## 2 Materials and Methods
### Experiment Design  
We designed two experiments, including passive (somatosensory only) and active (motor plus somatosensory), in order to investigate the most effective stimulation parameters in evoking HR. Continuous wave fNIRS data was recorded at a sampling frequency of 7.8 Hz via a NIRX NIRScout(tm) [39] system. Figure 1a shows the optode probe layout based on the standard 10-20 system which consists of 16 dual-tip LED sources and 20 detectors that were configured over both hemispheres for a total of 104 channels (52 HbO and 52 HbR links). The associated wavelengths with HbR and HbO in the absorption spectra were 760 nm and 850 nm, respectively. The sources and detectors were bilaterally placed over the primary somatosensory (S1, postcentral gyrus) and primary motor (M1, precentral gyrus) cortices. For both passive and active experiments, the Galileo(tm) pneumatic tactile stimulator [40], shown in Figure 1b, was programmed to deliver the different velocities of saltation on the glabrous skin of the dominant hand propagated through an array consisting of 14 TAC-Cell nodes. The saltatory wave consisted of pneumatic pulses (60 ms duration, 10 ms rise/fall) that were delivered directionally, as shown by the arrows in Figure 1c, starting from four distal fingertip phalangeal nodes (D2-D5), traversing over the corresponding four proximal phalangeal nodes, and terminating at the distal phalynx of the thumb (D1). The velocity of the saltatory pneumotactile wave was presented in a random-balanced design at 10, 25, and 45 cm/s. During the active task, the participants were instructed to mimic the saltatory pattern immediately upon the detection of the pulsed stimuli at their D2-D5 fingertips, by flexing their fingers at a matching velocity, and remain still otherwise. During the passive task, the participants were instructed to relax and remain still throughout the experiment. The participants had 10 minutes of rest between the active and passive tasks. Every recording session began with 60 s of rest, followed by 30 velocity trials (10 per velocity) each lasting 40 s, consisting  
Figure 1: Experiment Design: (**a**) fNIRS optode layout. Blue dots are light sources, red dots are detectors and grey dots are five-percent distance increments in the standard 10–20 system. Each line is a link (channel). (**b**) Galileo(tm) pneumotactile stimulus control system housed in a sound-isolated room adjacent to the fNIRS recording suite to minimize its acoustic noise. (**c**) Color-coded TAC-cells and the arrows show the order and direction of the saltation wave at each node. Each wave starts at D2–D5 fingertips (red nodes) and terminates at the thumb (blue node). The time interval between two consecutive nodes depends on the velocity and hand measurements. (**d**) Participant configured with the fNIRS cap while receiving the somatosensory stimulus. (**e**) Stimulus pulse train color-coded to match channel assignment. Each node delivers a 60 ms pulse. Each stimulus pulse train consists of five 60 ms pulses at varying intervals based on the velocity and hand measurements. Each trial lasts 40 s, consisting of 20 s of stimulus in the form of repeated pneumatic pulse trains with 2 s interburst intervals, followed by 20 s of no stimulation. There are 10 trials per velocity, resulting in 30 trials per task. Each trial is immediately followed by another in a random-balanced velocity design. of 20 s stimulus followed by 20 s of no stimulation, as illustrated in Figure 1e. The interval between two consecutive saltatory waves was 2 s, while the internode intervals varied based on the velocity and measured node distances. The number of saltatory waves across subjects was consistent; however, it was different across velocities, e.g., up to three more stimulus trains could fit in a 20 s window for the fastest velocity (45 cm/s) when compared to the slowest (10 cm/s).