On the Spectrum

Chapter 8
gyrification changes in postnatal life as well (White, Su, Schmidt, Kao and Sapiro 2010). In this
thesis, we found widespread reductions in gyrification to be associated with autistic traits
and we found a trend toward the same effect in a small subset of children that met criteria
for ASD according to the golden standard instruments. This association held in a subset of
regions when we excluded the most severely affected children, indicating a true continuum
in the neurobiology. Our finding of reduced gyrification is in line with some studies of young
children with ASD (Schaer, Ottet, Scariati, Dukes, Franchini et al. 2013; Bos, Merchan-Naranjo,
Martinez, Pina-Camacho, Balsa et al. 2015). However, there have also been studies that found
the opposite (Kates, Ikuta and Burnette 2009; Wallace, Robustelli, Dankner, Kenworthy, Giedd
et al. 2013). This discrepancy can partly be understood from differences in methodology, as
well as gender, age and IQ of the study sample. However, a recent clinical study in children of
a similar age range, using the same methodology reported the opposite finding in children
with ASD (Yang, Beam, Pelphrey, Abdullahi and Jou 2016). Considering this heterogeneity, we
cannot be sure of the nature of gyrification differences in young children with ASD.
One of the theories of gyrification suggests that it reflects connection strength between
spatially separated regions of the brain (Van Essen 1997), and two previous studies that
combined gyrification and white matter measurements revealed white matter disruptions
alongside reduced gyrification in children with ASD (Schaer et al. 2013; Bos et al. 2015).
Based on our previous findings of reduced gyrification, we hypothesized a continuum in
white matter microstructure related to autistic traits, so that white matter differences related
to autistic traits would mimic those most commonly reported in children with ASD. Studies
of children with ASD show alterations in white matter microstructure, most notably in tracts
that facilitate long-range connections: corpus callosum, the left uncinated fasciculus, and the
left superior longitudinal fasciculus (Aoki, Abe, Nippashi and Yamasue 2013). We detected
a localized association between autistic traits and lower white matter integrity in a small
region in the superior longitudinal fasciculus. In this long-range white matter tract, we
found a continuous association with autistic traits, that remained after excluding children
with ASD. However, we did not find major differences in all of the hypothesized long-range
white matter connections in these children. White matter integrity in some of these tracts
may not relate to this quantitative measure of social impairment, thus disruptions may occur
more exclusively at the severe end of the spectrum. Alternatively, associations may only be
revealed in samples where children generally have more severe symptoms.
Our findings in functional connectivity offer more support for the conceptualization of
ASD as a ‘disconnection syndrome’, where symptoms are thought to arise from disturbances
in neural circuitry (Geschwind and Levitt 2007). Using a novel type of resting state fMRI
analysis, where assumptions of stationarity of functional connectivity over time were
relaxed, we found that children with more autistic traits showed alterations in these dynamic
connectivity characteristics. Specifically, they spent less time in the more heavily connected,
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