Corrcoef

DCS and LDF measurements of blood flow during hypoxia-hypercarbia were down-sampled to similar time bins, normalized to a baseline period prior to each manipulation of ventilation, pooled across pigs and time points, then correlated (corrcoef, Matlab 2019a, Mathworks). In the present analysis, due to the very small study population (5 pigs), we did not correct for repeated hypoxia-hypercarbia cycles in the same pig or multiple time-series measurements.
Blood flow in the spinal cord during aortic occlusions were averaged over 3 minutes after the balloon was fully inflated. Data from multiple pigs and serial inflations in the same pig were pooled. The median (prctile, Matlab 2019a) and 95% confidence intervals [40 ] were calculated without statistical correction for non-independent data. A receiver operating characteristic (ROC) curve and area under the ROC curve (AUC) were calculated using average change from baseline of DCS-measured blood flow over 3 minutes following balloon inflation at each of the 3 DCS sensor locations, using radiographically-determined location of the aortic balloon and sensor as the ground truth (perfcurv, Matlab 2019a).

The method for calculating the task-based functional connectivity of brain activity in the RJA and IJA conditions was similar to that of previous studies examining functional connectivity in infants using fNIRS (48 (link), 49 (link)). First, for each infant, the Matlab function “corrcoef” (The MathWorks Inc., Natick, MA, USA) was used to calculate the pairwise correlation coefficient (r) between the averaged time course of oxy-Hb in all 22 measurement channels. The correlation coefficients (r) were then subjected to Fisher's z-transformation [z (r)] to make the statistical distribution of correlation coefficients for each condition close to a normal distribution. Individual z (r) values in each of the RJA or IJA conditions were examined using a one-sample t-test against zero for the measurement channels. The calculated t-values were transformed into z-statistics according to the equation z = (t - t)/σ (t and σ represent the mean and SD, respectively). Functional connectivity between all channel pairs (22 × 21 pairs) except the same channel pair were examined. To compare the functional connectivity between RJA and IJA conditions, the z-values were subjected to paired-sample t-test for the channels that showed significant functional connectivity in each condition.