuroimaging approaches to uncover the neural basis of emotion and personality. The first section of this paper reviews studies from the domains of affective and forensic neuroimaging. These studies illustrate some of the topics likely to be the subject of future ethical debates. The second section relates limitations of the neuroimaging approach to ethical considerations in predicting future psychopathology on the basis of brain state analysis.

GABAergic regulation of the central nucleus of the amygdala: implications for sleep control

It is becoming established that the amygdala has a strong influence on arousal state, with most evidence indicating a role in the regulation of rapid eye movement sleep (REM). Electrically activating the central nucleus of the amygdala (CNA) can increase subsequent REM and enhance REM-related phenomena. However, drugs that may be inhibitory to CNA have been typically reported to reduce REM. This suggests that enhancing activity in CNA could promote REM, and that inhibiting activity in CNA could suppress REM. We reversibly inactivated CNA using the GABA(A) agonist, muscimol, or blocked GABAergic inhibition with the GABA(A) antagonist, bicuculline, and examined the effects on sleep and wakefulness. Rats (90-day-old male Sprague-Dawley) were implanted with electrodes for recording EEG and EMG. Cannulae were aimed into CNA for microinjecting muscimol (0.001, 0.3 and 1.0 microM/0.2 microl saline) or bicuculline (56 and 333 pM/0.2 microl saline). Each animal received bilateral microinjections of muscimol, bicuculine or saline alone followed by 6-h sleep recordings. Microinjections of low concentrations of muscimol into CNA produced relatively selective decreases in total REM and number of REM episodes that lasted up to 6 h. In contrast, microinjections of bicuculline into CNA produced significant increases in REM. There were no significant reductions in NREM or wakefulness. These findings demonstrate that inactivating CNA can produce a relatively selective suppression of REM. The possible role that spontaneous activity in CNA may play in REM initiation and/or maintenance is discussed.

Dopamine modulates the response of the human amygdala: a study in Parkinson's disease

In addition to classic motor signs and symptoms, Parkinson's disease (PD) is characterized by neuropsychological and emotional deficits, including a blunted emotional response. In the present study, we explored both the neural basis of abnormal emotional behavior in PD and the physiological effects of dopaminergic therapy on the response of the amygdala, a central structure in emotion processing. PD patients and matched normal controls (NCs) were studied with blood oxygenation level-dependent functional magnetic resonance imaging during a paradigm that involved perceptual processing of fearful stimuli. PD patients were studied twice, once during a relatively hypodopaminergic state (i.e., > or =12 hr after their last dose of dopamimetic treatment) and again during a dopamine-replete state. The imaging data revealed a robust bilateral amygdala response in NCs that was absent in PD patients during the hypodopaminergic state. Dopamine repletion partially restored this response in PD patients. Our results demonstrate an abnormal amygdala response in PD that may underlie the emotional deficits accompanying the disease. Furthermore, consistent with findings in experimental animal paradigms, our results provide in vivo evidence of the role of dopamine in modulating the response of the amygdala to sensory information in human subjects.

Molecular targets in the treatment of anxiety

Although the cathecholamine systems have long been the focus of drug therapy in anxiety and depression, the development of novel drugs specifically aimed at new targets within these traditional neurotransmitter systems and at targets outside of these systems is now propelling the field of drug development in anxiety. A greater understanding of regional brain networks implicated in stress, anxiety, and anxious behaviors has provided localized targets for anxiolytics. Within the serotonin and norepinephrine systems, increased understanding of postsynaptic receptor regulation with chronic treatment and cross-system effects of drug therapy have been critical in furthering our understanding of effective pharmacological interventions. Receptors within the glutamate, gamma-aminobutyric acid, and neuropeptide systems provide a rich diversity of drug targets, both in localization and function. While acknowledging significant clinical and biological differences between the various anxiety disorders, an important aspect of modern neurobiological research is to look for similarities among these disorders, given that they are highly comorbid with each other and often respond to the same spectrum of treatments. Here we review current views on both traditional and new molecular targets in the treatment of anxiety, realizing that the ultimate challenge in effective anxiolytic drug development may be achieving specificity in brain regions important in generating and sustaining anxiety.

Fear memory formation involves p190 RhoGAP and ROCK proteins through a GRB2-mediated complex

We used fear conditioning, which is known to alter synaptic efficacy in lateral amygdala (LA), to study molecular mechanisms underlying long-term memory. Following fear conditioning, the tyrosine phosphorylated protein p190 RhoGAP becomes associated with GRB2 in LA significantly more in conditioned than in control rats. RasGAP and Shc were also found to associate with GRB2 in LA significantly more in the conditioned animals. Inhibition of the p190 RhoGAP-downstream kinase ROCK in LA during fear conditioning impaired long- but not short-term memory. Thus, the p190 RhoGAP/ROCK pathway, which regulates the morphology of dendrites and axons during neural development, plays a central role, through a GRB2-mediated molecular complex, in fear memory formation in the lateral amygdala.

Alzheimer's disease and the basal forebrain cholinergic system: relations to beta-amyloid peptides, cognition, and treatment strategies

Alzheimer's disease (AD) is the most common form of degenerative dementia and is characterized by progressive impairment in cognitive function during mid- to late-adult life. Brains from AD patients show several distinct neuropathological features, including extracellular beta-amyloid-containing plaques, intracellular neurofibrillary tangles composed of abnormally phosphorylated tau, and degeneration of cholinergic neurons of the basal forebrain. In this review, we will present evidence implicating involvement of the basal forebrain cholinergic system in AD pathogenesis and its accompanying cognitive deficits. We will initially discuss recent results indicating a link between cholinergic mechanisms and the pathogenic events that characterize AD, notably amyloid-beta peptides. Following this, animal models of dementia will be discussed in light of the relationship between basal forebrain cholinergic hypofunction and cognitive impairments in AD. Finally, past, present, and future treatment strategies aimed at alleviating the cognitive symptomatology of AD by improving basal forebrain cholinergic function will be addressed.

Functional magnetic resonance imaging technology and traumatic brain injury rehabilitation: guidelines for methodological and conceptual pitfalls

OBJECTIVES

To illuminate the current methodological and conceptual pitfalls inherent in conducting functional magnetic resonance imaging (fMRI) research with individuals who have sustained traumatic brain injury (TBI) and to discuss appropriate remedies. The aim is describe fMRI research, its limitations, and how to best use this technology to examine TBI.

DISCUSSION

The topics discussed in this article include issues regarding signal detection, brain activation measurement, head movement, and sources of signal artifact. Issues surrounding data interpretation and the importance of analyzing the brain as a connected neural network is also discussed. Finally, problems with spatial normalization when examining individuals with TBI are reviewed.

CONCLUSIONS

To date, there is a scarcity of research applying fMRI technology to the study of TBI. However, because it is a noninvasive procedure with high availability in hospital settings across the country, the next decade of TBI research will likely include a proliferation of this form of investigation. At this time, much work is needed to better understand how to optimally use this technology to examine the effects of TBI on behavior. For fMRI to enhance TBI research it will be imperative to establish valid research protocols and reliable methods of data interpretation.

Preclinical models: status of basic research in depression

Approximately one half-century ago several classes of medications, discovered by serendipity, were introduced for the treatment of depression and bipolar disorder. These highly effective medications revolutionized our approach to mood disorders and helped launch the modern era of psychiatry. Yet our progress since those serendipitous discoveries has been disappointing. We still do not understand with certainty how those medications produce their desired clinical effects. We have not introduced newer medications with fundamentally different mechanisms of action than the older agents. We have not identified the genetic and neurobiological mechanisms underlying depression and mania, nor do we understand the mechanisms by which nongenetic factors influence these disorders. We have only a rudimentary understanding of the circuits in the brain responsible for the normal regulation of mood and affect, and of those circuits that function abnormally in mood disorders. In approaching these gaps in our knowledge, this workgroup highlighted four major areas for future investment. These include developing better animal models of mood disorders; identifying genetic determinants of normal and abnormal mood in humans and animals; discovering novel targets and biomarkers of mood disorders and treatments; and increasing the recruitment of investigators from diverse backgrounds to mood disorders research.

Differential amygdala response during facial recognition in patients with schizophrenia: an fMRI study

Human lesion or neuroimaging studies suggest that amygdala is involved in facial emotion recognition. Although impairments in recognition of facial and/or emotional expression have been reported in schizophrenia, there are few neuroimaging studies that have examined differential brain activation during facial recognition between patients with schizophrenia and normal controls. To investigate amygdala responses during facial recognition in schizophrenia, we conducted a functional magnetic resonance imaging (fMRI) study with 12 right-handed medicated patients with schizophrenia and 12 age- and sex-matched healthy controls. The experiment task was a type of emotional intensity judgment task. During the task period, subjects were asked to view happy (or angry/disgusting/sad) and neutral faces simultaneously presented every 3 s and to judge which face was more emotional (positive or negative face discrimination). Imaging data were investigated in voxel-by-voxel basis for single-group analysis and for between-group analysis according to the random effect model using Statistical Parametric Mapping (SPM). No significant difference in task accuracy was found between the schizophrenic and control groups. Positive face discrimination activated the bilateral amygdalae of both controls and schizophrenics, with more prominent activation of the right amygdala shown in the schizophrenic group. Negative face discrimination activated the bilateral amygdalae in the schizophrenic group whereas the right amygdala alone in the control group, although no significant group difference was found. Exaggerated amygdala activation during emotional intensity judgment found in the schizophrenic patients may reflect impaired gating of sensory input containing emotion.

Mechanisms of Pavlovian fear conditioning: has the engram been located?

Uncertainty persists as to whether the amygdala is a crucial site of plasticity for classically conditioned fear or merely a sensory relay to structures generating fear responses. A recent Nature study suggests that associative synaptic changes take place in neurons of the amygdala during fear conditioning, and that these changes require dopamine-mediated modulation. Nevertheless, these findings do not prove that the amygdala is a sufficient site of plasticity for fear memory.

Isolation rearing-induced facilitation of Pavlovian learning: abolition by postsession intra-amygdala nafadotride

It has been shown previously in this laboratory that rats reared in social isolation acquire a Pavlovian-conditioned approach task much more rapidly than their respective controls. This study assessed the involvement specifically of the mesoamygdaloid dopamine pathway in this facilitated learning of isolates. Thus, animals were required to associate arbitrary stimuli with a pulsed light stimulus (unconditioned stimulus, US). The US, while without biological significance, was nevertheless capable of eliciting an intrinsic and sustained alerting response. Procedures ensured that the arbitrary stimuli (tone or clicker) did not elicit a response in the first instance, and were presented either paired (CS+) or unpaired (CS-) with the US. Isolates and socially reared controls received intra-amygdala infusions of the D3 dopamine receptor antagonist, L-nafadotride, or vehicle immediately following the end of each training session. The conditioned response increased over sessions in both groups of vehicle-infused rats during presentations of the CS+ stimulus, but not CS-, and isolates acquired this association more rapidly than controls. However, acquisition of this association was abolished by postsession intra-amygdala L-nafadotride. Responding to the US was largely unaffected by drug or rearing conditions. Hence, these data provide strong evidence for the specific involvement of the mesoamygdaloid dopamine projection in the facilitation of associative learning by isolation rearing.

Serotonin transporter genetic variation and the response of the human amygdala

A functional polymorphism in the promoter region of the human serotonin transporter gene (SLC6A4) has been associated with several dimensions of neuroticism and psychopathology, especially anxiety traits, but the predictive value of this genotype against these complex behaviors has been inconsistent. Serotonin [5- hydroxytryptamine, (5-HT)] function influences normal fear as well as pathological anxiety, behaviors critically dependent on the amygdala in animal models and in clinical studies. We now report that individuals with one or two copies of the short allele of the serotonin transporter (5-HTT) promoter polymorphism, which has been associated with reduced 5-HTT expression and function and increased fear and anxiety-related behaviors, exhibit greater amygdala neuronal activity, as assessed by BOLD functional magnetic resonance imaging, in response to fearful stimuli compared with individuals homozygous for the long allele. These results demonstrate genetically driven variation in the response of brain regions underlying human emotional behavior and suggest that differential excitability of the amygdala to emotional stimuli may contribute to the increased fear and anxiety typically associated with the short SLC6A4 allele.

The amygdala and reward

The amygdala -- an almond-shaped group of nuclei at the heart of the telencephalon -- has been associated with a range of cognitive functions, including emotion, learning, memory, attention and perception. Most current views of amygdala function emphasize its role in negative emotions, such as fear, and in linking negative emotions with other aspects of cognition, such as learning and memory. However, recent evidence supports a role for the amygdala in processing positive emotions as well as negative ones, including learning about the beneficial biological value of stimuli. Indeed, the amygdala's role in stimulus-reward learning might be just as important as its role in processing negative affect and fear conditioning.

Deficits in trace cued fear conditioning in galanin-treated rats and galanin-overexpressing transgenic mice

Galanin inhibits the release of several neurotransmitters and produces performance deficits in a variety of spatial and aversive learning and memory tasks. The experiments in this study investigated the role galanin has in emotional learning and memory using a standard delay cued and contextual fear conditioning task. Rats were administered galanin into the lateral ventricles before training, and scored for freezing behavior in the same context and in a novel context with and without an auditory cue (CS) that had been paired previously with an aversive stimulus (US). Galanin-overexpressing transgenic mice were tested in an identical behavioral protocol. The galanin-administered rats and the transgenic mice were not significantly different from their respective controls on this task. A more challenging trace cued and contextual fear conditioning procedure was administered to separate groups of galanin-treated rats and galanin-overexpressing transgenic mice. Subjects were trained with the same CS and US, however, a 2.5-sec delay was inserted between CS offset and US onset. Following the trace conditioning, rats administered galanin and mice overexpressing galanin both exhibited significantly less freezing to the CS in the novel context as compared with their control groups. These results indicate that the observed disruption of cued fear conditioning was specific to the more difficult trace conditioning task. These findings are the first demonstration that galanin impairs performance on an emotional memory task and support the hypothesis that galanin-induced deficits are specific to more difficult cognitive tasks.

The central nucleus of the amygdala and the wake-promoting effects of modafinil

Modafinil, a novel non-amphetamine stimulant recently approved for the treatment of narcolepsy, has been shown to increase waking in both animals and humans. However, its mechanism of action is currently unknown. Earlier research into the brain structures responsible for the wake-producing actions of modafinil implicated the central nucleus of the amygdala (ACe) as a possible site of action [Neuroscience 87 (1998) 905-911; Neurosci. Lett. 241 (1998) 95-98]. The present experiments were designed to test the hypothesis that the ACe is, at least in part, involved in the wake-producing actions of modafinil. In the first experiment, rats with lesions of the ACe were injected systemically with varying doses of modafinil and sleep was recorded. At the highest dose, modafinil significantly increased waking and decreased sleep. However, there was no interaction between the lesion and the effect of the drug. In the second experiment, varying doses of modafinil were injected directly into the ACe and sleep was recorded. Injection of modafinil into the ACe did not affect sleep architecture. Thus, ACe does not play a simple role in modafinil's wake-promoting action. We suggest that more complex testing will be required to elucidate its role.

Complexity management theory: motivation for ideological rigidity and social conflict

We are doomed to formulate conceptual structures that are much simpler than the complex phenomena they are attempting to account for. These simple conceptual structures shield us, pragmatically, from real-world complexity, but also fail, frequently, as some aspect of what we did not take into consideration makes itself manifest. The failure of our concepts dysregulates our emotions and generates anxiety, necessarily, as the unconstrained world is challenging and dangerous. Such dysregulation can turn us into rigid, totalitarian dogmatists, as we strive to maintain the structure of our no longer valid beliefs. Alternatively, we can face the underlying complexity of experience, voluntarily, gather new information, and recast and reconfigure the structures that underly our habitable worlds.

Functional neuroanatomy of emotion: a meta-analysis of emotion activation studies in PET and fMRI

Neuroimagingstudies with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have begun to describe the functional neuroanatomy of emotion. Taken separately, specific studies vary in task dimensions and in type(s) of emotion studied and are limited by statistical power and sensitivity. By examining findings across studies, we sought to determine if common or segregated patterns of activations exist across various emotional tasks. We reviewed 55 PET and fMRI activation studies (yielding 761 individual peaks) which investigated emotion in healthy subjects. Peak activation coordinates were transformed into a standard space and plotted onto canonical 3-D brain renderings. We divided the brain into 20 nonoverlapping regions, and characterized each region by its responsiveness across individual emotions (positive, negative, happiness, fear, anger, sadness, disgust), to different induction methods (visual, auditory, recall/imagery), and in emotional tasks with and without cognitive demand. Our review yielded the following summary observations: (1) The medial prefrontal cortex had a general role in emotional processing; (2) fear specifically engaged the amygdala; (3) sadness was associated with activity in the subcallosal cingulate; (4) emotional induction by visual stimuli activated the occipital cortex and the amygdala; (5) induction by emotional recall/imagery recruited the anterior cingulate and insula; (6) emotional tasks with cognitive demand also involved the anterior cingulate and insula. This review provides a critical comparison of findings across individual studies and suggests that separate brain regions are involved in different aspects of emotion.

Depression: perspectives from affective neuroscience

Depression is a disorder of the representation and regulation of mood and emotion. The circuitry underlying the representation and regulation of normal emotion and mood is reviewed, including studies at the animal level, human lesion studies, and human brain imaging studies. This corpus of data is used to construct a model of the ways in which affect can become disordered in depression. Research on the prefrontal cortex, anterior cingulate, hippocampus, and amygdala is reviewed and abnormalities in the structure and function of these different regions in depression is considered. The review concludes with proposals for the specific types of processing abnormalities that result from dysfunctions in different parts of this circuitry and offers suggestions for the major themes upon which future research in this area should be focused.

Fear conditioning in humans: the influence of awareness and autonomic arousal on functional neuroanatomy

The degree to which perceptual awareness of threat stimuli and bodily states of arousal modulates neural activity associated with fear conditioning is unknown. We used functional magnetic neuroimaging (fMRI) to study healthy subjects and patients with peripheral autonomic denervation to examine how the expression of conditioning-related activity is modulated by stimulus awareness and autonomic arousal. In controls, enhanced amygdala activity was evident during conditioning to both "seen" (unmasked) and "unseen" (backward masked) stimuli, whereas insula activity was modulated by perceptual awareness of a threat stimulus. Absent peripheral autonomic arousal, in patients with autonomic denervation, was associated with decreased conditioning-related activity in insula and amygdala. The findings indicate that the expression of conditioning-related neural activity is modulated by both awareness and representations of bodily states of autonomic arousal.

The primate amygdala and the neurobiology of social behavior: implications for understanding social anxiety

The amygdala has long been implicated in the mediation of emotional and social behaviors. Because there are very few human subjects with selective bilateral damage of the amygdala, much of the evidence for these functional associations has come from studies employing animal subjects. Macaque monkeys live in complex, highly organized social groups that are characterized by stable and hierarchical relationships among individuals who engage in complex forms of social communication, such as facial expressions. Understanding the role of the amygdala in animals that display a level of social sophistication approaching that of humans will help in understanding the amygdala's role in human social behavior and in psychopathology such as social anxiety. Selective bilateral lesions of the amygdala in mature macaque monkeys result in a lack of fear responses to inanimate objects and a "socially uninhibited" pattern of behavior. These results imply that the amygdala functions as a protective "brake" on engagement of objects or organisms while an evaluation of potential threat is carried out. They also suggest that social anxiety may be a dysregulation or hyperactivity of the amygdala's evaluative process. Finally, recent data from developmental studies raise the possibility that, at least at some developmental stages, fear in social contexts may be subserved by different brain regions than fear of inanimate objects.

Affective modulation of multiple memory systems

The hippocampus and caudate nucleus are anatomical components of relatively independent memory systems and recent research has focused on the nature of the interaction between these two systems. The amygdala exerts a general modulatory influence on memory storage processes related, in part, to an organism's level of affective or emotional arousal. Moreover, affective state can influence the use of different memory systems, and the amygdala may mediate this effect of emotion on memory. Recent evidence indicates that the amygdala modulates the separate types of memory mediated by the hippocampus and caudate nucleus. Recent human brain imaging studies also point to both sex- and hemisphere-related asymmetries in amygdala participation in emotionally influenced memory.