2.1. Experimental protocol

Subjects sat in a reclining chair with a gas mask placed over their mouth and nose. The mask was sealed by transparent film dressing (Tegaderm, 3M, St. Paul, USA) and connected to a computer-controlled gas delivery circuit (RespirAct^TM, Thornhill Research Inc, Toronto, Canada) [19]. A non-invasive monitoring system was secured to the participant’s left arm (Finapres Medical Systems, Netherlands) to record heart rate (HR) and arterial blood pressure continuously (sampling rate = 200 Hz) during the experiment. A custom-designed probe holder was placed on the forehead and secured by a velcro headband. The holder was designed using SpaceClaim (ANSYS Inc., Canonsburg, USA), and 3D printed (TAZ 5, LulzBot, Loveland, USA) from a thermoplastic polyurethane material (Sapphire NinjaFlex, NinjaTek, Manheim, USA). Detection fibers for both subsystems were placed at a short and long source-detector separation (r_SD): 1.0 and 3.0 cm for TR NIRS, and 1.0 and 2.7 cm for DCS.

The experimental protocol consisted of three 2-min periods of hypercapnia. The first period started after two minutes at normocapnia, and each was followed by five minutes of normocapnia. The hypercapnia P_ETCO₂ target was set to 15 mmHg above each subject’s normocapnic P_ETCO₂ value, which was defined by the automated gas controller. TR-NIRS and DCS data were acquired continuously throughout the cyclical P_ETCO₂ protocol at a sampling rate of 3.33 Hz.