Age-related reduced prefrontal-amygdala structural connectivity is associated with lower trait anxiety.

Anxiety is an unpleasant emotional state that involves physical, cognitive, and behavioral manifestations such as impaired emotion regulation, especially in interpersonal, ambiguous, and dangerous situations (Endler et al., 1992). Studies investigating the neurobiological basis of anxiety have implicated the amygdala in various negative affective responses, including the fear and vigilance that are characteristic of anxiety (Kapp et al., 1994; Oler et al., 2009; Pessoa and Adolphs, 2010; Whalen, 1998). An increasing number of studies have linked anxiety to complex functional and structural neural circuitry that is centered on the amygdala (Kim et al., 2011a). With regard to structural networks, the amygdala is densely and reciprocally connected with many brain regions involved in emotion and stress regulation, such as the prefrontal cortex (PFC; Dolcos et al., 2011). It has been suggested that an altered balance between activity in the amygdala and PFC could be a neural mechanism underlying anxiety (Bishop, 2008). Converging findings from animal and human research suggest that anxiety-related behaviors involve top-down regulatory influences from the vPFC to the amygdala (Banks et al., 2007; Ghashghaei et al., 2007; Stein et al., 2007a). This framework is supported by functional magnetic resonance imaging (fMRI) studies showing that successful emotion regulation is associated with lower amygdala activity and greater vPFC activity (Hariri et al., 2000; Hartley and Phelps, 2010; Wager et al., 2008). Other fMRI studies have linked amygdala-vPFC circuitry to the negative interpretation of emotionally ambiguous expressions, a threat-related perceptual bias that manifests in normal and pathological levels of anxiety (Bishop, 2007). Since these affective processes are affected in both normal and pathological anxiety, it follows that the amygdalar circuitry implicated in these behaviors is also implicated in anxiety (Milad et al., 2006). Several fMRI studies have demonstrated a negative association between trait anxiety and amygdala-vPFC functional connectivity, both at rest and during task-based paradigms (Bishop et al., 2004; Hare et al., 2008; Simpson et al., 2001). Another study found that greater resting-state functional connectivity (rsFC) between the amygdala and vPFC predicted lower trait and state anxiety in psychiatrically healthy individuals (Kim et al., 2011a). Together, these studies suggest that higher anxiety is associated with reduced functional connectivity between the amygdala and vPFC. Studies also suggest that anxiety-related decreases in amygdala-vPFC functional connectivity are associated with underlying structural declines. In accordance with functional findings, both human and animal studies suggest that structural connectivity between the amygdala and ventral prefrontal cortex (vPFC) also plays an important role in behaviors that are compromised in anxiety, including emotion regulation and fear conditioning (Cisler et al., 2009; Hare et al., 2008; Phelps et al., 2004). In psychiatrically healthy individuals, trait anxiety has been shown to be negatively associated with both the white matter integrity and volume of the left uncinate fasciculus, a dense white matter bundle that connects the amygdala to the vPFC (Baur et al., 2012; Bracht et al., 2009; Kim and Whalen, 2009). Patients with social anxiety disorder (SAD) also exhibit lower uncinate fasciculus integrity compared with healthy, age-matched controls (Phan et al., 2009). To account for these findings, Kim et al. (2011a) proposed that the vPFC exerts a “top-down” influence on the amygdala, wherein better regulatory control is facilitated by both increased structural connectivity and functional connectivity between these regions (Ghashghaei and Barbas, 2002; Kim and Whalen, 2009; Kim et al., 2011b). However other neuroimaging studies have reported findings that contradict this regulatory model. Several diffusion tensor imaging (DTI) studies have demonstrated that higher trait anxiety scores are associated with increased rather than decreased structural integrity of the uncinate fasciculus (Modi et al., 2013; Montag et al., 2012). Similarly, another DTI study showed that the severity of anxiety in individuals with obsessive-compulsive disorder (OCD) was positively associated with the integrity of the internal capsule, a pathway that may encompass some amygdala-vPFC connections that are not part of the uncinate fasciculus (Lochner et al., 2012). Thus, while the amygdala-vPFC pathway is consistently implicated in anxiety, the significance of the structural integrity of this circuitry to anxiety remains unclear. Whereas most of these findings were derived from younger adults, less is known about how changes in this pathway relate to anxiety in older adults. While anxiety is characterized by an attentional bias towards processing aversive information (Mathews and MacLeod, 2002), older adults demonstrate a positivity effect in which, relative to younger adults, they show preferences for positive over negative information in both attention and memory (Charles et al., 2003; Mather and Carstensen, 2003). Indeed, several investigations have reported age-related reductions in trait anxiety and anxiety disorders such as post-traumatic stress disorder (PTSD) and SAD (Lindesay et al., 1989; Regier et al., 1990). Given evidence that successful emotion and anxiety regulation relies on prefrontal control over the amygdala, a logical assumption would be that older adults have greater structural integrity in this circuitry than younger adults. Yet previous DTI studies have instead revealed significant age-related white matter decline between the amygdala and prefrontal cortex (Burzynska et al., 2010; Davis et al., 2009a), particularly in measures of fractional anisotropy (FA) and radial diffusivity (RD), which are sensitive to changes in myelin integrity (Schmierer et al., 2004). Age-related white matter decline is particularly robust in the frontal lobe, with pathways linking the amygdala to the vPFC declining early on (Head et al., 2004; Malykhin et al., 2011; Salat et al., 2005). However, if the regulatory model is correct, age-related decline in amygdala-vPFC structural connectivity should be associated with increased anxiety. Here, we used aging as a model of amygdala-vPFC structural decline to test the hypothesis that the structural integrity of this pathway relates to trait anxiety. In order to test this hypothesis, we utilized probabilistic tractography, a structural imaging technique that enables white matter pathways to be traced in vivo (Behrens et al., 2007). Tractography is performed by tracing white matter “streamlines” between a pair of brain regions based on patterns of water diffusion in brain tissue. Since water diffusion is fastest along the length of axons, this technique can be used to estimate the topography and integrity of neuroanatomical connections. In turn, these tract estimates can be used to extract white matter indices, such as FA, RD, and axial diffusivity (AD), which serve as scalar measures of structural integrity. White matter tractography has been successfully implemented in previous DTI studies to examine white matter connections between the amygdala and frontal cortex (Bach et al., 2011; Saygin et al., 2011). To date, the majority of anxiety studies have focused on white matter decline in the uncinate fasciculus. However, some tractography studies have delineated additional structural connections between the amygdala and vPFC that course through the extreme capsule and more medially through peri-striatal regions, such as the substantia innominata (Croxson et al., 2005; Johansen-Berg et al., 2008; Kim and Whalen, 2009). These findings suggest that the uncinate fasciculus region-of-interest might not capture the full extent of this anxiety circuit. Thus, we used probabilistic tractography to trace participant-specific white matter pathways between the amygdala and vPFC. In the present study, we used aging as a model of structural decline in the amygdala-vPFC pathway to test whether structural changes contribute to higher trait anxiety. Trait anxiety represents an individual’s generalized and long-lasting predisposition to respond to stress with apprehension and foreboding (Lueck, 2007), suggesting that it may be linked to more stable brain characteristics, such as the structural rather than functional integrity of circuits underlying stress and emotion processing. To quantify age-related changes in amygdala-vPFC structural connectivity, we analyzed tract FA, RD, AD and volume in three age groups ranging from younger to older adults. Only healthy, non-clinically-diagnosed participants were selected for analysis in order to compare our results to earlier findings in healthy younger adults (Kim and Whalen, 2009). We predicted that mean amygdala-vPFC FA and RD would decline with age. To test the regulatory anxiety model (Kim et al., 2011a), we also examined whether this structural change was associated with a concomitant increase in trait anxiety.