Interparental relationship dynamics and cardiac vagal functioning in infancy

Abstract

This study examined associations between interparental relationship dynamics and vagus system functioning in infancy. The functioning of the vagus system, part of the parasympathetic nervous system, indexes emotional reactivity and regulation. Interparental avoidance and dyadic adjustment constitute the focus of this study in order to bring attention to relationship dynamics not subsumed under overt conflict. Infants’ baseline vagal tone and change in vagal tone in response to a novel toy were assessed at 5 months in a sample of high-risk mother–infant dyads (n = 77). Maternal report of interparental avoidance demonstrated an association with infants’ baseline vagal tone, while interparental dyadic adjustment was associated with change in infants’ vagal tone from baseline to the novel toy. Infant gender moderated these associations. Maternal sensitivity did not mediate interparental relationship dynamics and infants’ vagal functioning. Results are discussed in the context of emotional security theory.

Introduction

The development of emotion regulation during infancy has received considerable scientific attention. A more recent focus on parents’ relationship dynamics and their possible influences on developing emotion regulatory capabilities has broadened the scope of investigation from a largely exclusive focus on dyadic infant–caregiver interactions. While links between interparental relationship dynamics and adjustment in older children have been established (Davies and Cummings, 1994, Davies et al., 2007), evidence of connections between interparental dynamics and infants’ adjustment are only beginning to emerge. Moreover, little of this work has been conducted with high-risk samples in which greater variability in family dynamics and conflict may hold different implications for infants’ developing regulatory psychophysiology.

The extant literature suggests that infants’ emerging emotion regulatory capabilities are related to levels of interparental aggression and positive affect (Crockenberg et al., 2007, Moore, 2010, Porter et al., 2003). Inconsistencies regarding the impact on physiological systems underlying emotion regulation, however, require further investigation. For example, whereas Porter et al. (2003) reported a negative correlation between marital conflict and infants’ baseline vagal tone (an index of parasympathetic nervous system functioning) Moore (2010) found instead that marital conflict only predicted infants’ vagal modulation (an index of parasympathetic nervous system response to environmental stimuli) in response to an interaction with a caregiver. The parasympathetic nervous system constitutes a particularly important target of investigation as highlighted by studies showing that baseline vagal tone and vagal modulation moderate the impact of interparental conflict on adjustment in older children (El-Sheikh et al., 2001, El-Sheikh and Whitson, 2006, Katz and Gottman, 1995, Katz and Gottman, 1997). The present study seeks to clarify and extend our understanding of how interparental relationship dynamics contribute to individual difference in infants’ vagal tone as well as to vagal modulation in response to novel environmental demands. Differences in vagal tone and modulation may precede exposure to conflict, thus operating as buffers or vulnerabilities in the context of future conflict. Alternatively, vagal tone and modulation may develop under the sway of interparental processes beginning in infancy.

The sympathetic and parasympathetic branches of the autonomic nervous system function in a complementary manner to maintain bodily homeostasis, with parasympathetic nervous system activity associated with growth and restorative processes and sympathetic nervous system activity associated with meeting external demands. Vagal tone, or vagus nerve activity, reflects the influence of the parasympathetic nervous system on a number of organs including the heart (Porges et al., 1982). The coordinated activity of efferent and afferent fibers of the vagus nerve, or tenth cranial nerve, causes increases in heart rate during inhalation, and decreases in heart rate during exhalation. Respiratory sinus arrhythmia (RSA), the rhythmic change in heart rate associated with respiration, is therefore used as an index of cardiac vagal tone (commonly referred to as vagal tone), which in turn provides an index of parasympathetic nervous system activity.

One critical indicator of vagal functioning is baseline vagal tone, which provides a measure of the ability to maintain bodily homeostasis during periods with little external challenge or stimulation. Higher baseline vagal tone during infancy is associated with increased capacity for engagement with the environment, as evidenced by increased positive, as well as negative, emotional responding to a range of stimuli (Beauchaine, 2001, Propper and Moore, 2006). During childhood and adulthood, higher baseline vagal tone is a negative correlate of both externalizing and internalizing symptoms (Beauchaine, 2001). Modulation of vagal tone, or change in vagal tone in response to environmental stimuli, provides an index of emotional functioning that is somewhat distinct from baseline vagal tone. Withdrawal of vagal tone in response to challenge demonstrates associations with increased emotion regulation capabilities from infancy (Calkins and Dedmon, 2000, DeGangi et al., 1991) through later childhood (El-Sheikh et al., 2001). However, application of, or increased, vagal tone in response to novelty predicts sustained attention to, and exploration of novel stimuli during infancy (Bazhenova et al., 2001, DiPietro et al., 1992). Therefore, whether a stimulus challenges or captures attention determines how vagal tone changes in response to environmental stimuli. Atypical patterns of vagal modulation in response to environmental challenges (cognitive, sensory or emotional) have been linked to problems in regulating behavior and emotions (Porges, 1995). For example, Porges, Doussard–Roosevelt, Portales, and Greenspan (1996) reported that infants who evidenced little withdrawal of RSA during administration of the Bayley Scales of Infant Development (Bayley, 1969), were reported to have higher levels of internalizing and externalizing symptoms at 3 years of age.

Rapid changes in indices of cardiac functioning between 4 and 9 months of age suggest the possibility of a sensitive period for physiological systems that innervate the heart (Bar-Haim, Marshall, & Fox, 2000). In line with research indicating that infants emerging emotion regulatory capabilities are contingent upon sensitive caregiving (Propper & Moore, 2006), parenting behaviors appear to influence infants’ baseline vagal tone and vagal modulation. Infants’ baseline vagal tone correlates positively with symmetrical co-regulation in infant–mother dyads (Porter, 2003), and patterns of vagal modulation in response to a social stressor differ based on levels of synchronized affect in infant–mother dyads (Moore & Calkins, 2004). Longitudinal work indicates that the normative developmental pattern of increasing vagal tone during the first year of life is altered in infants with depressed mothers (Field, Pickens, Fox, & Nawrocki, 1995). However, family dynamics beyond the dyadic infant–parent relationship have received less attention as a potential influence on infants’ developing vagus system.

As mentioned, possible associations between interparental dynamics and baseline vagal tone in infancy are mixed, with some finding a negative correlation between marital conflict and baseline vagal tone in 6-month-olds (Porter et al., 2003), and others finding no association (Moore, 2010). Findings regarding vagal modulation are limited; only one study, focused on a dyadic interaction between infant and mother, has examined connections between interparental dynamics and infants’ vagal modulation. Moore (2010) examined vagal modulation across the still-face paradigm (Tronick, Als, Adamson, Wise, & Brazelton, 1978), finding that infants from high conflict families evidenced withdrawal of vagal tone from baseline to each episode, regardless of whether the episode was neutral (play), stressful (still face) or recuperative (reunion) and, to some extent, irrespective of maternal behavior. In comparison, infants from low conflict families evidenced an increase in vagal tone from baseline to the play episode, followed by vagal withdrawal from play to still face, and a subsequent increase in vagal tone from the still face to reunion. These results suggest that for some infants, high familial conflict may have a direct effect on vagal functioning, perhaps independent of infant–caregiver interactions.

Crockenberg et al. (2007) examined associations between interparental aggression and infants’ emotion regulation by observing infants’ behaviors in response to a novel toy. They found that higher levels of withdrawal or escape behaviors were associated with higher levels of interparental aggression. In general, infants’ response to novelty appears to have implications for future emotion regulation capacity; higher levels of withdrawal behavior at 6 months of age predict symptoms of anxiety at 2.5 years of age (Crockenberg & Leerkes, 2006). Novel situations during infancy are both common and critical to cognitive and socioemotional development (DiPietro et al., 1992), with individual differences in temperamental fearfulness providing a partial explanation for why some infants might approach and others withdraw from new experiences (Crockenberg et al., 2007). As Crockenberg and colleagues’ research suggests, interparental conflict may create unpredictable or overwhelming contexts within which infants develop regulatory behaviors designed to avoid novel albeit potentially useful situations; at present, possible links to infant's developing regulatory physiology have not been explored.

Emotional security theory (EST) posits that children's adjustment depends upon feeling safe and supported not only in the context of the parent–child relationship, but also in the context of the interparental relationship (Cummings & Davies, 2002). According to EST, relationship dynamics that create an environment of instability predict negative outcomes for children. Observational studies of couple interactions suggest that disengagement from conflict predicts marital dissolution (Gottman & Levenson, 1992). Self-report of avoidance of conflict also significantly predicts the likelihood of marital dissolution (McGonagle, Kessler, & Gotlib, 1993). Although the literature has focused largely on overt interparental conflict, there appear to be distinct pathways from covert (defined by withdrawal behaviors and lack of responding to one another) versus overt conflict to children's internalizing and externalizing problems (Ablow, 2005). Avoidance of conflict or disengagement during conflict may actually be a more potent predictor of child maladjustment than overt conflict (Sturge-Apple, Davies, & Cummings, 2006). Sturge-Apple and colleagues (2006) interpreted their findings with regard to avoidance as indicative of parents’ inability to manage negative affect, and of relationship instability.

Infants’capacity to distinguish between various emotional states portrayed by adult actors (Caron, Caron, & MacLean, 1988), combined with a tendency to allocate higher levels of attention to angry emotional tone of voice (Grossmann, Striano, & Friederici, 2005), suggests that negative affect in the interparental relationship would likely be detected by infants. Avoidance of conflict likely engenders chronic tension, and negative affect as conflicts remain unresolved (Ablow, 2005). Increased startle response reported for 5-month-old infants viewing angry versus happy faces (Balaban, 1995) indicates that infants’ level of arousal may be increased in the presence of negative affect, even in the absence of overt vocal expressions of anger. Looking to animal research it appears that chronic stimulation of brain regions involved in processing negative affect leads to dysregulation of biobehavioral emotional systems (Kalynchuk & Meaney, 2003).

Dyadic adjustment constitutes another important predictor of relationship stability, which has received little attention in the literature regarding child outcome. Spanier (1976) conceptualized dyadic adjustment as a measure of the overall health of the interparental relationship including partners’ satisfaction, agreement on fundamental issues, cohesion in terms of engaging in shared interests, and expressions of affect. Levels of dyadic adjustment reliably distinguish between separated versus non-separated couples and predict relationship dissolution (Möller and Van Zyl, 1991, Prouty et al., 2000). Dyadic adjustment also maps onto the domain of constructive relationship behaviors, such as problem solving and expressions of affection, which have been shown to have a positive impact on children's behavioral regulation (Goeke-Morey, Cummings, Harold, & Shelton, 2003). Aspects of dyadic adjustment also appear relevant for infants’ adjustment as maternal report of positive emotion and behaviors aimed at maintaining a marriage's health have been associated with higher emotion regulatory capacity during infancy (Porter et al., 2003). Infants observing a dynamic range of well regulated emotions (from negative to positive) in the context of the interparental relationship may be more likely to effectively express and regulate their own emotions.

This study examined the associations between interparental relationship dynamics and infants’ vagal functioning as indexed by RSA. Baseline RSA was measured to assessed infants’ capacity to engage with the environment, whereas RSA modulation (change form baseline) was evaluated as an index of emotion response and regulation. We examined RSA modulation in response to a novel toy, which we expected would elicit either fear or attention/curiosity from most infants. This study extends previous work by focusing on parents’ covert conflict dynamics and dyadic adjustment, and by examining a physiological index of infants’ emotion regulation with known links to critical behavioral outcomes.

We predicted that dynamics indicative of maladjustment in the interparental relationship (higher interparental avoidance and lower dyadic adjustment) would correlate with lower baseline vagal tone in infants, indicative of lower levels of overall engagement with the environment. Regarding RSA modulation in response to the novel toy, we predicted that interparental relationship dynamics indicative of maladjustment would be associated with a withdrawal of RSA, commensurate with a fear response. However, consistent with decreased emotion regulatory capacity, we also expected that greater interparental difficulties would be associated with a smaller magnitude of RSA response. In contrast, we expected infants from homes characterized by interparental adjustment to show clear RSA increases in response to the novel toy, indicative of engagement and directed attention.

Emotional security theory posits a set of necessary controls, as well as potential moderators or mediators of the impact of negative interparental dynamics on children's development that merit consideration (Cummings & Davies, 2002). Rothbart and Bates (1998) have argued that infants’ emotional and behavioral responses to novel situations have strong constitutional underpinnings in the temperamental traits of fear or withdrawal versus approach, including attention and positive engagement. Accordingly, measures of infants’ temperamental fear and approach were controlled in the present investigation.

Second, differences in how boys and girls express emotion regulatory difficulties in the context of interparental conflict (Ablow, 2005, Davies and Lindsay, 2004, El-Sheikh and Whitson, 2006), persuaded us to examine potential gender differences in RSA. For example, girls are more likely than boys to express internalizing problems in the context of interparental conflict, though it is not yet clear when such gender differences emerge, nor whether these differences are detectible early in life in infants’ psychobiology. Evidence for the effects of interparental dynamics on psychobiology are limited, though there is some evidence that boys exposed to interparental conflict may show autonomic responses more akin to vigilance than fear when exposed to conflict (El-Sheikh, 1994).

Third, the structural features of children's environments, such as parents’ current living arrangements, may also determine how distal or proximal interparental conflict is to their infants. Due to the range of living situations in this high-risk sample, mothers’ marital and living situation with their partners also was tested as a possible moderator. It is possible that being married, or at least cohabiting, increases stability of a relationship (Bouchard, 2006). Alternatively, being married or at least cohabiting may keep an infant exposed to developmental stress in the context of poor relationship adjustment.

Finally, maternal sensitivity was included as a potential mediator of the effects of interparental dynamics on infants’ vagal functioning as more proximal parenting constructs have previously been shown to change as a function of marital dynamics (Ablow, 2005), and have demonstrated associations with infants’ vagal functioning (Conradt & Ablow, 2010).