1 INTRODUCTION

It has been proposed that stress in association with negative affect can intensify the desire and urge to smoke, thus further contributing to the maintenance and chronification of nicotine addiction (Kassel et al., 2003). Higher levels of perceived stress are associated with heavy smoking in daily smokers (Stubbs et al., 2017). Stress regulation and smoking seem to rely on the recruitment of similar brain structures (Richards et al., 2011), further suggesting a strong relationship between stress exposure and smoking behavior. Studies using exposure to laboratory stressors with subsequent analysis of different aspects related to smoking behavior offer a fine-graded approach to study the relationship between stress and smoking behavior. To this end, psychological stress, which leads to significant hypothalamus–pituitary–adrenal (HPA) axis activation, has been shown to increase cigarette craving (Buchmann et al., 2010; Childs & Wit, 2010; Erblich et al., 2003; McKee et al., 2011; Niaura et al., 2002) and reduce the ability to resist smoking (McKee et al., 2011).

To fully appreciate the complex processes contributing to smoking behavior, dual-process models conceptualize nicotine addiction, and addictive behavior in general, as an imbalance between a reflective system and an automatic, impulsive system (Deutsch & Strack, 2006; Wiers, Gladwin, et al., 2013). Dual-process models of addiction may explain why smokers exhibit a tendency to show maladaptive behavior even when actively aware of the hazardous outcomes of smoking (McClure & Bickel, 2014). Interestingly, stress can potentially lead to a switch from deliberative to more automatic modes which may in turn exacerbate cigarette craving and nicotine intake (McClure & Bickel, 2014). Automatic modes can manifest as automatic approach bias toward cigarettes and smoking-related cues. Such automatic approach biases toward cigarettes and smoking-related cues can be assessed in the laboratory by means of the approach–avoidance task (AAT; Machulska et al., 2015, 2016; Wiers, Gladwin, et al., 2013; Wiers, Kühn, et al., 2013; Zlomuzica et al., 2016). Moreover, different training versions of the AAT have been developed as an attempt to modify approach bias in the context of nicotine addiction and reduce smoking behavior; however, the beneficial effects of these training AATs are still limited (Kong et al., 2015; Machulska et al., 2016; Wen et al., 2020).

In an explorative pilot study, we aimed to examine the effects of psychological stress on smokers’ approach biases toward smoking stimuli in the AAT. While previous studies investigated the effect of psychological stress on subjective craving (Buchmann et al., 2010; Childs & Wit, 2010; Erblich et al., 2003; McKee et al., 2011; Niaura et al., 2002), no study thus far has assessed the impact of exposure to acute psychological stress on implicit, smoking-related approach biases. We used a within-subject design to examine the effect of psychological stress on approach bias in the smoking-related AAT. Smokers were subjected either to a stress or control condition of the socially evaluated cold pressor task (SECPT; Schwabe et al., 2008). Immediately thereafter, the AAT was presented.

Attempts to provide evidence for the existence of approach biases in different substance use disorders have produced rather mixed results. For instance, some studies demonstrated approach biases in heroin (Zhou et al., 2012), cannabis (Cousijn et al., 2011), and alcohol addiction (Ernst et al., 2014). Conversely, studies which did not find approach biases in alcohol addiction (Barkby et al., 2012) or stronger approach biases in alcohol-dependent patients compared to healthy controls (Wiers et al., 2017) also exist. Similarly, the demonstration of approach biases for smoking-related cues in smokers by means of the AAT has not been shown consistently in previous studies (see Machulska et al., 2015, 2016; Zlomuzica et al., 2017; but see Woud et al., 2016; Larsen et al., 2014). Therefore, another aim of this study was to replicate previous demonstrations of significant approach biases for smoking-related cues via the AAT.

To summarize, the present study attempts to replicate prior findings of approach biases in smoking by means of the AAT, as well as additionally examine the effects of psychological stress on smokers’ approach biases toward smoking stimuli. In an exploratory approach, we were further interested to determine whether the relative smoking approach tendency was moderated by the participants' level of nicotine dependence, their stress hormone levels, and their exhaled carbon monoxide (CO) levels.

2 MATERIALS AND METHODS

2.6 General procedure

Each participant was subjected to two sessions which were separated by a 24 hr interval. The entire experimental procedure during the first and second session is illustrated in Figure 1. Each session started by measuring the level of exhaled CO using the Smokerlyzer. Subsequently, participants were asked to fill out subjective measures and the first saliva sample was collected. After another 5 min, participants were subjected to the stress condition or the control condition of the SECPT. After another 25 min from SECPT onset, the second saliva sample was collected. Immediately thereafter, the AAT was presented. One of the two existing versions of the AAT was presented during either the stress or control condition. Presentation of the AAT version was counterbalanced across the different conditions of the SECPT. At the end of the second session, all participants were fully debriefed and reimbursed.

3 RESULTS

3.1 Approach bias in the AAT

3.1.1 Data preparation

For each of the eight combinations of stress condition (stress, no stress), picture type (smoking, control), and movement (pull, push), each participant's median full-movement reaction time (RT) was computed after excluding extreme outliers (the fastest 1% and slowest 1% RTs overall), yielding individual median RTs per condition between 578 and 1,312 ms. For the overall mean RTs and SDs per experimental condition, see Table 3. Using the medians, response tendencies for smoking pictures and tooth cleaning control pictures were computed for each participant and stress condition by deducting the RT for pulling from the corresponding RT for pushing. Positive values of these difference scores indicate faster approach than avoidance, while negative values indicate faster avoidance than approach. Finally, so-called relative smoking approach tendencies were computed by deducting the response tendency for control pictures from the corresponding tendency for smoking pictures. Here, positive values indicate stronger approach of smoking pictures than of control pictures. The critical test here involved whether the relative smoking approach tendency would be larger in the condition with stress or without.

TABLE 3. Mean reaction time (and SDs) in the approach–avoidance task depending on socially evaluated cold pressor task (SECPT) condition, picture type, and movement direction

SECPT condition	Picture type and movement
	Smoking		Tooth cleaning control
	Pull	Push	Pull	Push
Control	772 (122)	800 (149)	806 (135)	773 (146)
Stress	773 (111)	790 (123)	791 (113)	771 (125)

3.1.2 Confirmatory analysis

Effects of stress. A 2 × 2 × 2 repeated-measures ANOVA with the within-subjects factors "Stress" (stress, no stress), "Picture Type" (smoking, control), and "Movement Direction" (pull, push) was computed, using the median RTs as the dependent variable. The only significant effect found in this ANOVA was a large effect of the picture type by movement direction interaction, F(1, 18) = 17.63, p = 0.001, c = 0.495. Inspection of the corresponding means revealed the expected relative smoking approach tendency: Participants were relatively faster to pull smoking pictures closer, F(1, 18) = 3.84, p = 0.066,  = 0.176, and relatively faster to push control pictures away, F(1, 18) = 6.43, p = 0.021,  = 0.26. No other main effect or interaction was significant (all p < 0.17). Most importantly, the size of this tendency did not depend on the stress condition, F(1, 18) = 1.97, p = 0.177,  = 0.099. The tendency was large and significant both with stress, F(1, 18) = 24.46, p < 0.001,  = 0.595, and without stress, F(1, 18) = 4.86, p = 0.041,  = 0.213.

3.1.3 Exploratory analyses: Potential moderators

A series of additional 2 × 2 × 2 ANCOVAs w​as computed to determine whether the relative smoking approach tendency was moderated by the participants' level of nicotine dependence, their cortisol levels before and after each AAT, their alpha amylase levels before and after each AAT, and the CO level in their breath before each AAT. In each ANCOVA, one of these variables was used as a covariate in addition to the within-subjects factors described above. Of main interest was the potential interaction of the covariate with the three-way interaction of picture type, motion direction, and stress condition: Such a four-way interaction would indicate that the covariate moderates the influence of stress on the relative smoking approach tendency. Table 4 shows the results of these ANCOVAs with regard to the four-way interaction. To summarize, only the CO level measured before the AAT with stress and the cortisol level measured before the AAT without stress yielded a significant four-way interaction. For the CO level, the nature of the interaction indicated that the higher the CO level was before the AAT with stress, the less the relative smoking approach tendency was affected by stress, r = −0.506, p = 0.027. For individuals with higher CO levels, the tendency was even slightly stronger with stress (65 ms) than without (48 ms), whereas stress greatly reduced the smoking approach tendency for individuals with lower CO levels (from 72 to 11 ms on average). This may suggest that smoking before the stress task—and thereby increasing the CO level—buffered against the approach reduction observed after stress exposure. For the cortisol level before the AAT without stress, the interaction followed a similar pattern, indicating that the higher the cortisol level was before the AAT without stress, the less the relative smoking approach tendency was affected by stress, r = −0.462, p = 0.046. For individuals with higher cortisol levels, stress did not reduce the approach tendency (changes from 66 to 59 ms, on average), whereas stress reduced the tendency for individuals with lower cortisol levels (from 55 to 12 ms, on average).

TABLE 4. Statistics of the four-way interaction involving various covariates

Covariate	Statistics of covariate × stress × picture type × movement interaction
Nicotine dependence (FTND score)	F (1, 17) = 0.07, p = 0.795,  = 0.004
CO level before AAT, control	F (1, 17) = 1.40, p = 0.254,  = 0.076
CO level before AAT, stress	F (1, 17) = 5.84, p = 0.027,  = 0.256
Cortisol level before AAT, control	F (1, 17) = 4.62, p = 0.046,  = 0.214
Cortisol level after AAT, control	F (1, 17) = 3.12, p = 0.095,  = 0.155
Cortisol level before AAT, stress	F (1, 17) = 2.98, p = 0.102,  = 0.149
Cortisol level after AAT, stress	F (1, 17) = 0.20, p = 0.657,  = 0.012
Alpha amylase level before AAT, control	F (1, 17) = 0.38, p = 0.544,  = 0.022
Alpha amylase level after AAT, control	F (1, 17) = 0.13, p = 0.726,  = 0.007
Alpha amylase level before AAT, stress	F (1, 17) = 0.49, p = 0.495,  = 0.028
Alpha amylase level after AAT, stress	F (1, 17) = 0.26, p = 0.618,  = 0.015

• Abbreviations: AAT, approach–avoidance task; CO, carbon monoxide; FTND, Fagerström Test for Nicotine Dependence.
• Significant (p<.05) interactions are displayed in bold.

4 DISCUSSION

In the present explorative pilot study, the effects of psychological stress administered via SECPT on approach bias for smoking-related cues in the AAT were examined. Our results indicate a significant approach bias for smoking-related stimuli in smokers. Prior exposure to stress relative to a control condition did not affect this general bias for smoking-related cues. However, exploratory analyses revealed a moderating effect of CO levels in expired air and cortisol levels which, in combination with stress, modulated the direction of approach bias for smoking-related cues. Higher CO levels followed by stress were associated with a significant approach bias for smoking-related cues. The finding of a significant approach bias for smoking-related cues in the AAT for smokers is noteworthy. Previous studies in this field either failed to demonstrate a significant approach bias toward smoking or nicotine-related cues in the AAT (Larsen et al., 2014; Woud et al., 2016), or provided evidence for a moderate approach bias (Machulska et al., 2015). Data from other studies suggest that the nicotine-related approach bias in the AAT might depend on specific characteristics related to the sample, such as smoking status and/or smoking severity (Wiers, Kühn, et al., 2013) or genetic variations in reward sensitivity (Zlomuzica et al., 2017). A closer inspection of the studies indicated that the level of nicotine addiction, assessed with the FTND, was comparable across existing studies and the present investigation. However, substantial differences in the pictorial stimuli used in the AAT exist across the studies. The AAT used in our study was adapted from Machulska et al. (2016) who succeeded to find a stronger approach bias for smoking-related pictures relative to tooth cleaning pictures. Both studies utilized picture categories (smoking-related and control stimuli), which were derived from Stippekohl et al. (2010) and did not differ in terms of dimension, shape, or color. The selection of both smoking-related and tooth cleaning pictures was based on the rationale to mimic similar arm movements in both picture categories (i.e., moving objects from real life toward the mouth). The inclusion of such ecologically relevant features might account for the difference in detection of nicotine-related approach bias across the studies. However, other factors (e.g., nicotine deprivation vs. a non-deprived state prior to the AAT assessment) might affect the approach bias for nicotine-related cues in the AAT (see Watson et al., 2013). The same is true for our findings showing that the CO and stress levels modulate the magnitude of approach bias for smoking-related cues. Nevertheless, by replicating findings from Machulska et al. (2016), our version of the AAT for smokers might represent a suitable paradigm for future smoking cessation interventions incorporating the AAT (Machulska et al., 2020; Smits et al., 2019).

Dual-process models of addiction implicate that stress might promote a switch from deliberative to automatic modes in smokers, leading to an increased urge to smoke (McClure & Bickel, 2014; Wiers, Gladwin, et al., 2013). In fact, there is some indirect evidence supporting this proposition showing that the exposure to laboratory stressors increases cigarette craving (Buchmann et al., 2010; Childs & Wit, 2010; Erblich et al., 2003; McKee et al., 2011; Niaura et al., 2002). The results from the present study are not in line with these findings. Stress alone did not affect the direction or magnitude of the approach bias in smokers. However, previous studies mainly used self-report measures of cigarette craving while we used a more implicit measure of nicotine addiction as the outcome variable. Thus, it is possible that the approach bias and craving represent somewhat different aspects related to nicotine addiction which are differentially affected by stress. Comparing the effects of stress on both subjective measures of craving and implicit measures related to nicotine addiction would be valuable. Interestingly, Field and Powell (2007) investigated the effects of stress on alcohol craving and attentional bias for alcohol-related cues in a group of heavy social drinkers. They showed that stress leads to an increase in alcohol craving. The effects of stress on attentional bias for alcohol-related cues, however, were moderated by a third variable (i.e., the level of coping motives). Similar to these findings, our exploratory analyses revealed that CO levels prior to the exposure to stress need to be considered to understand how stress modulates approach bias in smokers. In particular, smokers showing higher CO levels prior to the exposure to the stress condition of the SECPT show a significant approach bias for smoking-related cues. Conversely, a reduced approach bias for smoking-related cues was found in smokers showing low CO levels prior to the exposure to the stress condition of the SECPT.

How can we explain this pattern of findings? Acute cigarette smoking is associated with elevated cortisol levels (Steptoe & Ussher, 2006). Smokers show attenuated cortisol levels in response to psychological stress (Rohleder & Kirschbaum, 2006), probably due to a desensitization of the HPA axis following psychological stress. Thus, smoking compared to non-smoking before the addition of stress, via SECPT, may have induced different conditions prior to the commencement of the AAT. These conditions vary with respect to the timing of HPA axis activation (i.e., whether the HPA axis was already activated during smoking or later on during the SECPT challenge). Additionally, the relative cortisol levels over the course of experimentation might be different in these two conditions. Therefore, depending on the timing of HPA axis activation and relative cortisol levels, the approach bias was either unaffected or reduced. Interestingly, Watson et al. (2013) showed that only smokers who smoke prior to the AAT show significant approach bias for smoking-related cues. Further, Roelofs et al. (2005) provided evidence that cortisol responses are associated with either a decrease or no change in active approach–avoidance behavior. They investigated the impact of cortisol responses during the Trier Social Stress Test (TSST) on approach–avoidance behavior in a computerized AAT with positive and threatening social stimuli. Both the results from the Roelofs et al. (2005) study and the Watson et al. (2013) study are important to understand our findings, however, they cannot be fully extrapolated to our study design. For instance, both studies indicate that acute smoking and increases in cortisol levels can influence approach behavior, albeit this might happen in an interactive manner in our study. Acute smoking and the exposure to stress can both modulate attention and working memory capacity, along with other functions mediated by the prefrontal cortex (Lupien et al., 1999; Heishman et al., 2010). Stress exerts a detrimental effect on attentional regulation which is also mediated by the prefrontal cortex (Arnsten, 2009). Thus, in the present study, one might speculate that the exposure to stress decreases approach behavior (Roelofs et al., 2005) via disruption of attentional regulation (Arnsten, 2009). However, smoking prior to the stress exposure compensates for this effect (Heishman et al., 2010) and thus still leads to significant approach bias for smoking-related cues. This interpretation is highly speculative and needs to be addressed in future systematic studies. Therefore, additional studies are required to understand the interaction between acute smoking, psychological stress exposure, and HPA axis activation. Ideally, such studies could be combined with AAT tasks to examine to which extent acute smoking (in addition to stress) affects reward sensitivity in smokers (Ironside et al., 2018).

Limitations of this study include the relatively small sample size which may have been insufficient to detect moderator effects, or a more general effect of psychological stress on approach tendencies for smoking-related cues. The study was not preregistered and represents an exploratory analysis on the possible association among stress, CO levels, and AAT bias in smoking. Given the negative correlation between nicotine levels and craving for cigarettes (Jarvik et al., 2000), future studies might also include craving as an important moderator variable to analyze how stress affects AAT performance. Exploring for many different potential moderators, allowing for alpha inflation, should be controlled for in confirmatory analyses of future data. In the current study, none of the four-way interactions would have withstood Bonferroni correction (the corrected significance level would be p = 0.0045). It is noteworthy, however, that the approach bias for smoking-related cues in the AAT was strong, even in this small sample. The exposure to the stress relative to the control condition of the SECPT did not lead to a significantly higher increase in cortisol levels. As outlined already, such an aberrant cortisol response might be due to a desensitization of the HPA axis in response to psychological stress in smokers (Rohleder & Kirschbaum, 2006). This was demonstrated in studies using the TSST (Rohleder & Kirschbaum, 2006) which is considered a rather “strong” psychological stressor (Skoluda et al., 2015). To our best knowledge, no study assessed the HPA reactivity after exposure to SECPT in smokers. According to the general guidelines, robust increases in saliva cortisol concentrations occur 25 min after SECPT onset in healthy subjects (Schwabe & Schächinger, 2018). While we followed the general recommendations and collected saliva samples 25 min after onset, we did not include repeated measurements of saliva cortisol concentrations. The latter might have decreased the possibility to detect the expected peak in cortisol levels. However, our manipulation check clearly indicates that stress (relative to the control condition) led to a significant increase in systolic and diastolic blood pressure. Similarly, stress was rated as more “difficult, unpleasant, stressful, and painful” relative to the control condition by our participants. It is, therefore, unlikely that the stress and control condition of the SECPT exerted equal effects on the stress system level in smokers. Nevertheless, as the existing literature emphasizes that social and/or highly aversive stressors are especially relevant for smoking (Kassel et al., 2003), future investigations might consider using stressors which are more demanding in terms of emotional and/or social aspects than the SECPT (but see Skoluda et al., 2015).

To conclude, by replicating previous findings from our group (Machulska et al., 2016), we provided evidence for an approach bias for smoking-related cues in the AAT in smokers. Stress administered by the SECPT prior to the AAT in smokers did not affect the general bias. However, an interaction of CO and cortisol levels, and stress on approach bias for smoking-related cues was found. Higher CO levels, possibly due to recent smoking, prior to stress exposure were associated with an approach bias for smoking-related cues. Our findings provide novel cues to how stress influences implicit, automatic processes in nicotine addiction. The number of clinical trials which utilize approach bias retraining procedures to promote smoking cessation has been steadily growing (e.g., Machulska et al., 2020; Smits et al., 2019). Although our study was purely explorative, one might consider studying the boundary conditions (e.g., the influence of acute smoking and stress; Carey et al., 1993) of these novel interventions on smoking cessation.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

AUTHOR CONTRIBUTIONS

AZ designed the study. ML and SR performed the study. AM provided the AA task. AZ and MR wrote the manuscript. All authors contributed to the final version of the manuscript.