Background

Cold-water swimming has become increasingly popular (Griffiths and Turner, 2022). With much evidence suggesting this activity has risks (Tipton, 1989; Tipton et al., 2017; Knechtle et al., 2020), the beneficial effects of cold-water immersion (CWI) have been linked to enhancing a positive mood state (Massey et al., 2022), higher perceived psycho-physical wellbeing (Huttunen, et al., 2004; Demori et al., 2021), and ameliorating mood disorders (Lindeman et al., 2002). It has been suggested that whole-body exposure to cold triggers a release of neurotransmitters such as serotonin, cortisol, dopamine, norepinephrine, and β-endorphin (Hirvonen et al., 2002), which play a crucial role in emotion (Gu et al., 2016; Canli and Lesch, 2007) and stress regulation (Godoy et al., 2018). Furthermore, emerging evidence indicates that the therapeutic benefits of cold water, such as increasing positive and decreasing negative affects, may be gained even from a single exposure (Massey et al., 2020; Kelly and Bird, 2022). This evidence was derived from behavioural studies using a questionnaire design and, as such, the neural mechanisms of the effects of CWI on affective processes remain largely unknown.

Only a few studies investigating how CWI changes brain activity have been conducted in humans (Jarrahi et al., 2017; Bitar et al., 2020; Grouper et al., 2022). However, these studies focused on assessing brain activity and its association to cognitive-affective aspects of pain, by using an experimental protocol of administered tonic limb cooling. It must be noted that physiological responses to limb cooling using very low water temperatures (1–10 °C) constitute points of contrast that result in painful experiences, which are different from what we experience after taking a cold shower or swimming in higher temperature open waters.

We developed and tested a novel protocol to examine the effects of short-term, head-out, whole-body CWI on changes in brain functional connectivity. This protocol can be adapted for other research questions focusing on brain-behaviour interactions [e.g., linking personality traits, cognitive functions such as decision making, memory, and perception to the brain's functional architecture (Cole et al., 2014)]. A tight control of experimental factors in our protocol makes it easy to replicate and minimise confounding effects.