To date, only one known study has investigated how neural activity during a stress task is related to physiological responses in a purely sympathetic measure. This study examined how neural activity during a demanding Stroop task related to pupil diameter, a pure measure of SNS responding (Critchley et al., 2005
). Results indicated that neural activity in the dACC, a core region of the threat-processing system, during incorrect (relative to correct) trials of a Stroop task was related to greater pupil dilation, suggesting that the dACC may be important for stress-induced SNS activity.
The majority of research in this area has focused on the neural systems related to changes in physiological measures that are correlates of SNS-related activity—reflecting some combination of sympathetic increases and parasympathetic decreases (e.g., blood pressure, heart rate, baroreflex sensitivity). In an early study by Critchley and colleagues (2000)
participants completed challenging mental arithmetic and a difficult hand-grip task, and neural activity associated with both stressful tasks was related to changes in mean arterial pressure (MAP; a weighted averaging of changes in systolic and diastolic blood pressure [SDP/DBP]) and heart rate. Neural activity in threat-related neural circuitry was related to higher blood pressure (dACC) and higher heart rate (AI) during stress, whereas activity in a safety-related neural region (VMPFC) was related to lower blood pressure.
Another study on the neural correlates of heart rate responses to stress examined how neural activity during a demanding working memory task was related to the low-frequency band of heart rate variability (LF-HRV; Critchley et al., 2003
), which is thought to represent a combination of SNS activation and PNS withdrawal. Once again, neural activity in threat-related neural regions (e.g., dACC and AI) was positively related to increases in LF-HRV.
More recently, a number of studies by Gianaros and colleagues (2005a
) have examined how neural activity during stress relates to blood pressure reactivity (for a quantitative review, see Gianaros & Sheu, 2009
). These studies all employed a modified Stroop color-word interference task as a form of psychological stress. Importantly, performance on incongruent trials of the Stroop was fixed so that correct responses were given on only 60% of trials, perhaps contributing to the already stressful nature of having to inhibit a pre-potent response. In addition to measuring neural activation to the stressful task, measures of blood pressure (SBP or MAP) were taken. In all four studies, neural activity in a region of the self/social cognition network (PCC) was related to greater stressor-evoked increases in blood pressure. Neural activity in other regions of the self and social cognitive processing system (pACC, MPFC) and in threat-related neural regions (dACC, AI, amygdala) was associated with greater increases in blood pressure in some, but not all, studies (Gianaros et al., 2005a
). Thus, activation in neural regions involved with thinking about the self and others, and in regions involved with processing threat, was related to increases in blood pressure.
In another set of recent studies, (Wager et al., 2009a
), participants performed a speech preparation task in which they were told they would have to mentally prepare to give a speech that they would subsequently deliver to a panel of experts. Subjects were scanned using fMRI while they prepared for the speech as well as during a baseline period before and after the speech prep. Measurements of heart rate were taken throughout the scan. The authors examined which neural regions mediated the relation between speech preparation stress and changes in heart rate. In one study, neural activity in a threat-related neural region (dACC) mediated the relationship between social threat and heart rate increases (Wager et al., 2009a
), while activity in a region implicated in thinking about the self and others (pACC) was a mediator of the stress-heart rate increase in another study (Wager et al., 2009b
). Finally, in both studies, the extent of activity in VMPFC, a safety-related neural region, was negatively related to heart rate, suggesting that participants who showed greater VMPFC activity during stress actually showed less of an increase in heart rate.
To date, only one study has examined the neural correlates of baroreflex suppression1
, another index of increased cardiovascular arousal (Gianaros et al., 2011
). In this study, participants completed a task similar to the Stroop task, and accuracy was fixed so that 40% of the time, participants received negative feedback indicating they had answered incorrectly. The task was completed twice: once during an fMRI scan, and once in a separate session during which beat-to-beat blood pressure and interbeat intervals were measured to form an index of baroreflex sensitivity. Greater suppression of the baroreflex (indicative of greater sympathetic arousal) was associated with heightened neural activation in a number of brain regions, including threat-processing related regions (dACC, AI, amygdala), and regions involved in thinking about the self and others (MPFC, pACC, PCC). Thus, activity in threat-processing and self/other-processing neural systems during stress was related to greater suppression of the baroreflex and thus greater sympathetic activity.
In sum, increases in neural activity in threat-related brain regions (dACC, AI, amygdala) in response to stress are associated with subsequent increases in SNS-related activity. This pattern of activation makes sense given that these regions are part of a threat-processing neural system, and increased perception of threat is associated with increased physiological responses. On the flip side, relatively greater activity in a safety-related neural region (e.g., VMPFC) is associated with relatively less of an increase in SNS activity, suggesting that greater perceptions of safety during stress may be associated with lower SNS responses. A number of studies also find neural activity in MPFC/pACC, and PCC associated with increases in SNS reactivity. These brain regions are part of a neural system involved in thinking about the self and others, suggesting that to the extent that individuals are thinking about themselves or others during a stressor (e.g., worrying about how they are perceived, or what others are thinking), they may show greater increases in SNS responses. Given that many of the SNS measures used in neuroimaging studies are not “pure” measures of sympathetic activity, future research should incorporate more direct measures of SNS activation to specify the relation between neural activity and purely sympathetic measures.