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Autism spectrum disorder (ASD) is a neurodevelopmental disorder emerging in the first few years of life. Currently, three characteristics are required for a diagnosis of ASD, impaired social interactions, impaired verbal communication and restricted and repetitive patterns of behaviour or interests. This last category can optionally include hyper- or hypo-reactivity to sensory input. Individuals with autism can also display superior performance on visual tasks where it may help to ignore global detail, behaviour sometimes described as ‘not seeing the forest for the trees’. At present, the exact mechanisms underlying the perceptual differences between autistic and neurotypical groups remain unknown, but they may reflect an imbalance in the contributions that bottom-up and top-down processing make in perceptual processing. Visual perception is thought to rely on interactions between the ‘bottom-up’ flow of ambiguous information from the retina and the ‘top-down’ flow of disambiguating information from higher cortical areas, via cortical circuits that have been shaped by a lifetime’s experience. These interactions lead to the activation of internal representations (of objects) which are necessary for the successful navigation of our environment. In order to investigate these perceptual differences, we employed three well-known experimental paradigms with a group of thirteen autistic participants and their matched controls. We investigated visual integration (involving bottom-up and top-down interactions) across low and intermediate stage neural mechanisms. A dim line (target) is easier to detect when flanked by two brighter collinear lines (flankers), an effect known as collinear facilitation, and we used two variations of this task to investigate low-level visual integration. In the first, we varied the orientation of the collinear flankers and found reduced integration for an autistic compared to a neurotypical group, a finding that conflicts with previous research. In a second collinear facilitation experiment with neurotypical participants, in which the target could be presented before, during or after flanker presentation, we were able to isolate facilitation that we believe was due to feedforward and feedback processing. However, in a subsequent study in which we compared autistic and neurotypical performance on this task, we found no significant difference. Moving onto intermediate level visual integration, we used a contour integration task consisting of open (lines) and closed (square) contours and found reduced integration for the autistic compared to the neurotypical groups when integrating closed contours. In our final study, we looked at global motion integration, and made use of a translating diamond. This is a bistable stimulus in which four lines can be perceived as independent line fragments moving vertically, or as a single integrated shape - a diamond moving horizontally. In this experiment, the autistic group showed an unexpected bias to perceiving the stimuli in its integrated form as a diamond. Perceptual processing of shapes based on squares or diamonds reflects visual integration at a global level, and so the differences we have found in shape processing between our experimental groups (reduced integration for the square and increased integration for the diamond in autism) are more likely to be the result of differences in top-down processing. |
