Procedure

The study protocol was reviewed and approved by the IRB of the Harvard T.H. Chan School of Public Health. Testing occurred over a single 30-min session between the daytime hours of 09:00 and 16:00. A Polar H7 heart rate monitor (Polar Electro Oy, Kempele, Finland) was used as a validated research device to measure R–R intervals with accuracy comparable to electrocardiograms (Barbosa et al., 2016; Giles et al., 2016). A logging application on iPad (Apple Inc., Cupertino, CA, United States) recorded the R–R interval signals from the chest strap which were further analyzed.

Participants sat upright quietly for 5 min while reading the instructions for the study, and then, heart rate, cuff blood pressure, and respiration rate were measured. To generate varying baseline autonomic states among participants, subjects were randomized to one of four preconditioning groups: (1) nasal respiration at 0.1 Hz (inhale nose 5 s, exhale mouth 5 s) for 5 min, (2) contracting arm muscles by grasping a tennis ball at 0.1 Hz for 5 min (alternating contractions in left and right arms every 5 s), (3) performing contraction and nasal respiration in synchrony at 0.1 Hz for 5 min, and (4) reading consecutively four articles rated as emotionally neutral for 5 min. The contraction tasks have been demonstrated to vary cardiac reactivity through differing levels of resonance (Lehrer et al., 2000, 2009; Vaschillo et al., 2011). A graphical timer application on iPad was used to visually cue the breathing/contraction. The articles were Scientific American excerpts that were previously validated as emotionally neutral (van den Broek et al., 2001).

To assess executive task function, a computerized version of the Stroop test¹ was run for 5 min. The Stroop test has been demonstrated to produce a mild sympathetic response through dissonant executive task function (Salahuddin et al., 2007; Visnovcova et al., 2014). Participants were asked to indicate the color of the word (and not its meaning) by keystroke, as quickly as possible, while minimizing their errors. For congruent trials, the displayed word and the color described by the word were the same. For incongruent trials, the displayed word and color presented were not the same. The reaction time for each word pair is recorded by the computer program with the premise of the Stroop test that incongruent pairs have longer reaction times when compared to congruent pairs (Dyer, 1973). As a marker for performance, a reaction time gap was calculated for each participant from the difference between congruent and incongruent pair reaction times. A shorter reaction time gap was considered to indicate higher performance.

At the end of this task period, a 5-min questionnaire Short Flow State Scale-2 (SFSS-2) was then administered to assess degree of flow engagement (Jackson et al., 2008). Flow engagement is the degree of perceived task immersion, and the SFSS-2 has been validated for evaluation of performance engagement. Respiration rates were monitored during all phases of testing to ensure they were within the 9–24 cycles/min range required to correspond accurately to vagal tone (Laborde et al., 2017).