Participants

Thirty-two Leiden University undergraduate students (22 females, 10 males, mean age = 21.34 years, range 18–28) took part in the study. Participants were enrolled via an online recruiting system and were given either course credits or a financial reward of 10 euros for participating in a study on the effects of brain stimulation on decision-making. Once recruited, all participants were screened individually by the same lab-assistant using the Mini International Neuropsychiatric Interview (M.I.N.I.; Sheehan et al. 1998). The M.I.N.I. is a short, structured interview of about 15 min that screens for several psychiatric disorders and drug use, and it is often used in clinical and pharmacological research (Colzato et al. 2012, 2013). Following previous published protocols (Colzato et al. 2017; Sellaro et al. 2015; Steenbergen et al. 2015; Beste et al. 2016) participants took part in the experiment only if they met the following criteria: (1) age between 18 and 30 years; (2) no history of neurological or psychiatric disorders; (3) no history of substance abuse or dependence; (4) no history of brain surgery, tumors, or intracranial metal implantation; (5) no chronic or acute medications; (6) no pregnancy; (7) no susceptibility to seizures or migraine; (8) no pacemaker or other implanted devices. All participants never experienced tVNS before this study. Before the beginning of the testing session, they were given a verbal and written description of the procedure and of the usual adverse effects (i.e., itching and tingling skin sensation, skin-reddening, and headache). Participants received no information about the different types of stimulation (active vs. sham) or about the assumptions regarding the study. The experiment conformed to the ethical standards of the Declaration of Helsinki and the protocol was approved by the local ethical committee (Leiden University, Institute for Psychological Research). Written informed consent was obtained from all participants.

Apparatus and procedure

A single-blind, sham/placebo-controlled, randomized cross-over within-subject design with counterbalanced order of conditions was used to assess the effect of online (i.e., stimulation overlapping with the critical task) tVNS on flow performance in healthy young volunteers.

All participants took part in two sessions (active vs. sham), separated by 1 week, and were tested individually. Twenty minutes after the onset of stimulation, participants performed for 30 min an emotion recognition task that requires participants to assess someone’s emotions based on images of whole faces and bodies. Participants were asked to choose which of four emotions (i.e., happy, fear, anger, and sad) better described what the person in the image was feeling. The four emotional labels were displayed at the four corners of an imagined square surrounding the target picture, and participants had to click with the computer mouse on the chosen emotion. Because of technical failure, the data of the emotion recognition task have not been further analyzed or published elsewhere. After task completion, participants had to rate their flow experience on the Flow Short-Scale (Engeser and Rheinberg 2008). Afterwards, the stimulation was terminated and participants were asked to complete a tVNS adverse effects questionnaire requiring them to rate, on a five-point (1–5) scale, how much they experienced (1) headache, (2) neck pain, (3) nausea, (4) muscle contraction in face and/or neck, (5) stinging sensation under the electrodes, (6) burning sensation under the electrodes, (7) uncomfortable (generic) feelings, and (8) other sensations and/or adverse effects. None of the participants reported major complaints or discomfort during or after tVNS. They were explicitly asked if they could guess the stimulation received and no one reported to be aware of it.

Transcutaneous vagus nerve stimulation (tVNS)

We employed the NEMOS® tVNS neurostimulating device. This device is composed by a stimulation unit and a dedicated ear electrode, which can be worn like an earphone. Following previous published protocols for optimal stimulation (Colzato et al. 2017; Sellaro et al. 2015; Steenbergen et al. 2015; Beste et al. 2016), the tVNS® device was programmed to a stimulus intensity at 0.5 mA, delivered with a pulse width of 200–300 μs at 25 Hz. Stimulation alternated between on and off periods every 30 s. In the active condition, the stimulation electrodes were applied to the outer auditory canal. In the sham (placebo) condition, the stimulation electrodes were placed on the center of the left ear lobe. Indeed, the ear lobe has been found to be free of cutaneous vagal innervation (Peuker and Filler 2002; Fallgatter et al. 2003) and a recent fMRI study found that this sham condition produced no activation in the cortex and brain stem (Kraus et al. 2013).

Further, following safety criteria to avoid cardiac side effects, the stimulation was always applied to the left ear (Nemeroff et al. 2006; Cristancho et al. 2011). Indeed, although efferent fibers of the vagus nerve affect cardiac function, such an impact seems to relate only to the efferent vagal fibers connected to the right ear (Nemeroff et al. 2006). Consistent with this picture, a clinical trial reported no arrhythmic effects of tVNS when applied to the left ear (Kreuzer et al. 2012).

Flow Short-Scale

Flow experience was indexed by the two subscales absorption and fluency of the Flow Short-Scale (Engeser and Rheinberg 2008). The items are assessed on a seven-point Likert scale from 1 (I don’t agree) to 7 (I agree). Absorption consists of four items (e.g., “I do not recognize that time is going by”) and fluency is measured by six items (e.g., “I feel that everything is under control”). We found good to satisfactory reliabilities of absorption (Cronbach’s α = 0.65) and fluency (Cronbach’s α = 0.73) replicating previous protocols (Peifer et al. 2014). Participants filled in the Flow Short-Scale immediately after they finished the emotion recognition task and were instructed to refer to it when answering the questionnaire.

Statistical analyses

For each participant, and for both the active and the sham stimulation, the scores for the absorption and fluency subscales were calculated. To examine whether active tVNS, as compared to sham (placebo) stimulation, affected flow experience, two separate repeated measures analysis of variance (ANOVA) were carried out with absorption and fluency as dependent variables and session (active vs. sham) as within-participant factor.

A significance level of p < 0.05 was adopted for all statistical tests.