3.5 MRCPs by Age
To investigate the effect of participant age on MRCPs, participants were divided into three age bins (6-8.99 years, 9-11.99 years, and 12-15 years), and group averages were constructed by averaging individual participant traces from each age bin (Figure 5 ). Omnibus GLMMs revealed a 3-way interaction between group, electrode, and age, indicating that the developmental trajectory of sensorimotor maturation may differ as a function of participant group (Late BP: F(2,3818.017) = 24.442, p < 0.001; MP: F(2,3818) = 3.978, p = 0.019; RAP: F(2,3818) = 12.553, p < 0.001). Visualizations of this 3-way interaction are provided in figure S2A-C . Group differences are largest in the late BP period. For all components of interest, group differences were largest between the youngest group of participants.
To visualize MRCP modulation by age, average traces from each age bin are displayed for each group. Representative electrodes are shown from left frontal (5B ), frontal (5C ), left parietal(5D) from TD participants (left column) and ASD participants (right column) separately.
Statistical cluster plots were computed for each group separately to identify regions showing significant differences as a function of age across three age bins: 6-8.99 years, 9-11.99 years, and 12-15 years (5A ). Significant age-related differences were observed across fronto-central and parietal scalp sites between 25 and 200 ms post-response onset for both TD (left column) and ASD (right column) participants. Additional differences were present in ASD participants between -250 and -100 ms over left fronto-central and left central scalp sites, and between -150 and -50 ms over the left fronto-central and right parietal scalp (5A ).
Group differences in age-related effects were explored using statistical cluster plots and topographic visualizations (Figure 5E-G ). Representative topographic plots from 50 ms time bins encompassing -125 ms to -75 ms (5E ) and from 25 to 75 ms (5G ) provide an illustration of age-related differences in topography between ASD and TD participants. For participants aged 6 – 8.99, significant and widespread differences emerged over fronto-central scalp between -300 and -100, and over right central-parietal scalp (5F.1 ). For children aged 9 – 11.99, significant differences emerged within left temporal electrodes between -300 and -100 ms prior to response-onset (5F.2 ). For participants between 12 – 15 years, significant differences were present over the central scalp between -350 and -100 ms prior to the button press, and again over the fronto-central and right parietal scalp between 0 and 100 ms following the response (5F.3 ).