The data was low-pass filtered and divided into blocks that consisted of 4 s of prestimulus onset baseline, followed by the experimental trial and, after that, a whole trial of baseline (9 to 12 s in length). Each block was detrended by fitting a straight line between the average signal value in the prestimulus onset period and the average signal value on the last four seconds at the end of the block, which correspond to the last part of the subsequent baseline trial. The detrending procedure brings the start and end points of each block to zero, so the and HHb values reflect increase or decrease from that reference value.3 Measurements for each infant were analyzed, and trials, channels, or participant data were rejected from further analysis in a two-step preprocessing protocol: first, by looking time measures, and second, by the quality of the signals as assessed by artifact-detection algorithms (which either excluded the data of whole channels per infant or data from individual trials within a channel, according to the magnitude of the artifact).3,42 Criteria for channel rejection included: (1) measuring the coefficient of variation (CV) of the signal (channels were excluded if the CV of the attenuation measurement for each wavelength exceeded 10%, possibly due to movement of the arrays and hat) and/or (2) high-frequency noise beyond the limits of physiological effects, where the normalized high-frequency power is greater than 35% of the total power of the signal.43 For each infant, the channels that survived these rejection criteria were analyzed for trial selection. The trial selection analysis identified sharp changes in the signal caused by sudden movements. This was applied following data conversion from attenuation to concentration data. Trials that contained changes in concentration that exceeded a predefined range ( during baseline and during the experimental trials where artifacts in the signal may occur in addition to activation), were removed from the data set. These thresholds were set according to experience with the current array design over the past 8 years. The minimum number of valid experimental trials for each channel was 3. At group level, the grand averaged hemodynamic responses () of all infants were calculated and the maximum change (or amplitude) in (increase in chromophore concentration) and/or HHb (decrease in chromophore concentration) was assessed during the experimental condition relative to baseline within a time window selected between 8 and 16 s poststimulus onset for each trial. This period of time was selected to include the range of maximum concentration changes observed across infants for and HHb. Two-tailed t-tests were used to test the statistical significance of the change. Either a significant increase in concentration or a significant decrease in HHb is commonly accepted as an indicator of cortical activation in infant work.3 During the channel by channel t-tests and subsequent spatial reliability analyses, if and HHb were either to increase or decrease significantly in unison, the signal was considered inconsistent with a hemodynamic response to functional activation40 and not reported in the analyses (for further discussion of physiological changes reported in infant fNIRS work, see Refs. 3 and 4). To identify these channels, the statistical analyses were reviewed and those channels with an increase or decrease in both chromophores were excluded. For the group level, no channels evidenced this pattern in either session. For the individual level, in Session 1, three participants had one channel excluded and one participant had eight channels excluded from the activation maps; and in Session 2, two participants had channels excluded (one channel and four channels). This exclusion criterion was not applied during the signal reliability analysis. Throughout the text, the terms “significant increase of ,” “significant decrease in HHb,” or “significant channel” will be used considering these criteria. To resolve statistical problems of multiple measurement sites for these group analyses, we applied the false discovery rate (FDR) test for multiple comparisons.44,45 The channels that did not survive the test are highlighted in Table 1 with an asterisk. results were unaffected by FDR correction; however, none of the channels with significant HHb decrease survived the test.