# Assessment of fNIRS Signal Processing Pipelines: Towards Clinical Applications
## 3 Results
### Block Average and Group-Level Results  
An example of the block average in a representative subject (i.e., subject #4, which is the same one whose preprocessing steps were provided over Figures 4-6) is shown in Figure 7 relevant to left-hand motor task and a measurement channel placed over the right motor region (i.e., contralateral hemisphere to the hand performing the task). As expected, a clear HRF is shown by the peak in \(\Delta[HbO_{2}]\) (i.e., red curve) kept by the whole task duration and leading and trailing slope with the foreseen delayed dynamics. A concomitant drop in \(\Delta[HbR]\) (i.e., blue curve) is also seen, but with a two-/three-fold scale reduction, which testifies to the regional cerebral blood flow increases. Dispersion of the 10 task repetitions is represented by the thickness of the respective lines (mean \(\pm\) SE), which demonstrates a good SNR. Interestingly, the whole cortex monitoring offered by the CW-fNIRS allows us to detect of secondary effects such as the inhibition of the ipsilateral (i.e., left) motor cortex, as if the activation of the left hand required to actively prevent the right hand from mirroring the left hand (Figure 8). Such specular inhibitions were already observed in other cortical areas, e.g., in the visual cortex under visual tasks [47; 48] and push for further research under the hypothesis of being a core mechanism of brain organization and cerebrovascular coupling. It is worth remarking that the inhibitory HRF is consistently more than two-fold lower than the contralateral excitatory one (peak value in 0.3-0.4 A.U. range in Figure 7, 0.1-0.2 A.U. range in Figure 8). This effectively impacts the inhibitory \(\Delta[HbR]\) amplitude, which is very small. Therefore, the fMRI BOLD signal, based on the sole \(HbR\) and blind to \(HbO_{2}\), might miss such inhibitory effects. Further considering the example of subject #4 and the left motor cortex (i.e., Figure 7), Figure 9 displays the comparison between the block average response and the GLM fitting over the motor channel referred to Figure 7. The primary contralateral activation response is shown, which highlights the good superposition in the peak phase of \(\Delta[HbO_{2}]\) and in the drop phase of \(\Delta[HbR]\). However, some differences are displayed concerning the recovery phase, which is shorter in the GLM fitting due to the temporal trend of the canonical HRF (cHRF), which is derived from fMRI literature [49]. The latter displays broader plateaus centered around the block average positive \(\Delta[HbO_{2}]\) and negative \(\Delta[HbR]\) peaks. Such secondary shape differences are well explained by the GLM a-priori assuming a fixed standard HRF shape, which in turn is derived from the convolution of the cHRF with the on-off square wave of task onset-offset. Clearly, such an a priori regressor locks the  
Figure 7: Subject-specific block-averaged responses across pipelines of \(\Delta[HbO_{2}]\) and \(\Delta[HbR]\) related to left-hand motor grasp and a measurement channel placed over the right motor region (i.e., contralateral hemisphere). (**a**) SC-MARA-spline, (**b**) MC-MARA-spline, (**c**) SC-MARA-tPCA, (**d**) MC-MARA-tPCA, (**e**) HybridMARA-spline SG, (**f**) HybridMARA-tPCA SG, (**g**) TDDR, and (**h**) Wavelet method. estimated shape and adapts only its amplitude. In this way, it retains details that are not detected in the single subject or may be smeared by the block average computation. Importantly, all methods showed similar correspondences between the block-average and the GLM fitting. Nonetheless, the TDRR and Wavelet algorithms displayed slightly decreased response amplitudes (i.e., 0.4 A.U. vs. 0.3 A.U. ), while keeping with the lesser power of their preprocessed signals. However, such a signal decrease was not accompanied by SNR drops. Table 3 provides a numerical evaluation of the SNR of block averaged responses across subjects. Results refer to measurement channels placed over the motor region, following single subject representation of Figures 7 and 8.