Enhanced orientation discrimination and higher peak gamma frequency in autism spectrum conditions.

This thesis investigates sensory processing in autism spectrum conditions (ASC), specifically focusing on low-level visual perception. Previous literature indicates that sensory problems are prevalent in ASC, and affect individuals across a range of modalities and in a variety of ways.

Study one of this thesis examines a low-level visual process, orientation discrimination, in neurotypical individuals (n=94) and demonstrates that orientation discrimination thresholds are lower (enhanced) in those with higher levels of autistic traits compared to those with lower levels of autistic traits. Study two (n=96) confirms that enhanced orientation discrimination is also present in individuals with an autism spectrum diagnosis, as individuals with an ASC show significantly lower orientation discrimination thresholds than matched control participants.

As orientation discrimination is closely linked to neural inhibition, the possibility that disruption in the balance of neural excitation and inhibition (E/I) may be present in those with enhanced orientation discrimination was investigated in studies three and four. Peak gamma frequency, a metric said to capture the balance of neural E/I, was measured in both individuals with higher levels of autistic traits and those with a clinical ASC diagnosis. The results of study three (n=33) and study four (n=80) show that visually-induced peak gamma frequency is higher in individuals with higher levels of autistic traits, and those with an ASC diagnosis.

This thesis employs both a psychophysical measure and a neural measure which tap into neural inhibition. In the context of previous literature (e.g. Muthukumaswaramy et al., 2009) the results of the research presented in this thesis suggests, in direct contrast to the increased excitation model of ASC (Rubenstein & Merzenich, 2003), that neural inhibition levels may actually be increased in at least some individuals with ASC.