Higher- and lower-order personality traits and cluster subtypes in social anxiety disorder.

Social anxiety disorder (SAD) can come in different forms, presenting problems for diagnostic classification. Here, we examined personality traits in a large sample of patients (N = 265) diagnosed with SAD in comparison to healthy controls (N = 164) by use of the Revised NEO Personality Inventory (NEO-PI-R) and Karolinska Scales of Personality (KSP). In addition, we identified subtypes of SAD based on cluster analysis of the NEO-PI-R Big Five personality dimensions. Significant group differences in personality traits between patients and controls were noted on all Big Five dimensions except agreeableness. Group differences were further noted on most lower-order facets of NEO-PI-R, and nearly all KSP variables. A logistic regression analysis showed, however, that only neuroticism and extraversion remained significant independent predictors of patient/control group when controlling for the effects of the other Big Five dimensions. Also, only neuroticism and extraversion yielded large effect sizes when SAD patients were compared to Swedish normative data for the NEO-PI-R. A two-step cluster analysis resulted in three separate clusters labelled Prototypical (33%), Introvert-Conscientious (29%), and Instable-Open (38%) SAD. Individuals in the Prototypical cluster deviated most on the Big Five dimensions and they were at the most severe end in profile analyses of social anxiety, self-rated fear during public speaking, trait anxiety, and anxiety-related KSP variables. While additional studies are needed to determine if personality subtypes in SAD differ in etiological and treatment-related factors, the present results demonstrate considerable personality heterogeneity in socially anxious individuals, further underscoring that SAD is a multidimensional disorder.

Association between amygdala neurokinin-1 receptor availability and anxiety-related personality traits.

Animal studies indicate that substance P (SP) and its preferred neurokinin-1 (NK1) receptor modulate stress and anxiety-related behavior. Alterations in the SP-NK1 system have also been observed in human anxiety disorders, yet little is known about the relation between this system and individual differences in personality traits associated with anxiety propensity and approach-avoidance behavior, including trait anxiety, neuroticism, and extraversion. Exploring this relation could provide important insights into the neurobiological underpinnings of human anxiety and the etiology of anxiety disorders, as anxious traits are associated with increased susceptibility to develop psychopathological conditions. Here we examined the relationship between central NK1 receptor availability and self-rated measures of trait anxiety, neuroticism, and extraversion. The amygdala was chosen as the primary region of interest since this structure has been suggested to mediate the effect of the SP-NK1 system on anxiety. Anxious traits and NK1 receptor availability, determined with positron emission tomography and the radiotracer [11C]GR205171, were measured in 17 healthy individuals. Voxel-wise analyses showed a significant positive correlation between bilateral amygdala NK1 receptor availability and trait anxiety, and a trend in similar direction was observed for neuroticism. Conversely, extraversion was found to be negatively associated with amygdala NK1 receptor availability. Extraversion also correlated negatively with the NK1 measure in the cuneus/precuneus and fusiform gyrus according to exploratory whole-brain analyses. In conclusion, our findings indicate that amygdala NK1 receptor availability is associated with anxiety-related personality traits in healthy subjects, consistent with a modulatory role for the SP-NK1 system in human anxiety.

Distance to threat and risk of acute and posttraumatic stress disorder following bank robbery: A longitudinal study.

Identifying pathways through which environmental risk factors influence PTSD is important for understanding PTSD etiology. Here, we hypothesized that the physical proximity to threat influences PTSD risk by increasing ASD following trauma. One hundred six bank employees who had experienced a bank robbery participated in the study. A longitudinal design assessing ASD at day 2 and PTSD at day 30 was used to test the hypothesis. Participants also indicated their location in the bank at the time of the robbery. ASD was identified in 40 (38%) and PTSD in 16 (15%) of the robbery victims. Distance to the robber had a strong effect on ASD (OR 3.51, 95% CI 1.94-6.34) and a somewhat lesser effect on PTSD (OR 2.15, 95% CI 1.04-4.46), indicating that the effect of proximity to threat on PTSD 1 month following trauma could be mediated by its effect on ASD 2 days following trauma. Using structural equation modeling, we confirmed that the effect of distance on PTSD was fully mediated by ASD. These findings suggest that proximity to threat may increase PTSD risk by enhancing the acute stress response following trauma.

Neuroticism-related personality traits are associated with posttraumatic stress after abortion: findings from a Swedish multi-center cohort study.

BACKGROUND: Most women who choose to terminate a pregnancy cope well following an abortion, although some women experience severe psychological distress. The general interpretation in the field is that the most consistent predictor of mental disorders after induced abortion is the mental health issues that women present with prior to the abortion. We have previously demonstrated that few women develop posttraumatic stress disorder (PTSD) or posttraumatic stress symptoms (PTSS) after induced abortion. Neuroticism is one predictor of importance for PTSD, and may thus be relevant as a risk factor for the development of PTSD or PTSS after abortion. We therefore compared Neuroticism-related personality trait scores of women who developed PTSD or PTSS after abortion to those of women with no evidence of PTSD or PTSS before or after the abortion. METHODS: A Swedish multi-center cohort study including six Obstetrics and Gynecology Departments, where 1294 abortion-seeking women were included. The Screen Questionnaire-Posttraumatic Stress Disorder (SQ-PTSD) was used to evaluate PTSD and PTSS. Measurements were made at the first visit and at three and six month after the abortion. The Swedish universities Scales of Personality (SSP) was used for assessment of Neuroticism-related personality traits. Multiple logistic regression analyses were performed to investigate the risk factors for development of PTSD or PTSS post abortion. RESULTS: Women who developed PTSD or PTSS after the abortion had higher scores than the comparison group on several of the personality traits associated with Neuroticism, specifically Somatic Trait Anxiety, Psychic Trait Anxiety, Stress Susceptibility and Embitterment. Women who reported high, or very high, scores on Neuroticism had adjusted odds ratios for PTSD/PTSS development of 2.6 (CI 95% 1.2-5.6) and 2.9 (CI 95% 1.3-6.6), respectively. CONCLUSION: High scores on Neuroticism-related personality traits influence the risk of PTSD or PTSS post abortion. This finding supports the argument that the most consistent predictor of mental disorders after abortion is pre-existing mental health status.

Serotonin Synthesis and Reuptake in Social Anxiety Disorder: A Positron Emission Tomography Study.

IMPORTANCE: Serotonin is involved in negative affect, but whether anxiety syndromes, such as social anxiety disorder (SAD), are characterized by an overactive or underactive serotonin system has not been established. Serotonin 1A autoreceptors, which inhibit serotonin synthesis and release, are downregulated in SAD, and serotonin transporter availability might be increased; however, presynaptic serotonin activity has not been evaluated extensively. OBJECTIVE: To examine the serotonin synthesis rate and serotonin transporter availability in patients with SAD and healthy control individuals using positron emission tomography (PET) with the radioligands 5-hydroxytryptophan labeled with carbon 11 ([11C]5-HTP) and 11C-labeled 3-amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile [11C]DASB. DESIGN, SETTING, AND PARTICIPANTS: We performed a cross-sectional study at an academic clinical research center. Eighteen patients with SAD (9 men and 9 women; mean [SD] age, 32.6 [8.2] years) and 18 sex- and age-matched healthy controls (9 men and 9 women; mean [SD] age, 34.7 [9.2] years) underwent [11C]5-HTP PET imaging. We acquired [11C]DASB PET images for 26 additional patients with SAD (14 men and 12 women; mean [SD] age, 35.2 [10.7] years) and the same 18 sex- and age-matched healthy controls. Participants were recruited through newspaper advertisements. Data were acquired from March 12, 2002, through March 5, 2012, and analyzed from March 28, 2013, through August 29, 2014. MAIN OUTCOMES AND MEASURES: The influx rate of [11C]5-HTP as a measure of serotonin synthesis rate capacity and [11C]DASB binding potential as an index of serotonin transporter availability were acquired during rest. We used the Liebowitz Social Anxiety Scale to measure severity of social anxiety symptoms. RESULTS: The PET data were not available for analysis in 1 control for each scan. Increased [11C]5-HTP influx rate was observed in the amygdala, raphe nuclei region, caudate nucleus, putamen, hippocampus, and anterior cingulate cortex of patients with SAD compared with healthy controls (P < .05 corrected), supporting an enhanced serotonin synthesis rate. Increased serotonin transporter availability in the patients with SAD relative to healthy controls was reflected by elevated [11C]DASB binding potential in the raphe nuclei region, caudate nucleus, putamen, thalamus, and insula cortex (P < .05 corrected). CONCLUSIONS AND RELEVANCE: Neurotransmission in SAD is characterized by an overactive presynaptic serotonin system, with increased serotonin synthesis and transporter availability. Our findings could provide important new insights into the etiology of anxiety disorders.

Posttraumatic stress among women after induced abortion: a Swedish multi-centre cohort study.

BACKGROUND: Induced abortion is a common medical intervention. Whether psychological sequelae might follow induced abortion has long been a subject of concern among researchers and little is known about the relationship between posttraumatic stress disorder (PTSD) and induced abortion. Thus, the aim of the study was to assess the prevalence of PTSD and posttraumatic stress symptoms (PTSS) before and at three and six months after induced abortion, and to describe the characteristics of the women who developed PTSD or PTSS after the abortion. METHODS: This multi-centre cohort study included six departments of Obstetrics and Gynaecology in Sweden. The study included 1457 women who requested an induced abortion, among whom 742 women responded at the three-month follow-up and 641 women at the six-month follow-up. The Screen Questionnaire-Posttraumatic Stress Disorder (SQ-PTSD) was used for research diagnoses of PTSD and PTSS, and anxiety and depressive symptoms were evaluated by the Hospital Anxiety and Depression Scale (HADS). Measurements were made at the first visit and at three and six months after the abortion. The 95% confidence intervals for the prevalence of lifetime or ongoing PTSD and PTSS were calculated using the normal approximation. The chi-square test and the Student's t-test were used to compare data between groups. RESULTS: The prevalence of ongoing PTSD and PTSS before the abortion was 4.3% and 23.5%, respectively, concomitant with high levels of anxiety and depression. At three months the corresponding rates were 2.0% and 4.6%, at six months 1.9% and 6.1%, respectively. Dropouts had higher rates of PTSD and PTSS. Fifty-one women developed PTSD or PTSS during the observation period. They were young, less well educated, needed counselling, and had high levels of anxiety and depressive symptoms. During the observation period 57 women had trauma experiences, among whom 11 developed PTSD or PTSS and reported a traumatic experience in relation to the abortion. CONCLUSION: Few women developed PTSD or PTSS after the abortion. The majority did so because of trauma experiences unrelated to the induced abortion. Concomitant symptoms of depression and anxiety call for clinical alertness and support.

The prevalence of posttraumatic stress among women requesting induced abortion.

OBJECTIVES: To describe the prevalence and pattern of traumatic experiences, to assess the prevalence of posttraumatic stress disorder (PTSD) and posttraumatic stress symptoms (PTSS), to identify risk factors for PTSD and PTSS, and to analyse the association of PTSD and PTSS with concomitant anxiety and depressive symptoms in women requesting induced abortion. METHODS: A Swedish multi-centre study of women requesting an induced abortion. The Screen Questionnaire - Posttraumatic Stress Disorder was used for research diagnoses of PTSD and PTSS. Anxiety and depressive symptoms were evaluated by the Hospital Anxiety and Depression Scale (HADS). RESULTS: Of the 1514 respondents, almost half reported traumatic experiences. Lifetime- and point prevalence of PTSD were 7% (95% confidence interval [CI]: 5.8-8.5) and 4% (95% CI: 3.1-5.2), respectively. The prevalence of PTSS was 23% (95% CI: 21.1-25.4). Women who reported symptoms of anxiety or depression when requesting abortion were more likely to have ongoing PTSD or PTSS. Also single-living women and smokers displayed higher rates of ongoing PTSD. CONCLUSIONS: Although PTSD is rare among women who request an induced abortion, a relatively high proportion suffers from PTSS. Abortion seeking women with trauma experiences and existing or preexisting mental disorders need more consideration and alertness when counselled for termination.

Cognitive abilities of street children: low-SES Bolivian boys with and without experience of living in the street.

OBJECTIVES: This study compared results on cognitive tests measuring nonverbal visualization and reasoning, executive functions, and creativity between 36 boys with experience of living in the street and 31 housed yet socioeconomically equivalent boys, in Bolivia. RESULTS: The street children scored significantly higher on the creativity measure, which is discussed in relation to contextual relevance. No significant differences were found on the other cognitive tests. Time elapsed after living in the street and drug use were strongly associated with cognition, while age was not. Both groups scored below average compared to Western norms. The results are discussed in terms of the cultural relevance of the tests and the impact of socioeconomic status, stress, and stimulation on cognition.

Amygdala subregions tied to SSRI and placebo response in patients with social anxiety disorder.

The amygdala is a key structure in the pathophysiology of anxiety disorders, and a putative target for anxiolytic treatments. Selective serotonin reuptake inhibitors (SSRIs) and placebo seem to induce anxiolytic effects by attenuating amygdala responsiveness. However, conflicting amygdala findings have also been reported. Moreover, the neural profile of responders and nonresponders is insufficiently characterized and it remains unknown whether SSRIs and placebo engage common or distinct amygdala subregions or different modulatory cortical areas. We examined similarities and differences in the neural response to SSRIs and placebo in patients with social anxiety disorder (SAD). Positron emission tomography (PET) with oxygen-15-labeled water was used to assess regional cerebral blood flow (rCBF) in 72 patients with SAD during an anxiogenic public speaking task, before and after 6-8 weeks of treatment under double-blind conditions. Response rate was determined by the Clinical Global Impression-Improvement scale. Conjunction analysis revealed a common rCBF-attenuation from pre- to post-treatment in responders to SSRIs and placebo in the left basomedial/basolateral and right ventrolateral amygdala. This rCBF pattern correlated with behavioral measures of reduced anxiety and differentiated responders from nonresponders. However, nonanxiolytic treatment effects were also observed in the amygdala. All subgroups, including nonresponders, showed deactivation of the left lateral part of the amygdala. No rCBF differences were found between SSRI responders and placebo responders. This study provides new insights into the brain dynamics underlying anxiety relief by demonstrating common amygdala targets for pharmacologically and psychologically induced anxiety reduction, and by showing that the amygdala is functionally heterogeneous in anxiolysis.

Age, sex and NK1 receptors in the human brain -- a positron emission tomography study with [¹¹C]GR205171.

The substance P/neurokinin 1 (SP/NK1) system has been implicated in the processing of negative affect. Its role seems complex and findings from animal studies have not been easily translated to humans. Brain imaging studies on NK1 receptor distribution in humans have revealed an abundance of receptors in cortical, striatal and subcortical areas, including the amygdala. A reduction in NK1 receptors with increasing age has been reported in frontal, temporal, and parietal cortices, as well as in hippocampal areas. Also, a previous study suggests sex differences in cortical and subcortical areas, with women displaying fewer NK1 receptors. The present PET study explored NK1 receptor availability in men (n=9) and women (n=9) matched for age varying between 20 and 50years using the highly specific NK1 receptor antagonist [¹¹C]GR205171 and a reference tissue model with cerebellum as the reference region. Age by sex interactions in the amygdala and the temporal cortex reflected a lower NK1 receptor availability with increasing age in men, but not in women. A general age-related decline in NK1 receptor availability was evident in the frontal, temporal, and occipital cortices, as well as in the brainstem, caudate nucleus, and thalamus. Women had lower NK1 receptor availability in the thalamus. The observed pattern of NK1 receptor distribution in the brain might have functional significance for brain-related disorders showing age- and sex-related differences in prevalence.

Arousal modulation of memory and amygdala-parahippocampal connectivity: a PET-psychophysiology study in specific phobia.

Phobic fear is accompanied by intense bodily responses modulated by the amygdala. An amygdala moderated psychophysiological measure related to arousal is electrodermal activity. We evaluated the contributions of electrodermal activity to amygdala-parahippocampal regional cerebral blood flow (rCBF) during phobic memory encoding in subjects with spider or snake phobia. Recognition memory was increased for phobia-related slides and covaried with rCBF in the amygdala and the parahippocampal gyrus. The covariation between parahippocampal rCBF and recognition was related to electrodermal activity suggesting that parahippocampal memory processes were associated with sympathetic activity. Electrodermal activity further mediated the amygdala effect on parahippocampal activity. Memory encoding during phobic fear therefore seems contingent on amygdala's influence on arousal and parahippocampal activity.

The effects of medial temporal lobe resections on verbal threat and fear conditioning.

A left hemisphere advantage in the processing of verbal threat has previously been reported, whereas both hemispheres seem equally important in fear conditioning. Here, we compared the effects of unilateral medial temporal lobe (MTL) resections on verbal threat as well as delay and trace fear conditioning. During verbal threat, right and left MTL-resections attenuated fear potentiated startle in comparison with controls. In contrast to previous studies, MTL-resections did not attenuate delay conditioning of skin conductance responses. Left and right resectioned patients did not differ in psychophysiological responses to verbal threat or delay fear conditioning. Trace conditioning was not observed in any group. Results suggest a bilateral MTL hemispheric involvement in the processing of verbal threat, whereas one intact hemisphere seems sufficient for delay conditioning.

Disentangling the web of fear: amygdala reactivity and functional connectivity in spider and snake phobia.

The objective was to study effects of fear on brain activity, functional connectivity and brain-behavior relationships during symptom provocation in subjects with specific phobia. Positron emission tomography (PET) and (15)O water was used to measure regional cerebral blood flow (rCBF) in 16 women phobic of either snakes or spiders but not both. Subjects watched pictures of snakes and spiders serving either as phobic or fear-relevant, but non-phobic, control stimuli depending on phobia type. Presentation of phobic as compared with non-phobic cues was associated with increased activation of the right amygdala and cerebellum as well as the left visual cortex and circumscribed frontal areas. Activity decreased in the prefrontal, orbitofrontal and ventromedial cortices as well as in the primary somatosensory cortex and auditory cortices. Furthermore, amygdala activation correlated positively with the subjective experience of distress. Connectivity analyses of activity in the phobic state revealed increased functional couplings between voxels in the right amygdala and the periamygdaloid area, fusiform gyrus and motor cortex. During non-phobic stimulation, prefrontal activity correlated negatively with amygdala rCBF, suggesting a phobia-related functional decoupling. These results suggest that visually elicited phobic reactions activate object recognition areas and deactivate prefrontal areas involved in cognitive control over emotion-triggering areas like the amygdala, resulting in motor readiness to support fight or flight.

Chronic whiplash symptoms are related to altered regional cerebral blood flow in the resting state.

The neural pathogenic mechanisms involved in mediating chronic pain and whiplash associated disorders (WAD) after rear impact car collisions are largely unknown. This study's first objective was to compare resting state regional cerebral blood flow (rCBF) by means of positron emission tomography with (15)O labelled water in 21 WAD patients with 18 healthy, pain-free controls. A second objective was to investigate the relations between brain areas with altered rCBF to pain experience, somatic symptoms, posttraumatic stress symptoms and personality traits in the patient group. Patients had heightened resting rCBF bilaterally in the posterior parahippocampal and the posterior cingulate gyri, in the right thalamus and the right medial prefrontal gyrus as well as lowered tempero-occipital blood flow compared with healthy controls. The altered rCBF in the patient group was correlated to neck disability ratings. We thus suggest an involvement of the posterior cingulate, parahippocampal and medial prefrontal gyri in WAD and speculate that alterations in the resting state are linked to an increased self-relevant evaluation of pain and stress.

A link between serotonin-related gene polymorphisms, amygdala activity, and placebo-induced relief from social anxiety.

Placebo may yield beneficial effects that are indistinguishable from those of active medication, but the factors underlying proneness to respond to placebo are widely unknown. Here, we used functional neuroimaging to examine neural correlates of anxiety reduction resulting from sustained placebo treatment under randomized double-blind conditions, in patients with social anxiety disorder. Brain activity was assessed during a stressful public speaking task by means of positron emission tomography before and after an 8 week treatment period. Patients were genotyped with respect to the serotonin transporter-linked polymorphic region (5-HTTLPR) and the G-703T polymorphism in the tryptophan hydroxylase-2 (TPH2) gene promoter. Results showed that placebo response was accompanied by reduced stress-related activity in the amygdala, a brain region crucial for emotional processing. However, attenuated amygdala activity was demonstrable only in subjects who were homozygous for the long allele of the 5-HTTLPR or the G variant of the TPH2 G-703T polymorphism, and not in carriers of short or T alleles. Moreover, the TPH2 polymorphism was a significant predictor of clinical placebo response, homozygosity for the G allele being associated with greater improvement in anxiety symptoms. Path analysis supported that the genetic effect on symptomatic improvement with placebo is mediated by its effect on amygdala activity. Hence, our study shows, for the first time, evidence of a link between genetically controlled serotonergic modulation of amygdala activity and placebo-induced anxiety relief.

Symptom provocation in specific phobia affects the substance P neurokinin-1 receptor system.

BACKGROUND: Animal studies demonstrate that stress and negative affect enhance the release of the neuropeptide substance P (SP), which binds to the neurokinin 1 (NK1) receptor. This positron emission tomography (PET) study evaluated how the activity in the SP-NK1 receptor system in the amygdala was affected by fear provocation in subjects with specific phobia. METHODS: Sixteen adult women with DSM-IV-defined specific phobia for either snakes or spiders but not both viewed pictures of feared and non-feared animals while being PET-scanned for 60 min with the highly specific NK1 receptor antagonist [(11)C]GR205171 as the labeled PET tracer. RESULTS: The uptake of the labeled NK1 receptor antagonist was significantly reduced in the right amygdala during phobic stimulation. In the left amygdala no significant differences were found between phobic and non-phobic conditions. There was a negative correlation in the right, but not left, amygdala between subjective anxiety ratings and NK1 tracer binding. CONCLUSIONS: Fear provocation affects the SP-NK1 receptor system in the right amygdala. This reflects reduced NK1 receptor availability during fear and could mirror an increased release of endogenous substance P.

Amygdala and anterior cingulate cortex activation during affective startle modulation: a PET study of fear.

The human startle response is modulated by emotional experiences, with startle potentiation associated with negative affect. We used positron emission tomography with 15O-water to study neural networks associated with startle modulation by phobic fear in a group of subjects with specific snake or spider phobia, but not both, during exposure to pictures of their feared and non-feared objects, paired and unpaired with acoustic startle stimuli. Measurement of eye electromyographic activity confirmed startle potentiation during the phobic as compared with the non-phobic condition. Employing a factorial design, we evaluated brain correlates of startle modulation as the interaction between startle and affect, using the double subtraction contrast (phobic startle vs. phobic alone) vs. (non-phobic startle vs. non-phobic alone). As a result of startle potentiation, a significant increase in regional cerebral blood flow was found in the left amygdaloid-hippocampal region, and medially in the affective division of the anterior cingulate cortex (ACC). These results provide evidence from functional brain imaging for a modulatory role of the amygdaloid complex on startle reactions in humans. They also point to the involvement of the affective ACC in the processing of startle stimuli during emotionally aversive experiences. The co-activation of these areas may reflect increased attention to fear-relevant stimuli. Thus, we suggest that the amygdaloid area and the ACC form part of a neural system dedicated to attention and orientation to danger, and that this network modulates startle during negative affect.

Neurofunctional correlates of posttraumatic stress disorder: a PET symptom provocation study.

SUMMARY: Patients with combat-related posttraumatic stress disorder (PTSD) show altered cognitive and affective processing and symptomatic responding following exposure to trauma reminders. Previous symptom provocation studies using brain imaging have involved Vietnam veterans. In this study neural correlates were investigated in patients with PTSD resulting from trauma in more recent war zones. (15)Oxygen water and positron emission tomography were used to measure regional cerebral blood flow (rCBF) in patients with war- and combat-related chronic PTSD during exposure to combat and neutral sounds. Self-reports and heart rate confirmed symptomatic responding during traumatic stimulation. The war-related condition, as compared to the neutral, increased rCBF in the right sensorimotor areas (Brodmann areas 4/6), extending into the primary sensory cortex (areas 1/2/3), and the cerebellar vermis. RCBF also increased in the right amygdala and in the periaqueductal gray matter adjacent to the pons. During provocation rCBF was lowered in the right retrosplenial cortex (areas 26/29/30 extending into area 23). Symptom provocation in PTSD promote sensorimotor, amygdaloid and midbrain activation. We conclude that perceptually induced symptom activation in PTSD is associated with an emotionally determined motor preparation and propose that subcortically initiated rather than cortically controlled memory mechanisms determine this pattern.

The human startle reflex and pons activation: a regional cerebral blood flow study.

Using positron emission tomographic measurements of regional cerebral blood flow, we report activation of a medial pons area in humans during acoustic startle stimulation. Eight healthy volunteers were scanned during rest and when presented startle-eliciting stimuli. We performed a theory-driven directed search for activity in the nucleus reticularis pontis caudalis, situated in the pons. Because habituation of cerebellar activity during acoustic startle repetition has been reported [Timmann, D., Musso, C., Kolb, F.P., Rijntjes, M., Juptner, M., Muller, S.P., Diener, H.C. & Weiller, C. (1998) J. Neurol. Neurosurg. Psychiatry 65, 771-773], we also predicted habituation in the cerebellum and in the pons as a function of startle repetition. Measurements of eye electromyography validated the presence of a startle response and its habituation. Analysis of regional cerebral blood flow revealed higher neural activity during startle stimulation than at rest in a medial pons area consistent with the location of the pontine reticular nucleus. As a consequence of startle repetition, regional cerebral blood flow increased in the medial cerebellum, and habituated in the ventral cerebellum and in a ventral pons area separate from the pontine reticular nucleus. In the ventral pons, but not in the pontine reticular nucleus, regional cerebral blood flow and the startle reflex were positively correlated. In the cerebellum both positive and negative correlations with the startle reflex were observed. Thus we conclude that the neurofunctional correlates of the startle circuit and its habituation in humans are similar to that previously described in animals.