A growing body of literature has shown that positive affect (PA) is related to health outcomes. High levels of PA are found to be associated with better concurrent and future health prospects, and the influence of PA on health is independent of negative affect (NA) and other disease risk factors (Chida and Steptoe, 2008, Dockray and Steptoe, 2010, Pressman and Cohen, 2005 and Steptoe et al., 2009). Accumulating studies have indicated some specific biological processes that may mediate the association between PA and health, including neuroendocrine, autonomic and immune systems (Chida and Steptoe, 2008, Dockray and Steptoe, 2010, Pressman and Cohen, 2005 and Steptoe et al., 2009). Of these studies, Pressman and Cohen (2005) proposed that trait PA might maintain homeostasis to benefit health through autonomic nervous system regulation, such as dampening sympathetic nervous system activity and activating parasympathetic nervous system activity.
Cardiac vagal tone, an index of the functional status of the parasympathetic nervous system, reflects the vagal influences on heart rate variability at the rate of respiration (i.e., respiratory sinus arrhythmia [RSA]) (Berntson et al., 1997 and Grossman and Taylor, 2007). At a basic level, high vagal tone (basal RSA) reflects the organism's ability to maintain homeostasis and its capacity to adaptively react to the environment (Porges, 2003 and Porges, 2007). It is therefore indicative of physiological flexibility and is found to be related to decreased rates of physical illness (Porges, 1992, Thayer and Lane, 2007 and Thayer and Sternberg, 2006). Emerging evidence suggests that PA is associated with basal vagal tone. For example, Oveis et al. (2009) examined the association between basal vagal tone and positive emotionality (extraversion, agreeableness, positive mood and optimism) and found that basal vagal tone was stably associated with trait PA (positive mood). Wang et al. (2013) also found that basal vagal tone was stably associated with trait PA. These findings implicate that there is a close relationship between trait PA and basal vagal tone, and the higher trait PA individuals have, the higher their basal vagal tone. Thus, if the trait PA levels of individuals with low trait PA could be enhanced, it is possible that their basal vagal tone levels would be increased as well. Recently, a study examined the association between changes of state PA and changes of basal vagal tone and showed that basal vagal tone prospectively predicted increases in positive emotions over the span of nine weeks and, reciprocally, that these increases in positive emotions prospectively predicted increases in vagal tone (Kok and Fredrickson, 2010). However, whether trait PA levels can be changed and whether increases of trait PA are related to increases of basal vagal tone still remains an open question.
Intervention studies that focused on alleviation of negative emotionality such as depression and panic disorder by cognitive behavioral treatment (CBT) have found that decreases of negative emotionality corresponded to simultaneously increases in cardiac vagal tone (Carney et al., 2000, Chambers and Allen, 2002 and Garakani et al., 2009). These findings suggest that trait-like emotionality and basal vagal tone might be changed by psychotherapy. Inspired by these studies, the present study will adopt Positive Psychotherapy (PPT) to examine whether trait PA can be changed, and whether changes of trait PA are associated with changes of basal vagal tone. PPT, drawn from positive psychology, is a successive treatment technique in raising PA (Seligman et al., 2006 and Seligman et al., 2005). Compared to other treatments, such as CBT, PPT not only focuses on effectively reducing negative symptoms, but also directly and primarily builds positive emotions, engagement, and meaning (Froh et al., 2009 and Seligman et al., 2006). Thus, the present study aims to conduct PPT on individuals with low trait PA to examine whether their trait PA would increase, and whether increases in trait PA would be associated with improvement of basal vagal tone.
Overall, the present study hypotheses are as follows: (1) high trait positive affect (HPA) group would have higher basal RSA than low trait positive affect (LPA) group. (2) Compared to the control group, the trait PA levels of LPA participants who receive PPT would be significantly enhanced, and increases in trait PA would be associated with increases in basal RSA.
