  Over the last years, increasing evidence has fuelled the hypothesis that
Autism Spectrum Disorder (ASD) is a condition of altered brain functional
connectivity. The great majority of these empirical studies rely on functional
magnetic resonance imaging (fMRI) which has a relatively poor temporal
resolution. Only a handful of studies have examined networks emerging from
dynamic coherence at the millisecond resolution and there are no investigations
of coherence at the lowest frequencies in the power spectrum - which has
recently been shown to reflect long-range cortico-cortical connections. Here we
used electroencephalography (EEG) to assess dynamic brain connectivity in ASD
focusing in the low-frequency (delta) range. We found that connectivity
patterns were distinct in ASD and control populations and reflected a double
dissociation: ASD subjects lacked long-range connections, with a most prominent
deficit in fronto-occipital connections. Conversely, individuals with ASD
showed increased short-range connections in lateral-frontal electrodes. This
effect between categories showed a consistent parametric dependency: as ASD
severity increased, short-range coherence was more pronounced and long-range
coherence decreased. Theoretical arguments have been proposed arguing that
distinct patterns of connectivity may result in networks with different
efficiency in transmission of information. We show that the networks in ASD
subjects have less Clustering coefficient, greater Characteristic Path Length
than controls -indicating that the topology of the network departs from
small-world behaviour- and greater modularity. Together these results show that
delta-band coherence reveal qualitative and quantitative aspects associated
with ASD pathology.
