In response to rapidly presented threats, youth with GAD show specific disturbances in neural activation. Consistent with our first hypothesis, when viewing briefly displayed, masked angry faces, GAD youth have greater amygdala activation relative to comparison subjects. Moreover, there is a positive correlation between degree of amygdala activation and anxiety-symptom severity. Results also supported our second hypothesis. As predicted, right amygdala and right ventrolateral prefrontal cortex activation did exhibit negative connectivity during threat trials specifically, which was evident in both groups. Post-hoc analysis revealed reduced negative coupling in GAD relative to healthy youth at a liberal statistical threshold. Finally, although both groups show an attention bias of similar magnitude to masked angry faces, attention bias correlates with amygdala activation in patients but not in healthy subjects.
The present findings and our previous work4
indicate that GAD youth process threat faces atypically at both behavioral and neural levels. Behaviorally, when angry faces are presented briefly as in the present study, youth with GAD and comparison subjects both show an initial attention bias toward the spatial location of threat. However, when angry faces were presented for longer periods (500 ms), GAD youth, relative to comparison subjects, showed an attention bias away from threat4
. Neurally, when threat is presented briefly, GAD youth show increased amygdala activation which positively correlates with anxiety severity. In contrast, when angry faces were displayed for 500 msec, GAD youth showed no difference from healthy peers in the amygdala, but they did show greater right ventrolateral prefrontal cortex activation. Moreover, when using the 500 msec threat exposures, GAD patients with mild symptoms showed greater ventrolateral prefrontal cortex activation than GAD patients with severe symptoms, suggesting that right ventrolateral prefrontal cortex compensates for a GAD-related disturbance in functioning elsewhere, potentially in the amygdala.
Presently, little is known about the development of the amygdala-ventrolateral prefrontal cortex circuit and how it relates to the emergence of anxiety disorders. Work from animal models indicates that the developmental timing of alterations to the amygdala-prefrontal cortex circuit greatly impacts anxiety-related behavior44,45
. Turning to humans, the question is how do neural disturbances relate to the onset of anxiety during development. Presently, it is not known if disturbances in this circuit precede the onset of GAD and are, therefore, risk markers, or if such disturbances arise with the disorder. Consistent with a risk-marker hypothesis, recent findings indicate that amygdala hyperactivation does relate to risk
for depression and anxiety in youth46,47
. Clearly, more work is needed to understand how the development of this circuit relates to the emergence of anxiety and other disorders that increase in prevalence during adolescence.
Work with a non-clinical sample of adults found that the right ventrolateral prefrontal cortex modulates amygdala responses to briefly presented, masked threat cues5
. Extending these findings, our psychophysiological interaction connectivity analysis indicates that the strength of amygdala activation varies as a function of right ventrolateral prefrontal cortex activity in youth and that the negative coupling may be weaker in GAD than comparisons. Consistent with neurobiological models of emotion8,48,49
, our results suggest that GAD in youth is associated with dysfunction in a threat detection system, involving a balance between sub-cortical and cortical regions, in particular, the amygdala and ventrolateral prefrontal cortex.
Some research examining the relationship between the amygdala and right ventrolateral prefrontal cortex in response to threat has emphasized the role of the right ventral prefrontal cortex in modulating amygdala responses in relation to strategic emotion regulation processes likely to be engaged over relatively long time periods10
. Other work has emphasized the amygdala-ventrolateral prefrontal cortex relationship in terms of emotion regulation processes that are engaged, even when a threat stimulus is briefly presented5,48
. The present findings are compatible with the latter view.
Of note, although only detected at a liberal statistical threshold, patients exhibited less negative coupling between the amygdala and ventrolateral prefrontal cortex relative to comparisons. Given that patients show greater amygdala response to threat, the reduced negative coupling in patients relative to comparisons may represent a sign of impaired amygdala modulation. Thus, from this perspective, GAD may relate more to the balance between amygdala and right ventrolateral prefrontal cortex activation, as opposed to overall increases in amygdala activation. Further research is required to clarify these relationships.
In addition, we performed a connectivity analysis across all task conditions. In contrast to the psychophysiological interaction connectivity analysis, this approach showed that both groups had a stronger positive coupling between the same regions and that GAD had greater positive connectivity. Such positive amygdala-ventrolateral prefrontal cortex connectivity has been observed previously in various populations and age groups studied with the standard approach used here34,50
. Differences in the approaches of these two connectivity procedures may provide insight into the discrepant findings. The goal of the psychophysiological interaction analysis was to examine task-dependent interactions, specifically related to threat. In contrast, this connectivity analysis reveals association in activation across the entire course of the task. Thus, it is not surprising that these procedures yield different results. Further work using both connectivity approaches is necessary to confirm and understand the manner in which threat content modulates amygdala-ventrolateral prefrontal cortex connectivity in healthy and abnormal development.
In another line of research, we recently found that when stimuli are presented for a relatively long duration in specific attention conditions (e.g. participants subjectively evaluate threat-related facial expressions shown individually for several seconds), youth with GAD selectively show greater amygdala activation34
. Thus, taken together with the current findings, these investigations suggest that differential amygdala response profiles are task dependent. The cognitive correlates of amygdala hyperactivation in these two studies are likely to differ. The present study may map neural correlates of threat orienting and detection related to vigilance in clinical anxiety. These correlates appear to involve the amygdala as well as disturbances in the balance between the amygdala and ventrolateral prefrontal cortex. Other tasks may map neural correlates of psychological processes distinct from threat orienting and detection, such as the subjective experience of fear. Work in this area with anxious youth demonstrates that amygdala hyperactivation, in tandem with enhanced ventral prefrontal activation and amygdala-prefrontal coupling, may be correlates of subjective fear34
. Further work is necessary to understand exactly what situations lead to normal and abnormal neural activation in youth with GAD.
Finally, even though both anxious and comparison groups show an attention bias toward masked angry faces, amygdala activation only correlates with attention bias in GAD and not in comparisons. This suggests that different neural processes underlie the common behavioral result of attention bias to masked angry faces. For patients, it may be that the amygdala mediates processing rapidly presented threats and this is part of a profile of cognitive responses to threat that underlies GAD. Since the comparison subjects also show attention toward masked angry faces, the bias toward masked angry faces is not a unique feature of GAD in youth.
The present study of GAD youth included patients with comorbid MDD and other anxiety disorders, particularly social phobia. However, follow-up analyses showed no group differences in right amygdala activation among patients with different diagnoses. Moreover, each patient subgroup showed greater activation in the same area of the amygdala relative to comparison subjects. These analyses indicate that the group differences in amygdala activation were not due to MDD or social phobia. Another limitation is the small sample. However, since small samples lead to reduced power and the hypothesized findings were confirmed, this limitation is less problematic. A final limitation is that there was a wide age range in both groups and the small sample size made it unfeasible to examine interactions between age and diagnosis. Future work on youth with GAD may wish to select specific age groups to examine how developmental changes relate to anxiety-related influences on brain function.