Accumulating research from a diverse range of areas suggests that many of our cognitive and affective processes take place outside of conscious awareness. Research from our group and others further suggests that this is also true of stress. Proposed in the Neurovisceral Integration Model and elaborated in the Generalized Unsafety Theory of Stress (GUTS) is the idea that the default response to uncertainty, novelty, and threat is the sympathoexcitatory preparation for action, that is, the so-called fight or flight response (Thayer & Lane, 2009; Thayer et al., 2012; Brosschot et al., 2018). This default response is thought to take place without conscious processing or appraisal and is thus believed to be largely unconscious. Importantly, this is due to largely unconscious perceptions of unsafety rather than stressors. However, the measurement of such unconscious processes is an ongoing challenge in all areas. With respect to stress, few studies have examined the influence of unconscious stress on stress-related physiology such as cortisol and blood pressure. Here, we summarize what we know and highlight what is in need of further research.
A number of tasks have been developed over the years to measure unconscious or, sometimes called, automatic cognition. These tasks include selective attention tasks, implicit memory tasks, lexical decision tasks, and categorization speed tasks. Perhaps the most well-known of these types of tasks is the implicit associations task (IAT). This task has gained much attention as a measure of implicit bias in many domains, including negative affect (Egloff et al., 2002; Schnabel, Banse, & Asendorpf, 2006; Brosschot et al., 2014; Verkuil, Brosschot, & Thayer, 2014). A more recently developed task is the implicit positive and negative affect task (IPANAT; see Quirin et al., 2009 and Quirin et al., 2014 for a full description including psychometric properties). With the exception of the IPANAT these tests have not typically been used with stress-related physiology. The IPANAT however has been related to cortisol, blood pressure, and heart rate reactivity and recovery. In one study that included both the IAT and the IPANAT only the IPANAT showed a relationship with stress-related physiology (Brosschot et al., 2014).
Another approach to investigate unconscious processes involves presenting information briefly and ostensibly outside of awareness. This approach involves presenting information very briefly (i.e., order of magnitude of tens of milliseconds) and examining behavioral responses such as reaction times as well as physiological responses such as startle responses, electrodermal responses, and phasic heart rate responses. Again, whereas this approach has been used across many areas, few studies have investigated the effects of subliminally presented stimuli on stress-related physiology. The literature on the effects of subliminally presented negative stimuli on peripheral physiology has been recently reviewed (van der Ploeg et al., 2016). These researchers concluded that whereas the literature was quite mixed, some consistent effects were reported especially for electrodermal responses and systolic blood pressure responses.
Most of the above approaches to investigating unconscious stress involve laboratory-based paradigms. However, it may also be possible to investigate the effects of unconscious stress in more ecologically valid ambulatory studies. One methodology that might be used in such studies involves so-called additional physiology studies. Blix, Stromme, and Ursin (1974) published a paper in which they examined the heart rate responses of helicopter pilots. The observed responses were compared to those that would have been predicted on the basis of their metabolic demand as assessed by their oxygen consumption. Heart rates in excess of the predicted values were termed “additional heart rate” and were associated with psychological activation. Over the years this methodology has been used by a number of researchers to examine associations with self-reported responses and in situations where self-reported responses were either unavailable or not recorded. Two recent studies applied this methodology to heart rate variability (HRV) and found that HRV decreases that exceeded those predicted on the basis of metabolic demand were associated with explicit measures of worry, negative affect, or stress (Verkuil et al., 2016; Brown et al., 2018). Interestingly, in these studies, more episodes of “additional HRV reduction” were detected than the number of episodes of explicitly reported worry, negative affect, or stress suggesting that those episodes that were not associated with explicit stress may be tapping unconscious stress.
This additional physiology methodology might turn out to be the most important method, since it is still an open question whether and to what extent unconscious stress (or unsafety) is cognitively represented in a way that is assessable by methods such as the IPANAT. As we point out elsewhere (Brosschot et al., 2018), the stress response is phylogenetically old, developed far before the representational features typical for the human brain. The additional physiology approach circumvents this problem by starting on the physiological side, measuring responses that are indicative of stress, without the need to directly assess the latter.
In summary, there currently exist several approaches that might be used to measure unconscious stress. However, to date, few studies using these methods have assessed traditional measures of stress-related physiology. Future studies are needed that further validate these methods with indices of stress-related physiology, as well as replicate and extend the current small number of studies that suggest the utility of these approaches.
This summary was prepared by Julian F. Thayer, PhD and Jos F. Brosschot, PhD, and reviewed by Cristina Ottaviani, PhD, Omer Van den Bergh, PhD, and other members of the Stress Measurement Network. If you have any comments on these measures, email Thayer.firstname.lastname@example.org. Version date: September 2018.
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