The present study examined the relationship between resting CVC and depression in a large unmedicated sample of patients with MDD. The primary goal was to investigate whether resting CVC is a psychophysiological marker of global depression severity. Resting CVC was not associated with concurrent depression severity at intake or at subsequent points, nor was change in CVC prospectively associated with change in the severity of depression. Although null findings are generally difficult to interpret, the large sample size, and the consistency of the null findings across three separate time points increase confidence in the conclusion that in absence of comorbidity or concurrent pharmacotherapy, depression severity is not associated with low levels of resting CVC. These results do not address potential differences in CVC between patients with MDD and nondepressed controls, as the entire sample was depressed, but within the broad range of depression (HRSD range 14–36), it would appear there is no robust relationship between CVC and overall severity. In the context of mixed results on the association between CVC and depression severity (e.g., Chambers and Allen, 2002
; Rottenberg et al., 2002
), the current report supports an overall conclusion that studies with large unmedicated non-comorbid samples are unlikely to find that resting CVC is a strong autonomic correlate of global indices of depression severity.
A second goal in this study was to follow intriguing preliminary indications that resting CVC levels might be related to a few specific MDD symptoms. Although the analyses were explicitly exploratory in nature, and hence did not correct alpha for multiple comparisons, relationships were observed between resting CVC and symptom measures of sadness, crying, and poor sleep, results that replicated prior findings (Miller and Wood, 1997
; Rottenberg et al., 2003 Bonnet and Arnand, 1998
). The previous finding of an association with CVC and suicidality (Crowell et al., 2005
) did not replicate, however. Because individual symptoms were both negatively (disturbed sleep) and positively (sadness and crying) correlated with resting CVC, these data raise the possibility that inconsistency in the results of prior studies of depression may relate to the clustering of different depression symptom profiles in different patient samples. For example, based on these data, one might predict that MDD samples with significant insomnia would be more likely to exhibit an inverse relationship between CVC and depression severity than would MDD samples characterized by marked sadness and crying behavior.
Because persistent sad mood is a cardinal symptom and a defining emotional feature of MDD, the significant positive association between RSA and reports of acute sadness is particularly intriguing and warrants replication. A positive association between sadness and RSA is consonant with theory that highlights the importance of the vagal pathway in emotion expression and social bonding (Porges, 1995
). The association of greater sadness and crying with higher levels of resting CVC in MDD is also consistent with empirical findings both in within-subject experimental research demonstrating that CVC increases with self-reported sad mood state (Miller and Wood, 1997
) and with crying behavior more specifically (Rottenberg et al., 2003).
4.1. Future Research Directions
In light of these findings, two future directions are identified for extending understanding of the CVC-depression relation. First, these findings demonstrate the importance of making fine-grained reliable measurement of specific symptoms of depression. Indeed, results for poor sleep suggest that even within “a single symptom”, there may be variation in the relationship to CVC based on the type of poor sleep experienced. The most consistent association between CVC and poor sleep in this sample emerged in relation to late-insomnia. Whereas late insomnia, measured with the HRSD and the sleep diary, was inversely related to CVC, neither measure of early insomnia was related to CVC. Such convergence when using multiple measures of the same symptoms increases confidence in findings. In contrast, the relationship between CVC and the two measures of middle insomnia were inconsistent, rendering those results relatively equivocal. It is possible that the inconsistency stemmed from measurement artifact. A larger error in the diary as compared with an interview measure of middle insomnia might have arisen in this study because without sufficient training in completion of sleep diaries, people often include the time lying in bed awake in the morning before their planned rise time as part of their response to the diary item measuring time awake after sleep onset. This is less likely to occur in response to the clinical HRSD interview, during which interviewer typically make a clear distinction between the middle and the end of the night.
The second direction for future research is to test whether or not the results from this study are specific to the way CVC was measured in this study. Specifically, this study, like most previously published studies, used resting CVC as an autonomic correlate of depression. Although this study found no relationship between resting CVC and globally measured depression, the results might differ when the measurement of CVC is done in the context of environmentally challenging conditions (cf. Coan et al., 2006
). Because CVC is a dynamic system, and CVC dynamism is theoretically meaningful (Porges 1995
), it will be important to assess the extent to which short-term fluctuations in CVC (e.g., vagal withdrawal as elicited by laboratory stressors) are indicative of concurrent or future depression. Indeed, there are preliminary indications that fluctuations in CVC may predict the course of depression in patient samples (Rottenberg et al., 2005
). Therefore, the use of laboratory challenge paradigms to examine the predictive power of CVC fluctuations in depression is strongly warranted.