Vertical but Not Oblique Line Orientation Discrimination Is Disturbed in ASD Children

Background: Neurophysiological studies suggested that excitation/inhibition (E/I) imbalance might underly visual perception abnormalities observed in some children with Autism Spectrum Disorder (ASD). Behaviorally, the E/I imbalance in visual cortex can affect the “oblique effect” - better discrimination ability for cardinal compared to oblique line orientations. Previous studies on typical developing subjects (TD) failed to find association between autistic traits and cardinal orientation discrimination thresholds, but found it for lines with oblique orientation (Brock et al., 2011, Dickinson et al., 2014). No studies examined the “oblique effect” in ASD children.

Objectives:  The aim of our study was to examine the orientation discrimination threshold for oblique and cardinally orientated lines in TD and ASD children.

Methods:  Subjects were 16 ASD boys and 32 TD boys, aged between 6 and 15 years with IQ > 71: intellectual ability was assessed by the Kauffman Assessment Battery (Kauffman&Kauffman, 2004). The experimental procedure was similar to those used by Dickinson et al., 2014.  The orientation of the first grating was held fixed at 0° in the “vertical” condition and 45° in the “oblique” condition, the second grating was rotated either clockwise or counterclockwise. The orientation difference between the gratings was adjusted using two interleaved one-up two down staircases that converged on 71% correct performance. Participants were asked to judge the direction of this rotation. The logarithm of orientation discrimination threshold for vertical and oblique lines assessed in degrees was taken as a dependent variable.

Results: The ANCOVA with Orientation (Vertical vs. Oblique) as a within-subject factor, Group (ASD vs. TD) as a between-subject factor, and Age and IQ as the covariates was applied to the data. The effects of interaction Orientation*Group and Orientation*Age were significant, and the univariate ANCOVA was further applied separately for Vertical and Oblique conditions. For the Oblique condition the only significant effect was the effect of Age: the oblique lines’ orientation discrimination improves with age in both groups. Noteworthy, the ASD and TD groups had similar oblique discrimination thresholds (10.8±1.5º vs. 9.0±0.6º). However, the significant Group effect was present in the Vertical condition: ASD boys had enhanced discrimination threshold as compared to TD boys (4.7±1.1º vs. 1.4±0.1º). The significant Group*Age interaction suggested that difference between ASD and TD diminished with age. 

Conclusions:  Our study provides evidence for a reduced “oblique effect” in ASD boys comparing to their TD peers. Unexpectedly, this was due to ASD difficulties with orientation discrimination of vertical lines, whereas the threshold for oblique line orientation did not differentiate ASD and TD boys. Previously, it was shown, that “oblique effect” increases with age, potentially reflecting the tuning of neuronal network to the environmental statistics through GABAergic modulation (Liang et al., 2007, Girshick et al. 2011, Shen et al., 2014). Therefore, the deficits in vertical orientation processing, seen in ASD children, might represent the E/I imbalance, which delays adequate visual system tuning and reduces the natural bias toward cardinal orientation.