3.4. MRCPs by RT
To understand the relationship between MRCPs and RT (whether a
particular response elicited a prompt or a slow response), individual
participant data trials for each individual were binned by RT quartile
into the fastest 25% of trials, middle 250-50%, middle 50-75%, and
slowest 25%. Group averages were constructed by averaging individual
participant traces for each RT quartile (Figure 4 ). Omnibus
GLMM analyses indicated a three-way interaction between participant
group (ASD vs. TD), RT quartile, and electrode site (Fz vs. P3) for all
three time windows of interest (Late BP: F(6,3818) = 2.284, p = 0.034;
MP: F(6,3818) = 9.161, p < 0.001; RAP: F(6,3818) = 3.466, p =
0.002) (Table 3 ). Visualizations of these three-way
interactions are given in Figure S2D-F . For each RT bin at each
electrode site (Fz and P3), at each timewindow of interest (BP, MP,
RAP), individuals with ASD show lower-amplitude cortical responses
relative to TD. For the MP, an additional 3-way interaction between
group, RT quartile, and age was identified, indicating that RT
differences in MP amplitude were most prominent in the youngest ASD
participants and are most clearly visible in the slowest 25% of trials.
To visualize MRCP modulation by RT, group average traces for ASD and TD
participants were constructed by averaging individual participant traces
for each RT quartile (Figure 4 ). Statistical cluster plots
depicting within-group differences as a function of RT bin are shown
separately for TD (4A ) and ASD (4B ) participants.
Representative electrodes are shown from right frontal (TD: 4C ,
ASD: 4D ) and left parietal ( TD: 4E, ASD:4F ). In general, faster RTs were associated with larger
amplitude responses for both participant groups.
For each RT quartile, SCPs were computed to contrast ASD and TD
participants (4I ), and scalp topographies were rendered to
visualize results (4G, 4H, 4J ). For the fastest 25% of trials,
significant differences between ASD and TD participants were evident
between -75 and -25 ms over fronto-central, parietal, and
parieto-occipital scalp regions, as well as between 50 and 100 ms after
the response over fronto-central, left temporal, and left parietal scalp
regions (4I.1 ). For trials showing an intermediate RT between
25-50th percentile, significant differences emerged
between -150 and -50 ms over fronto-central, left parietal, and right
parietal scalp regions, and after the response between 0 and 50 ms
primarily over the fronto-central scalp (4I.2 ). For trials with
an intermediate RT between the
50th-75th percentile, between-group
differences emerged primary between -200 and -100 ms prior to the button
press over left parietal and bilateral occipital scalp sites
(4I.3 ). Minimal differences were observed between groups for
the slowest 25% of trials (4I.4 ). Topographic maps are
presented for each RT quartile and group separately from -150 to -100 ms
(4G ), -25 to -75 ms (4H ) and 0 to 50 ms (4J )
to visualize group effects.