<div>Figure 2. A) Proportion of /da/ responses per group as a function of SOA
for congruent and incongruent trials. Here, negative SOAs indicate that
the voice was leading the lip movements, and vice versa. B) Proportion
of /da/ responses per group as a function of age-bin (bin size is 10
years) for incongruent trials (collapsed across SOAs). The error-bars
reflect the standard error of the mean.</div><div>We conducted a repeated measures ANOVA on the mean proportion of /da/
responses with SOA and congruency as within subjects variables and group
as a between subject factor using Just Another Statistical Program
(JASP) (Love et al., 2019). Here, and elsewhere in the manuscript, alpha
was set to .05, and <i>p</i> values were Hyunh-Feldt corrected to avoid
sphericity violations. As seen in Figure 2a, the rates of /da/ responses
varied significantly as a function of SOA (<i>F</i> (1,867) = 243.632,<i>p</i> &lt;.001) and congruency (<i>F</i> (1,867) = 529.059,<i>p</i> &lt;.001). What is more, there was a significant SOA ×
congruency interaction (<i>F</i> (1,867) =274.805,<i>p</i> &lt;.001), such that the McGurk/MacDonald illusion
occurred most frequently in incongruent trials with a slight visual
lead. However, follow-up two-tailed t-tests confirmed that there was a
significant congruency effect (and, hence, McGurk/MacDonald effect) at
all SOAs (minimum <i>t</i> (868) =10.471, all <i>p</i> &lt;.001).</div><div>Between the groups, autistic participants experienced the illusion more
frequently (<i>F</i> (1,867) = 11.448, <i>p</i> &lt;.001). There
were also interactions between group × congruency (<i>F</i> (867)=9.344,<i>p</i> =.002), group × SOA (<i>F</i> (867)=12.012,<i>p</i> =&lt;.001), and group × congruency × SOA
(<i>F</i> (867)=14.662, <i>p</i> &lt;.001). However, due to the
disparity in the age distributions between groups (see Table 1), we
conducted an exploratory analysis with age as a covariate, and found
that the group effect was no longer significant, nor were any of the
previously significant interactions including group. Instead, we
discovered a strong effect of age (<i>F</i> (1,866)=54.194,<i>p</i> &lt;.001), wherein older participants from both groups
tended to experience the illusion more frequently than younger ones (see
Figure 2b). Age also interacted with SOA (<i>F</i> (1,866)=37.815,<i>p</i> &lt;.001), congruency (<i>F</i> (1,866)=50.695,<i>p</i> &lt;.001), and SOA × congruency (<i>F</i> (1,866)=38.530,<i>p</i> &lt;.001), augmenting all of their effects. Further
evidence that age (rather than group) explains the pattern of results
above is provided using age matched and Bayesian approaches in the
Supplementary Materials (see Supplementary Tables 1 and 2).</div><div>Note that this section focused on the /da/ responses, as they reflect
the classic McGurk/MacDonald illusion, and rates of visual capture (/ga/
responses in incongruent trials) were extremely low for both groups. For
transparency, statistics and figures reflecting the rates of /ba/ and
/ga/ responses can be found in the Supplementary Materials.</div><div><i>Simultaneity judgments.</i> Figure 3a plots the mean proportion of
simultaneity judgments as a function of SOA and congruency for both
groups. Figure 3b shows the mean proportion of simultaneity judgments
collapsed across SOAs for both groups and congruency conditions as a
function of age, divided into bins of 10 years.</div>