While existing meditation research has mostly focused on

While existing meditation research has mostly focused on the functional connectivity between the PCC/precuneus and prefrontal regions, little evidence exists on whether the PCC/precuneus exhibits different functional connectivity patterns with subcortical affective networks following meditation training. In particular, the soluble guanylate cyclase stem raphe nuclei constitute the major source of serotonergic projections that target the cortical, limbic and striatal regions implicated in affective processing (Deakin and Graeff, 1991; Hajós et al., 2003). Existing evidence indicates that serotonergic transmission in the brain may influence an individual\'s cognitive and neural processing of negative stimuli (Murphy et al., 2002; Cools et al., 2008; Roiser et al., 2008), and that reduction of serotonergic functions may be associated with heightened rates of neuroticism (Sen et al., 2004), consistent with a proposed role of serotonergic activity in regulating negative affective processing and in affective disorders (Fischer et al., 2015; Fakhoury, 2016). Similarly, the raphe nuclei have been found to regulate activity in downstream cortical and limbic affective networks, as well as processing of aversive stimuli and distress (Hajós et al., 2003; Amat et al., 2004; Selvaraj et al., 2015). While the dorsal raphe nucleus (DRN) might have received the most attention on its serotonergic-based regulations of downstream cortico-limbic-striatal networks and affective processing (Graeff et al., 1996; Selvaraj et al., 2015), recent evidence indicates that the pontine raphe nucleus (PRN) may also play critical roles in affective regulatory functions (Cannon et al., 2006, 2007; Lee et al., 2015). Patients with bipolar disorders, which are fundamentally characterized by extreme euphoria and dysphoria states and affective dysregulations, exhibit reduced serotonergic functioning in the PRN (Cannon et al., 2006, 2007), and both the anatomical and functional connectivity of the PRN with other affective networks predicted the affective states and emotional reactivity of healthy participants (Lee et al., 2015). Further, the PRN is critically involved in regulating negative stress (Vollmayr et al., 2000). Thus, the functional interplay between the PRN and PCC/precuneus, which likely underlies the processes by which individuals regulate self-referential negative affective processing, may be strengthened by meditation training. Such hypothesis is further based on known anatomical connections between the PCC/precuneus and pontine nuclei (Leichnetz, 2001; Cavanna and Trimble, 2006), existing findings indicating a brain stem serotonergic influence on the PCC rs-FC (Hahn et al., 2012), and evidence indicating that 8-week meditation training increased the gray matter concentration in a pontine region that correlated with increase in self-reported psychological well-being, as well as in the PCC (Hölzel et al., 2011; Singleton et al., 2014). Based on the theoretical framework that meditation training leads to an increased affective regulatory influence of the PRN on PCC/precuneus self-referential processing, we further expected that causal interactions between the PCC/precuneus and PRN would be mostly directed from the latter to the former structure, as revealed by spectral dynamic causal modeling (spDCM) (Razi et al., 2015). In the present study, we investigated the effect of 8-week meditation versus relaxation training on affective processing and PCC/precuneus rs-FC patterns in elderly participants. We selected this age group because, despite evidence indicating beneficial cognitive and neural effects of meditation training among the elderly (Gard et al., 2014; Luders, 2014), no research has directly investigated the effect of short-term meditation training on affective processing networks in this age group. Given various existing studies have reported effects of meditation in reducing the valence (Ho et al., 2015; Lutz et al., 2016) and arousal (Menezes et al., 2013) processing of positive and negative stimuli, and based on the theorized impact of meditation in promoting non-judgmental observatory attitude (Tang et al., 2015), we hypothesized that following meditation versus relaxation training, participants would give less positive valence ratings to positive pictorial stimuli and more positive ratings to negative stimuli, as well as reduced arousal ratings for both positive and negative stimuli. On the neural network level, our primary hypothesis was that meditation relative to relaxation training would lead to increased rs-FC between the PCC/precuneus and brain stem regions containing the PRN, with the direction of the increased connectivity primarily from the latter to the former. Also, given the existing body of evidence implicating a role of the serotonergic transmission in the DRN in regulating the neural and behavioral patterns during processing negative stimuli (Graeff et al., 1996; Selvaraj et al., 2015), we additionally expected the rs-FC between the PCC/precuneus and the DRN to be affected by the meditation versus relaxation training. Furthermore, it has been proposed that meditation practice reduces self-referential processing of exteroceptive and interoceptive stimuli (Chiesa et al., 2013), which functions are respectively attributed to the somatosensory cortices and the posterior insula. Given the PCC/precuneus also has anatomical connections with other parts of the parietal cortices including the somatosensory cortices, we further hypothesized that meditation relative to relaxation training would lead to reduced PCC/precuneus rs-FC with the somatosensory cortex and/or posterior insula. Finally, the associations among meditation training, changes in affective ratings and changes in PCC/precuneus rs-FC would be accounted for by one or both of the following two models: i) rs-FC change mediates affective rating changes following meditation versus relaxation training, and ii) meditation training moderates the association between rs-FC and affective rating changes. As natural aging leads to particular functional declines in the dorsal prefrontal cortices (Shao and Lee, 2014), which might suggest that affective processing in the elderly is predominantly bottom-up and driven by subcortical networks (Chiesa et al., 2013), we did not explicitly hypothesize about changes in PCC/precuneus-prefrontal rs-FC following meditation training.