Participants

To determine the sample size, we performed a sample size calculation that adjusted for the correlated nature of the repeated measurements (Liu & Liang, 1997). The power analysis was performed with PASS (PASS 14, NCSS, Kaysville, USA) using a simulation approach. The significance level was fixed at p = .05 and the power level was fixed at .80. We inferred effect size from the study by Baden et al. (2004), in which the deception manipulation led to a 19.6% change in attentional focus. Using these parameters, and after 1,000 simulations, the power analysis indicated that at least 20 participants were needed for this study. Twenty-two students (15 males and seven females, M age = 21.27 ± 2.07 years) at the University of Nice Sophia-Antipolis were recruited in exchange for course credits. To be eligible for the study, the participants had to report at least one session of moderate long-duration exercise per week (e.g., biking or running for more than 1 h). Table 1 presents the participants’ anthropometric and physiological characteristics. All provided written informed consent, and the study was conducted according to the Declaration of Helsinki.

Table 1 Anthropometric and physiological characteristics of the participants Full size table

Experimental procedure

The participants came to the lab for three sessions. Before each session, they were instructed to abstain from any vigorous exercise for 24 h pre-session, to sleep at least 7 h the night before, and to avoid changes in caffeine ingestion in the morning before the experiment.

In the first session, they signed the informed consent form and were asked to perform an incremental test to exhaustion on an electromagnetically braked cycle ergometer (Brain-bike NeuroActive, Motion Fitness, Rolling Meadows, IL, USA), to identify their peak aerobic power (PAP) and peak heart rate (HR). The power output was increased by 20 W every minute after a 4-min warm-up at light intensity (women, 70 W; men, 80 W). Voluntary exhaustion was defined as the point at which participants voluntarily stopped or they could no longer maintain a pedaling frequency above 50 rotations per minute for more than 10 s, despite strong verbal encouragement. Heart rate was recorded continuously during the incremental test by a Polar system (Polar RS800CX, Polar Electror Oy, Kempele, Finland). After the test, the participants were also familiarized with the two psycho/physical scales to be used in the subsequent experimental trials (attentional focus and RPE).

The two experimental sessions were held at the same time of day, with a mean interval of 3.2 ± 1.4 days. The participants were informed that they would perform exercise at an intensity corresponding to 60% PAP on the cycle ergometer and that this would last for either 10 or 60 min, according to the condition. The order of the two trials was counterbalanced. Before starting the exercise, they were fit with the NIRS cap and the Polar chest belt. A 2-min baseline period followed in which they were seated without movement on the ergometer. They then performed a 1-min warm-up at low intensity (70 W for females and 80 W for males), which was followed by a 4-min period in which the intensity progressively increased to the target intensity. Although the ergometer adjusted the power output to the pedaling frequency, the participants were asked to stay at a fixed pedaling frequency all along the trial, which was similar for the two experimental sessions. Three sound probes were given at 200, 400, and 600 s after the trial onset. Each probe was immediately followed by two questions displayed on a screen located in front of the participants to assess their attentional focus and RPE. In both experimental sessions, the trial was stopped after 10 min. The experimenter justified the early stop in the 60-min condition by saying that a sufficient amount of data had been collected, so that the exercise could be stopped sooner than planned. At the end of the study, all participants were thanked and carefully debriefed about the aims of the study. The experimenter also explained the necessary use of deception in the method and asked the participants not to communicate this information to other possible participants, to avoid a contamination effect.

Measures

NIRS

Cerebral hemodynamics were measured in two sub-regions of the PFC using a continuous-wave multi-channel NIRS (Oxymon Mk II, Artinis Medical Systems, Zetten, the Netherlands). Optodes were placed into thermoplastic shells and mounted on a head cap (Easycap, Herrsching, Germany) to facilitate the positioning of the optodes on each participant’s head. To maintain sufficient pressure on the optodes at the skin surface, elastic bands were inserted on the probe holders to push the probes into the holders. NIRS sensitivity to gray matter depends to the source–detector distance and head region (Quaresima et al., 2012; Strangman, Zhang, & Li, 2014). Since penetration of light is elevated in the frontal region in comparison with other regions due to the absence of hair, the distance between the source and detector probe was fixed at 4.0 cm. This distance allows for deeper recording, which should ensure that a significant part of the NIRS signal comes from a cortical source. Strangman et al. determined that a 4.0-cm source–detector distance on the frontal cortex yields high NIRS sensitivity to gray matter in the frontal region (~15% of the NIRS signal). The area measured between the emitter and detector probes was defined as a channel. The positioning of the probes was arranged so that the two NIRS channels were centered on the AF2h and F6h sites of the extended 10–5 EEG system (Jurcak, Tsuzuki, & Dan, 2007). The cortical structure below the AF2h site corresponds to the anterior part of the right dorsomedial PFC (BA10). The cortical structure below the F6h site corresponds to the rdlPFC on the middle frontal gyrus (BA46). The projection of the positions of the two channels on the cortical surface (based on their stereotaxic coordinates) is illustrated in Fig. 1. The concentrations (in micromoles) of O 2 Hb and deoxyhemoglobin were provided using two wavelengths in the near-infrared range (764 and 858 nm) and an age-specific differential path length factor obtained using the modified Beer–Lambert equation (Obrig & Villringer, 2003). The data were sampled at 10 Hz and acquired with Oxysoft (version 3.0.43; Artinis Medical Systems, Zetten, The Netherlands). We computed an average O 2 Hb value for each of the three time periods during exercise (0–200, 200–400, and 400–600 s) at the target intensity (60% PAP). The moment during which the participants answered the two questions was excluded from these time windows.

Fig. 1 Locations of near-infrared spectroscopy (NIRS) channels, through projection on the cortical surface using the NIRS-SPM software (Ye, Tak, Jang, Jung, & Jang, 2009). The probes were arranged to measure hemodynamic activity in two regions of interest: the right dorsolateral prefrontal cortex (Channel 1) and the right medial frontal cortex (Channel 2). Full size image

Attentional focus

The attentional focus was assessed by adapting the procedure of Christoff et al. (2009) and Baden et al. (2004). After a sound probe, the following question was immediately displayed on a screen in front of each participant: “Where was your attention focused before the probe?” With a mouse click placed within reach of their right hand, the participants had to provide their response on a visual analog scale (VAS) displayed on the screen. The scale ranged from completely on task to completely off task. During the familiarization period, the participants were instructed that a minimal value on this scale indicated that they had spent all the period preceding the probe thinking about the exercise task they were performing, and that a maximal value indicated that they had spent all their time thinking about something else. Several examples were provided to ensure correct understanding of the scale. The position of the cursor was recorded in pixels and was then converted to a 0–100 scale to provide a meaningful score.

RPE

A second question then followed to assess the participants’ perceived exertion. To optimize the sensitivity of the measure and to avoid any memory of previous responses, participants also had to provide their response with a mouse click on a VAS, a valid measure of perceived exertion (Grant et al., 1999). The VAS had the same labels as the original 6–20 Borg scale (Borg, 1970). The cursor positions (in pixels) were then converted to the 6–20 RPE units in order to provide a meaningful interpretation of the recorded values.

Data analysis

We focused on O 2 Hb as the variable of interest to determine changes in PFC activity (Yamanaka, Yamagata, Tomioka, Kawasaki, & Mimura, 2010). Changes in O 2 Hb signals have been recognized to better reflect cortical activation than dHb due its superior contrast-to-noise ratio (Strangman et al., 2002). In addition, previous independent studies provide evidence for fNIRS O 2 Hb being an appropriate and better substitute for fMRI BOLD for studying cortical activity related to cognitive and motor tasks (Muthalib et al., 2013; Steinbrink et al., 2006; Strangman et al., 2002). To process the NIRS signals, we first checked for the quality of the signals by calculating the signal-to-noise ratio (SNR) using an adaptation of the formula used by Cui, Bray, and Reiss (2010):

$$ \mathrm{S}\mathrm{N}\mathrm{R}=\mathrm{mean}\left(\mathrm{exercise}\right)-\mathrm{mean}\left(\mathrm{baseline}\right)/\sqrt{\left\lfloor \mathrm{variance}\left(\mathrm{exercise}+\mathrm{baseline}\right)\right\rfloor }. $$