Maintaining upright stance, or postural control, is an adaptive process that relies on sensory, motor and cognitive processes (Balasubramaniam and Wing 2002) and can also be affected by emotional contexts (for a review, see Hagenaars et al. 2014). An emotional context that has been extensively studied in postural control is postural threat, which is a situation imposing increased challenge on balance. Postural threat has been primarily assessed in a series of studies by Carpenter and colleagues (Carpenter et al. 1999, 2001; Adkin et al. 2000) who showed that when participants stood on the edge of an elevated surface (0.8–3.2 m high), real, or virtual (Cleworth et al. 2012); they exhibited a reduction in postural sway and posterior body movement away from the platform’s edge compared with standing at ground level. This sway reduction was accompanied by an increase in sway frequency, muscle co-contraction, and ankle stiffness. Standing on an elevated surface also induced increases in fear of falling, sympathetic arousal, and stress response (Cleworth et al. 2012; Horslen et al. 2014). More recent studies focused on the underlying mechanisms of this adaptive response and showed that postural threat is likely to induce changes in two of the sensory channels involved in postural control, proprioceptive (Davis et al. 2011; Horslen et al. 2013), and vestibular (Horslen et al. 2014; Lim et al. 2017).
A reduction in body movement as a response to emotionally engaging, fear-inducing stimulation, however, is not unique to postural-threat contexts. Fear responses and their underlying mechanisms have been studied in animals and have been classified into two categories: defensive action, characterised by fight or flight behavior in response to impending attack, and defensive immobility, characterised by freezing, bradycardia, and hyper-attentiveness (Lang et al. 2000). For example, fear-freezing, measured as the amount of time rats in a cage were not moving has been observed in the context of Pavlovian-conditioned aversive responses using an auditory stimulus (LeDoux et al. 1988).
Using fear-freezing responses in animals as a starting point, freezing has also been assessed in humans using a paradigm comprising passive viewing of aversive or threatening images, for example, images of mutilation (Azevedo et al. 2005; Facchinetti et al. 2006). Similar to postural-threat research, these studies showed a reduction in sway, an increase in mean power frequency of sway but also bradycardia, the latter also being associated with freezing responses in animals. Similarly, Roelofs et al. (2010) showed a freezing response in reaction to angry faces, reflected in reduced heart rate and body sway, and Hillman et al. (2004) showed a backward body movement away from unpleasant stimuli in women which was not observed in the case of pleasant or neutral stimuli. This response was also affected by the previous experience of an aversive life event, with individuals who had experienced such an event showing a greater reduction in sway when exposed to aversive images compared with a control group (Hagenaars et al. 2012). However, another study manipulating arousal and valence using this paradigm (Horslen and Carpenter 2011) showed that only arousal affected postural sway similar to postural threat, and identified methodological limitations in some of the passive-viewing studies. The primary limitation involved the short duration of the postural trials (< 10 s) in some of these studies (Hillman et al. 2004; Stins and Beek 2007; Roelofs et al. 2010), which was not long enough for the full range of time scales present in postural sway time series to be identified (Van der Kooij et al. 2011).
Despite the methodological limitations identified in studies of freezing in humans, the consensus in the literature is that fear-related postural responses can be induced by at least two different types of emotion-specific paradigms, postural threat, and passive viewing of aversive or threatening images. Given that these two paradigms induce similar sway reduction responses, it would be reasonable to assume that this reduction is a general effect that goes beyond the two paradigms and could also be caused by other emotion-specific manipulations. In the psychology literature, fear-related emotional responses are primarily triggered using socially induced stress and anxiety (Dickerson and Kemeny 2004).
A well-established, effective method of inducing high levels of stress in humans is the presence of social evaluative threat (SET), in tasks primarily including mental arithmetic, public speaking, and singing (Kirschbaum et al. 1993; Frisch et al. 2015). SET is characterised by emotional responses observed during tasks performed in circumstances, where an evaluative audience or a negative social comparison is present. For example, SET has been successfully used to induce stress in combination with a mental arithmetic task, the Montreal imaging stress task (Dedovic et al. 2005). This study asked participants to perform arithmetic calculations and used a mock performance indicator combined with negative feedback by both the task software (after each trial) and the experimenter (between blocks, which was the SET element). When negative feedback was provided, an increase in cortisol was observed relative to the control and rest conditions, suggesting an increase in stress. Furthermore, a meta-analysis of over 200 studies showed that SET, together with uncontrollability, induced the largest increases in cortisol levels and the longest times to recovery compared to all other stressors (Dickerson and Kemeny 2004), thus making SET a very effective way of inducing stress in humans. The high effectiveness of this method makes it an excellent candidate to use as a novel manipulation to induce stress in the context of postural control, to see whether postural-threat- and passive-viewing-induced sway reduction can also be observed using SET-induced stress. Furthermore, performance of a mental arithmetic task while standing has been shown to affect both postural control and physiological arousal as measured by skin conductance (Maki and McIlroy 1996). Together, evidence suggests that SET in combination with mental arithmetic causes an increase in stress (Dedovic et al. 2013) and that mental arithmetic tasks affect physiological arousal and postural control (Maki and McIlroy 1996). However, a combination of these three tasks has not been previously used to induce stress in a postural control context.
The aim of this study was to assess whether a reduction in sway is observed when stress increases using a combination of mental arithmetic and SET manipulations. To this end, first, we assessed postural control and arithmetic separately and then concurrently. Subsequently, to increase stress and task demands, we added an element of time pressure by introducing a progress bar based on participants’ own performance in the arithmetic task. It was expected that under time pressure, participants would allocate more resources to the arithmetic task in an attempt to perform it more efficiently. Finally, to increase stress and task demand further, we added an SET manipulation, providing negative performance feedback. This approach was used to incrementally increase task demands and stress in each condition. This incremental increase was used to ensure that we could assess the contribution of each component of our design (adding the arithmetic task, time pressure, and SET) on stress, postural control, and performance on the arithmetic task.
Theories of anxiety and cognition, such as the processing efficiency theory (Eysenck and Calvo 1992) and its successor, attentional control theory (Eysenck et al. 2007), predict that during relatively simple tasks, performers can compensate for anxiety-related inefficiencies in processing information through increasing cognitive effort. However, as task demands increase, such inefficiencies can no longer be compensated for and performance starts to show deficits. We predicted that self-rated stress would increase incrementally with task demand. More importantly, we predicted that this increase in stress following the SET manipulation would be accompanied by a reduction in postural sway reflecting a stiffening or freezing response in line with the previous research, (Adkin et al. 2000; Azevedo et al. 2005; Facchinetti et al. 2006; Roelofs et al. 2010). These findings would suggest that the reduction in postural sway observed when using postural threat and aversive images can also be induced using cognitively and socially induced stress.
Performance on the arithmetic task was expected to improve following the inclusion of a time pressure manipulation, due to recruitment of additional resources to, or prioritization of the arithmetic task. However, we predicted that performance in this task would eventually decline under high levels of stress and task demands (e.g., Brooks 2014) as a consequence of anxiety-related reductions in processing efficiency. More specifically, we predicted that only in trials containing increased task demands and associated higher stress (i.e., trials including SET), would individuals demonstrate significant reductions in postural sway in conjunction with reduced performance in the cognitive task.