Divergence in cortical representations of threat generalization in affective versus perceptual circuitry in childhood: Relations with anxiety

Highlights

• After conditioning, children viewed a threat cue, safety cue, and blended stimuli.

• RSA was implemented to quantify children's threat generalization to ambiguous stimuli.

• Greater neural pattern differentiation in affective than perceptual brain regions.

• Anxiety symptomatology related to decreased representational similarity in vmPFC.

Abstract

Children at risk for anxiety display elevated threat sensitivity and may inaccurately classify safe stimuli as threatening, a process known as overgeneralization. Little is known about whether such overgeneralization might stem from altered sensory representations of stimuli resembling threat, especially in youth. Here we implement representational similarity analysis of fMRI data to examine the similarity of neural representations of threat versus ambiguous or safe stimuli in threat and perceptual neurocircuitry among children at varying levels of anxiety traits. Three weeks after completing threat conditioning and extinction, children underwent an fMRI extinction recall task, during which they viewed the extinguished threat cue (CS+), safety cue (CS-) and generalization stimuli (GS) consisting of CS-/CS+ blends. Multivoxel BOLD signal patterns were measured in seven regions of interest: four affective areas (ventromedial prefrontal cortex (vmPFC), anterior insular cortex (AIC), dorsomedial prefrontal cortex (dmPFC), and amygdala) and three perceptual areas (inferior temporal cortex (ITC) and visual areas V1 and V4). Compared to low anxious children, children with high trait anxiety evidenced less neural pattern differentiation between the CS+ and similar GS, particularly in the vmPFC. Together, these results demonstrate the utility of multivariate neuroimaging approaches in arbitrating the relative contributions of perceptual versus affective sources to threat generalization.

Introduction

Because threat can manifest in different forms, it is adaptive for an observer to be vigilant around exemplars that may similarly predict an aversive outcome. Threat generalization – a learning mechanism whereby threat responses extend to a range of stimuli resembling a past threat – enables a rapid response to novel and potentially dangerous stimuli (Armony et al., 1997; Lissek et al., 2008; Vansteenwegen et al., 2005). It has been proposed that developmental changes in threat learning and generalization may contribute to the emergence of anxiety disorders in late childhood and early adolescence (Britton et al., 2013; Lau et al., 2011). Several threat generalization studies in humans have considered how varying degrees of perceptual resemblance to a threat stimulus elicit graded univariate responses in emotion neurocircuitry (e.g. insula, ventromedial prefrontal cortex; Britton et al., 2013; Dunsmoor et al., 2011; Greenberg et al., 2013a, 2013b; Lissek et al., 2014; Michalska et al., 2016, 2019; for a review see Dymond et al., 2015). However, far fewer studies have examined whether ambiguous stimuli are similarly represented as threatening in object identification areas earlier in the visual processing stream. Further, limited work has examined threat generalization processes in children and adolescents, perhaps reflecting the challenge of finding a potent and biologically-relevant unconditioned stimulus as well as ethical considerations with threat induction in youth (see Shechner et al., 2014 for a review of key developmental considerations in threat conditioning). As a consequence, the neural mechanisms that mediate affective versus sensory-perceptual aspects of threat generalization and their relation to childhood anxiety traits remain unclear. The present study addresses this gap by integrating functional magnetic resonance imaging (fMRI) with multivariate analytical techniques to test how neural representation of perceptually similar stimuli contributes to threat generalization in children on a range of anxiety symptoms. Since perceptual networks have a less protracted developmental course than cognitive networks (Gogtay et al., 2004; Sowell et al., 2003), identifying perceptual contributions to threat overgeneralization at a relatively early developmental stage would bear implications for treatment of anxiety disorders.

Some stimuli are classified as dangerous based on inherent stimulus features, whereas other stimuli are classified as dangerous through effects of learning. Generalizability can be examined with either inherent or learned dangerous stimuli. Nevertheless, experimental work has focused more on entirely learned threats, as examined in Pavlovian conditioning and extinction paradigms, rather than inherently dangerous stimuli. In these Pavlovian conditioning paradigms, a neutral stimulus (conditioned stimulus, CS+) is paired with a threat (unconditioned stimulus, UCS) (Britton et al., 2013; Dunsmoor and Paz, 2015; Dymond et al., 2015; Milad et al., 2007; Quirk and Mueller, 2008), to examine the generalization of threat responses to generalization stimuli (GS): stimuli resembling the CS+ that have never themselves predicted an aversive experience (Dunsmoor and LaBar, 2013; Glenn et al., 2012; Michalska et al., 2016, 2019; Shechner et al., 2018). Differential conditioning paradigms also incorporate a second, unreinforced stimulus (CS-) to act as a safety cue. (Dunsmoor et al., 2009; Honig and Urcuioli, 1981; Lissek et al., 2008, 2010; Michalska et al., 2016, 2017, 2019; Shechner et al., 2018). Threat generalization can be tested by presenting individuals with the CS and GS either immediately after conditioning and extinction or even days or weeks afterwards. This process is known as extinction recall. Much of our current understanding of threat representations is grounded in computational models of learning during threat conditioning and extinction (Li et al., 2011; Pearce and Hall, 1980; Rescorla and Wagner, 1972), and findings from extinction recall may thus be more difficult to contextualize within this literature. Given our incomplete understanding of how threat generalization is instantiated in multivariate brain patterns during extinction recall, our study therefore represents a promising new research direction.

The neural sources of threat generalization, in both children and adults, remain poorly understood, though several brain networks have been reliably implicated in these processes. Among healthy adults, as stimuli become more similar to CS+, the amygdala, anterior insula, and dorsomedial prefrontal cortex (dmPFC) are recruited in forming threat associations and producing threat-conditioned behaviors (Davis, 1992; Dunsmoor et al., 2011; Resnik and Paz, 2015). Activation of the ventromedial prefrontal cortex (vmPFC), on the other hand, decreases as stimuli more closely resemble the CS+ and increases as stimuli more closely resemble the CS- (Lissek et al., 2014; Schiller et al., 2008). A failure to recruit vmPFC in response to safe stimuli has been associated with deficiencies in threat generalization (Cha et al., 2014; Greenberg et al., 2013b; Holt et al., 2012). In comparison to healthy individuals, children and adults with anxiety display hyperactivity in the amygdala and insula as well as aberrant engagement of the vmPFC upon viewing learned threat stimuli (Britton et al., 2013; Dunsmoor and Paz, 2015; Indovina et al., 2011; Milad et al., 2009). However, it is not known whether perturbations in these affective areas parallel neural response profiles in perceptual areas, or whether the generalization gradients in affective areas are decoupled from perceptual representations.

Two competing accounts frame the current investigation of threat generalization, the tendency to view neutral or ambiguous stimuli as threatening. Under the perceptual account, threat overgeneralization – the exaggerated tendency to view neutral or ambiguous stimuli as threatening – stems from a disordered ability to differentiate between threatening and ambiguous non-threatening stimuli at the perceptual level, possibly from a dysfunction that also manifests during threat learning (Dunsmoor and LaBar, 2013; Dymond et al., 2015; Lashley and Wade, 1946; Lim and Pessoa, 2008). This account generates the hypothesis of impaired perceptual discrimination of stimuli. If overgeneralization manifests in the context of intact perceptual discrimination, then generalization does not result from impaired perceptual discrimination. Research supporting the perceptual account finds that threat conditioning induces a wider generalization gradient through altering perceptual thresholds (Dunsmoor and LaBar, 2013; Dymond et al., 2015; Lim and Pessoa, 2008; Resnik et al., 2011; Resnik and Paz, 2015). Under the conceptual account, overgeneralization instead reflects abnormalities in non-perceptual, affective processes, including threat learning and memory mechanisms, separate from perceptual discrimination (Greenberg et al., 2013a; Kindt, 2014; Lissek et al., 2014; Shepard, 1987; Soeter and Kindt, 2015). Under this view, threat generalization is an active process leading to the generalization of threat even in the presence of accurate discrimination of the CS. In other words, the organism elicits behaviors to the GS appropriate for a CS, not from a failure to perceptually differentiate the GS from the CS but rather because they are functionally similar even if perceptually distinct. Thus, the classification would be based on psychological rather than physical similarity (Shepard, 1987). Primate research finds a shift in the tuning curves of neurons in the basolateral amygdala after conditioning (Resnik and Paz, 2015), but this study did not reveal whether these changes occurred at sensory encoding. The lack of studies probing generalization gradients across perceptual and affective brain regions simultaneously precludes strong inferences regarding sources of individual differences in threat generalization related to anxiety. Here we do not explicitly disambiguate between the two accounts; rather, we contribute to the ongoing debate by characterizing profiles of stimulus-evoked neural representations in both perceptual and affective neural regions following a threat conditioning and extinction procedure during late middle childhood, a period of developmental plasticity during which the risk for internalizing disorders is heightened (Kessler et al., 2005) and brain regions underlying threat learning are still developing (Gee et al., 2013; Gogtay et al., 2004).

Research studying the neural underpinnings of threat generalization has failed to delineate whether perceptual mechanisms play a role in threat generalization in part because univariate analytic methods are not well suited for addressing the question as to where in the cortical hierarchy ambiguous stimuli are encoded as threatening. However, recent advances in analytic methods have facilitated innovations in cognitive neuroscience, which may accelerate discoveries in the etiology of threat overgeneralization. One particularly promising approach is representational similarity analysis (RSA; Kriegeskorte et al., 2008), which leverages information contained in the patterns of activity across multiple voxels to characterize the unique neural representation of a stimulus within a given brain region (Davis and Poldrack, 2013; Haxby, 2012; Mahmoudi et al., 2012; see Ritchie et al., 2019 for counterpoint) rather than averaging the response across multiple voxels, as is standard in univariate techniques. Thus, whereas univariate methods compare a voxel's or region's signal strength between conditions, RSA recognizes the unique contribution of multiple voxels within a population. One application of RSA involves comparing or classifying the neural representations of different stimuli via examining their multivoxel patterns: the dissimilarity of patterns is taken to identify which representations of stimuli are alike and which diverge. When leveraged with other indices of threat responding, multivariate methods offer increased sensitivity and strengthen inferences that are not readily gleaned from univariate strategies alone, hence improving traction in the basic science of overgeneralization. However, these methods have rarely been applied in studies of threat overgeneralization in youth, leaving a significant gap in our understanding.

Here, in a comprehensive and targeted approach, we study a sample of youth in late middle childhood/early adolescence on a range of anxiety traits across visits in the psychophysiology laboratory and neuroimaging environment. We tested (1) Whether multivariate neural responses exhibited similar threat-tuning profiles across brain regions implicated in affective versus perceptual processes during extinction recall, and (2) Whether multivoxel patterns of neural response across both sets of regions varied as a function of children's anxiety symptoms. Based on prior work examining univariate responses in affective and cognitive regions using this dataset (Michalska et al., 2019), we hypothesized that high anxious children would show more similar representations between a threat cue and a novel, perceptually similar stimulus across emotional areas than do low anxious children, particularly in the vmPFC and anterior insula. Due to mixed evidence with regards to perceptual circuitry in the adult literature (Åhs et al., 2013; Dunsmoor et al., 2012; Dunsmoor and Murphy, 2015) and given the lack of empirical work on neural representation in inferior temporal and visual cortices in children, we remained agnostic about the direction of associations between pattern differentiation in perceptual regions and anxiety.