## 3 Results
### Attention Test with a Modified Probe for SCISCCO and fNIRS  
Having validated the SCISCCO system against the published literature and a commercial fNIRS system for hemodynamic response, we next moved to a pilot human study on 25 healthy human participants. The test was a cognitive attention test, where the expectation was that the active part of the brain would be in the forehead region, since the frontal lobe is central to attentional capabilities. For example, to complete an attentional task, the participant needed to focus on the target information, ignore distractors, and remember the necessary task rules or instructions to complete the task. The frontal lobe is essential in keeping all these aspects of attention \"on\" and operating cohesively to accomplish a given task [41; 42]. Because of the concern for the breath-holding test that fNIRS and SCISCCO system correlation may be affected by different probe locations, we modified the probe for the attention test to have the fNIRS and SCISCCO fibers interleaved and covering approximately the same region of the forehead. The new probe is shown in Figure 12, where the two SCISCCO fibers (light excitation and receiving fibers) were placed along one diagonal of a rectangle (green dots) while the two fNIRS fibers were placed along the other diagonal of the rectangle (red dots). The excitation fibers and the receiving fibers were placed approximately 3 cm apart. Figure 11: (**a**) Typical measurements of HbO, HbR, and the redox state of CCO using the SCISCCO system in a breath-holding test. (**b**) Expanded view of the changes in redox state of CCO using the SCISCCO system in a breath-holding test. The error bars indicate the variability between each run and between different subjects. Figure 13 illustrates the attention test protocol and the CCO probe location. The photograph shows that the CCO probe was placed with 3 cm of separation on the forehead. The fNIRS probe included a regular band at the left and right hemispheres and one reference channel at the forehead. The probe was taped to the forehead of the participant to minimize motion artifacts during the measurement. The attention test protocol shown at the top of Figure 13 has each test comprising three task blocks, and then the test was repeated three times (i.e., for each participant, there were nine blocks in total for the attention task). Each cycle of the protocol had a 60-s rest period followed by a 60-s attention test, which was repeated 3 times in the same test, and ended with a 60-s rest period. This protocol was then repeated three times. The description of the cognitive attention test is as follows. The participant would see a capital letter (e.g., A-, B-, C-, or D-) followed by a small letter (e.g., a-, b-, C-, or d-). Sometimes, the two letters were the same, and sometimes they were different. The participant would press a key when the small letter was observed, corresponding to a particular small letter (e.g., when the letter \"b\" was shown, the participant pressed the letter \"b\" on the keyboard). Since this is a cognitive test rather than a physiological test, we first wanted to verify that the fNIRS and SCISCCO systems still showed good correlation at least for HbO. Figure 14 shows the HbO comparison for the two systems, where the black curve is for the fNIRS system while the red curve is for the SCISCCO system, and the blue shaded regions correspond to the period of the attention test. We found that the HbO behavior for the two systems tracked each other and demonstrated an expected pattern. The slight phase shift  
Figure 12: Modified SCL probe for the attention tests, where the excitation and receiving fibers for the fNIRS system are interleaved with the fibers for the SCISCCO system. Figure 13: The attention test protocol and SCISCCO probe location. The excitation and receiving fibers were placed with 3 cm of separation on the forehead.