*3.1. Visual Orientation Task*

Infants' latency (in seconds) in orienting to the red lateral light was measured for every trial and averaged for each infant. In both groups, the majority of infants completed all 10 trials. In the DS group, 6 out of 7 infants completed 10 trials (M = 9.28, range from 5–10). In the TD sample, 16 out of 24 infants provided data for all trials (M = 8.95; range from 5–10 trials). Infants' orientation latency for the two groups is provided in Figure 2. Because sample size differed across groups, we performed a linear-mixed model analysis with infants' orientation latency as the dependent variable, group (DS and TD) as a fixed effect, while by-subject intercept was set as a random effect. Using the lmerTest [40] package in R, we observed that the DS group revealed significantly longer latency than the TD group (intercept = 6.29, DS estimate = 2.46, SE = 0.86, t = 2.85, *p* = 0.008, 95% CI: 0.77–4.16). Additionally, we compared the number of trials longer than 8 s across groups. A Wilcoxon-Mann-Whitney test revealed that DS infants exhibited more trials longer than 8 s (M = 3.0, SD = 2.0) than TD infants (M = 1.29, SD = 1.5; Z = 2.19, *p* = 0.028, r = 0.39). *Brain Sci.* **2021**, *11*, x FOR PEER REVIEW 6 of 12

**Figure 2.** Mean orientation latency (in seconds) in the visual attention task across the Down syndrome (DS) and typically developing (TD) groups. The values above the bars refer to the mean latency value for each group. Error bars represent 1 (+/−) standard error of mean. **Figure 2.** Mean orientation latency (in seconds) in the visual attention task across the Down syndrome (DS) and typically developing (TD) groups. The values above the bars refer to the mean latency value for each group. Error bars represent 1 (+/−) standard error of mean.

**Figure 3.** Mean proportion of looking time in the audiovisual task across the Down syndrome (DS) and typically developing (TD) groups. Error bars represent 1 (+/−) standard error of mean. Signif-

The looking pattern was analyzed for each group separately. First, we compared the 4 levels of AOIs (the eyes, the mouth, the arm, and the background) separately for each group. The results revealed that AOIs significantly differed in both groups (a Kruskal-Wallis test for the DS, H(3) = 17.7, *p* = 0.004, η2 = 0.61; a one-way-ANOVA for the TD

icant differences are signaled: 0.05 = \*, 0.01 = \*\*, 0.001 = \*\*\*.
