## 3 Results
### Breath-Holding Test Using SCISCCO on the Forehead  
As further confirmation of the validity of our measurements, we also compared our SCISCCO system results with a commercial fNIRS system (CW6, TechEn Inc., Milford, MA, USA). fNIRS only provides information regarding HbO and HbR (although typically the results for HbR are noisier and less reliable). Since the SCISCCO signal can be used to derive HbO, HbR, and CCO, in this test, we could at least compare the results for HbO and HbR. If those two measurements were relatively consistent, then we could reasonably assert that the CCO measurement was reliable. The set-up and protocol for the breath-holding test are illustrated in Figure 9. The probe from Figure 5b and the SCISCCO system from Figure 5a were then applied to the forehead of the healthy human participant. For the fNIRS system, the probes were placed on the left and right sides of the head. All of the fNIRS probes were secured with a silicone rubber band, and the source and detector separation was 3 cm. The protocol was for the participant to breath normally for about 5 min and hold his or her breath for 30 s, and then the sequence was repeated multiple times. The spectrum was measured continuously, with each spectral scan requiring about 30 s to collect due to the spectrometer's speed of tuning. Figure 10 shows the comparison between the fNIRS system (black curve) and the SCISCCO system (red for HbO and blue for HbR) for simultaneous measurements. The top curve shows the comparison for HbO, with the shaded regions corresponding to the breath-holding periods. Both systems show that the HbO increased during the breath-holding period, and there was reasonable correlation between the two measurements. The bottom curve shows the comparison for HbR, where the results were generally similar but the correlation was not as good. It should be noted that in fNIRS systems, the HbR tends to be noisier, in part because of less sensitivity to shorter wavelengths for the system. A similar rationale may also apply to the SCISCCO system. Moreover, the discrepancy between the two systems may also be attributable in part to changes in HbO concentration during the breath-holding periods and the low sampling rates. SCISCCO failed to capture this dynamic consistently. During breath holding, the HbO increased during the first 10-20 s, followed by a decline in HbO concentration [40]. Yet, as our system measured the levels about every 30 s due to hardware limitations, we may not have captured this dynamic in terms of its rise and fall during the breath-holding period. The measurement time limitation arose from the spectrometer's scanning speed. In the later experiments with the attention test (next section), we were able to optimize the system to reduce the measurement time down to ~17 s. Even shorter data acquisition times could be achieved using fast tunable filters, such as acousto-optic tunable filters. Figure 9: Set-up and protocol for the breath-holding test using the SCISCCO system. In Figure 11, the typical results from the SCISCCO system for the breath-holding test are illustrated. Figure 11a shows the overlap of the HbO, HbR, and CCO signals through the protocol, with the shaded regions corresponding to the periods of breath holding. Figure 11b shows in more detail the CCO measurements for the breath-holding test. This curve resulted from averaging five measurements from three different healthy human participants, with each participant being measured three times. What we found was that for at least three out of the four breath-holding periods, the CCO level decreased during the breath-holding period. Thus, we confirmed through the breath-holding test that we obtained a similar hemodynamic response for the fNIRS and SCISCCO measurements. The correlation for HbO was reasonably good, but the correlation with HbR was poorer due to higher noise levels. It should be noted that the hemodynamic responses we observed were consistent with the results published in the literature.