Four key findings emerged from our study. First, the current paradigm produced strong levels of differential conditioning, manifested as higher ratings to the CS+ relative to the CS− across all of the subjects. Second, in the context of similar levels of differential learning, overall fear collapsed across CS types was higher among anxious patients than healthy subjects. Furthermore, significant positive correlations between self-reported and parent-reported anxiety symptom scores and fear ratings to the CS+ following conditioning emerged, with similar sized correlations for the CS−. Third, among all of the subjects, fear following conditioning predicted avoidance of extinction session. Finally, fear ratings were resistant to extinction, showing strong stability across visits. These findings can be contextualized from the previous literature.
The present study used a novel fear-conditioning paradigm with CS− UCS pairings designed explicitly to address limitations in other paradigms used with juveniles. Air puffs, loud sounds, and aversive pictures have generally been ineffectual in eliciting fear or fear-related learning. Air puffs used by one study to examine children at risk of mood and anxiety disorders12,13
yielded inconsistent results across sexes and paradigms. Previous work with healthy subjects also found that this UCS elicits minimal fear in approximately 50% of subjects.14
Studies using loud sounds10,11
have struggled to demonstrate greater fear of the CS+ relative to the CS− among anxious children, whereas aversive pictures produce low levels of fear in children.25
In comparison, the present study showed higher fear levels to the CS+ relative to the CS−, with large effect sizes (d
> 0.8) in both anxious and healthy subjects. Thus, levels of differential self-reported fear in this experiment appears greater than those of previous conditioning experiments among children and adolescents.
Ethical questions concerning the tolerability of these procedures should be considered. Three factors speak to this. First, although levels of fear were markedly higher to the CS+ relative to CS−, overall fear levels were moderate and comparable to levels reported in other experimental procedures shown to be safe, such as blood draws and exposure to CO2
Second, fear levels appear commensurate with those experienced during everyday life experiences such as in various forms of media or entertainment. Third, all of the subjects in the present protocol (with one exception) continued to participate in our research and were followed over time by clinicians with no lasting impact noted.
These data comparing fear levels in anxious and healthy subjects can also be placed within a rich literature examining distress reactions in the laboratory among adult and pediatric anxious patients. At one level, as anxiety disorders are defined by high levels of reported distress, it is not surprising to observe greater fear in patients relative to healthy peers during conditioning. Yet work in other brain disorders, such as dementia, demonstrates the utility of paradigms evoking clinically relevant features of a disorder in controlled experimental settings.8
Previous experimental studies with both pediatric and adult anxiety disorders have not enjoyed consistent success in demonstrating expected patient-control differences using other methods, including fear-conditioning procedures.10
The present study documented large effect sizes of stimulus type within each subject group and in the comparison between anxious and healthy subjects. Furthermore, positive correlations emerged between concurrent anxiety symptoms and fear responses to the CS+ following conditioning. The effectiveness of the present procedure in eliciting disorder-relevant behaviors within remit of acceptable ethical standards makes it appealing to future investigations probing fear learning in pediatric anxiety.
The present data also form a developmental adjunct to research in adult anxiety.5
Both existing adult findings and the present data show comparable levels of differential conditioning to CS+ and CS− stimuli among anxious and healthy subjects, but with greater overall fear in patients collapsed across CS types. These between-group differences may arise from anxiety-related deficiencies in inhibiting fear responses, even in the presence of safety cues.27
As such, anxious individuals continue to exhibit high levels of fear to the stimulus unpaired with the UCS (CS−). In part, fear responses to the CS− may arise through a tendency to generalize fear across stimuli due to comparable visual properties (e.g., both CS + and CS− are photographs of female faces). However, as anxious individuals, like their healthy peers, report less fear of the CS−, some discrimination between these stimuli is present.
An alternative explanation for these results that does not rely on associative principles is that group differences may be due to enhanced sensitization among anxious relative to healthy subjects. That is, as a result of exposure to an anxiety-provoking UCS, patients exhibit heightened sensitivity to a range of stimuli. Because of a decision to collect ratings only following the conditioning procedures, no baseline indices of fear were available to verify this oversensitivity hypothesis. If preconditioning ratings to the CS+ and CS− were comparable in anxious patients and controls, then one could eliminate the possibility that enhanced fear responses were due to increased sensitization and attribute these directly to conditioning. Nevertheless, post hoc tests showed significant between-group differences in fear ratings to the CS+, but not to the CS−, following conditioning. Thus, although anxious individuals rate the CS+ as more fear provoking than healthy comparison subjects, this may not extend to the CS−. Future studies may establish baseline fear levels to the CS+ and CS− before conditioning to differentiate conditioning from sensitization accounts. Regardless, these data do show enhanced fear collapsed across CS− stimuli following conditioning in pediatric patients.
Findings that attrition rates between visits were not related to anxiety disorders per se but rather to acquired fear levels to the CS+ were also intriguing. These implied that subjects exhibiting higher distress to the CS+ following conditioning were less likely to attend visit 2. This finding is consistent with adult reports, showing higher avoidance rates (defined by refusal to return) among individuals who were unaware of the CS–UCS relationship during conditioning.4
Presumably for these individuals, as with our subjects, inability to predict occurrence of an aversive outcome was sufficiently fear provoking to warrant avoidance. Such instances of experimentally induced avoidance closely reflect clinically significant avoidance, a debilitating feature of anxiety disorders.
Finally, the present data on extinction extend previous findings. Recent work notes that extinction does not erase the memory of the CS+–UCS association but rather embeds this association in a new cognitive context.1
Thus, one would expect humans to show strong declarative memory for the CS+–UCS association, even after extinction. Consistent with this, previous studies using shock UCS in adults note strong persistence of reported fear.16
Similarly, the present data also show that fear of the CS+ after conditioning predicted fear of the CS+ after extinction, with stability comparable to that for clinical anxiety measures.
Four limitations of this study should be considered. First, the present study examined relatively small numbers of anxious and healthy subjects. Sample size may limit the ability to observe between-group differences in reactions to the CS− stimulus. Second, there was clear sample attrition across visits, biased among individuals reporting greater fear levels to the CS+ stimulus at visit 1. Although it is difficult to ascertain effects of attrition on stability of fear ratings, one might speculate underestimation of stability, given that subjects with higher fear ratings are expected to show consistently robust responses. Despite this, as it was our goal to design a procedure that elicits clinically meaningful fear among adolescents, significant levels of attrition are expected. A third caveat concerns a lack of baseline levels of fear to the CS+ and CS− before conditioning, precluding consideration of alternative explanations of the results. Finally, we obtained one set of fear ratings following sessions rather than continuous assessments. This is problematic because visit 1 included both acquisition and extinction trials, increasing ambiguity as to which process group differences are attributed to—visit 1 ratings may reflect fear associated with pure extinction trials toward the end of this session rather than fear during acquisition. Rating fear at the end of each session was driven by findings that self-focused evaluation of fear to facial stimuli yields differences between adolescent patients and healthy controls.7
To minimize confounds associated with self-focused evaluation of fear on group differences in fear generated by conditioning, we reduced the number of time points at which fear was assessed. A pertinent issue to arise is whether group differences in fear ratings at visit 1 can be attributed solely to acquisition. Two issues are relevant. First, given partial reinforcement rates during acquisition, in which only 75% of trials associated with the CS+ stimulus were followed by the UCS, participants may be less likely to attribute safety to the CS+ in the absence of the UCS during extinction. Thus, fear associated with extinction trials immediately following acquisition may be similar to the 25% of acquisition trials in which the CS+ is not followed by the UCS. Second, nonsignificant differences between anxious and healthy subjects in fear provocation following the extinction session makes it unlikely that greater fear provocation to CSs at visit 1 are driven by differences in fear to extinction. Future studies should derive cleaner indices of fear to distinguish these different processes.
Several aspects of the findings of this study may be relevant to clinical treatments. First, anxious patients reported higher overall fear to CSs compared with healthy individuals. Such greater fear provocation was also associated with subsequent patterns of avoidance, persisting after extinction. Interventions should therefore target reduction of fear to both the threat cue (CS+) and the safety cue (CS−) to attenuate later negative outcomes of avoidance and persistence of fear. Considerable work in basic science also provides clues concerning extinction factors that may reduce fear in humans. For example, a recent study demonstrated that extinction training after a short interval to acquisition is often associated with reduced reinstatement of fear at a later time point.28
Therapists may thus consider using exposure therapies soon after an anxiety-provoking event. The utility of such therapies may be further enhanced if patients are trained to discriminate actual threat cues (CS+) from safety cues (CS−). Although fear reduction is necessary to both cues, it may be beneficial for patients to learn specifically to reduce their fear to the latter. Finally, medications facilitating extinction in animal models may contribute to parallel efforts in humans.29
The current fear conditioning paradigm using novel CS and UCS stimuli induced strong, consistent distress reactions in both patients and healthy subjects. Moreover, these effects were large, both for within-subject and between-subject comparisons. Finally, initial distress reactions predicted avoidance and showed strong stability even after extinction. Given the suitability of this paradigm for engaging fear-related processes among patients and controls, application to neuroimaging studies, in which anxiety-linked differences in fear circuit engagement can be examined, represents a promising avenue.