Three Facets of Visual Orientation Processing in ASD

Background: It has been suggested that altered visual perception, such as superior visual abilities, underlies some of the social difficulties associated with the Autism Spectrum Disorder (ASD) phenotype (Dakin & Frith 2005; Behrmann, Thomas, & Humphreys 2006). The precise source for these atypicalities, however, remains a source of debate. The crux of the argument is whether the differences arise at the level of basic, low-level visual processing or further downstream at stages involving more complex, higher-level tasks and stimuli. A recent study has shown visual acuity in ASD to be equivalent to that of neurotypical controls. This suggests the earliest levels of retinal processing to be an unlikely candidate as the source of differences (Tavassoli et. al. 2011). The next potential level for divergent visual processing are those involved in processing simple aspects of visual stimuli, such as orientation and spatial frequency, considered to be functions of early visual cortical processing. One study supporting the low-level view of atypical perceptual abilities found that participants with ASD were superior in identifying the orientation for simple, first-order gratings at cardinal angles (Bertone, Mottron, Jelenic, & Faubert 2005). Yet, other, more rudimentary aspects of orientation perception, such as detection and discrimination, have not been assessed systematically within this population. Characterizing differences in the most basic visual abilities is necessary to determining the origins of these variances within the ASD phenotype, and what impact these alterations have for more complex levels of visual perception.

Objectives: The goal of this study was to characterize (i) sensitivity, (ii) precision, and (iii) accuracy for perceiving orientation in the same cohort of participants with ASD in comparison to age- gender- and IQ-matched neurotypical controls. Each experiment allows for both qualitative and quantitative assessment of the oblique effecti.e., the typical finding of superior performance at cardinal and as compared to oblique orientations.

Methods: In three psychophysical experiments we measured (i) contrast detection thresholds (ii) orientation discrimination thresholds and (iii) biases in perceived orientation of a 3 cycles/degree Gabor patch of varying base orientation.

Results: For all three experiments, our results indicated that participants with ASD perceive orientation of low-level stimuli in a qualitatively as well as quantitatively similar manner to neurotypical controls. Oblique effect was evident in both groups: at cardinal orientations (i) stimuli were easier to detect  (ii) precision was higher and (iii) perception was veridical as opposed to oblique orientations where perception is found to be biased.  Additionally, performance in all three tasks were quantitatively indistinguishable in the two groups of participants. We found no evidence of superior processing in detection, precision, and accuracy aspects of orientation perception in participants with ASD.

Conclusions: Our findings suggest that low-level visual processing of orientation is unremarkable for people with ASD. These results suggest that the source for altered perceptual abilities should be sought elsewhere, possibly in subsequent levels of visual processing.