Components of visual perceptual organisation in ASD: An overview of behavioural and fMRI evidence using Gabor patterns

Background: An atypical perceptual style, with a more locally oriented default processing level and an attenuated bias for global processing, is often described in ASD. Although extensive research has been conducted to pinpoint the specific differences with typical development, the literature is highly mixed and therefore difficult to integrate into clear-cut findings. Several stimulus- and task-related aspects appear to contribute to this diversity in research findings. 

Objectives: We performed a series of studies to evaluate the impact of stimulus- and task-related characteristics on visual perceptual organisation in individuals with ASD and typically developing controls. In particular, Study 1 and 2 investigated the influence of task instruction on perceptual organisation, Study 3 investigated the interplay between different processes involved in object identification, and Study 4 investigated the neural underpinnings of perceptual organisation in terms of texture segmentation and contour integration. All studies have in common that they used the same type of stimuli.

Methods: Three behavioural studies and one functional magnetic resonance imaging (fMRI) study were carried out. Participants were children and adolescents with ASD and well-matched typically developing controls. All four studies used stimuli composed of Gabor patches, which are highly controllable elements inspired by the tuning properties of visual neurons. In Study 1 and 2 we investigated the influence of (explicit versus implicit) task instruction on perceptual organisation of meaningless stimuli. In Study 3 we used dynamic Gabor patterns that gradually evolved from random towards organised to investigate contour- and texture-based object identification. In Study 4, dynamically evolving Gabor patterns were used to investigate the neural mechanisms behind texture segmentation and contour integration of nonexistent and everyday objects using fMRI.

Results: The behavioural studies demonstrated the presence of certain task- and stimulus-specific effects in both participant groups (e.g., a beneficial effect of symmetry upon bottom-up perceptual grouping). Differences between participant groups were subtle and depended on task demands and stimulus characteristics. Whereas individuals with ASD performed similarly to controls in a simple object detection task (Study 4), they performed more poorly in object identification, especially when more complex objects were presented (Study 3) or when implicit task instructions were given (Study 1). The fMRI findings revealed that more structured Gabor patterns systematically activated hierarchically higher areas along the ventral visual stream. However, the pattern of brain activity was similar for both groups.

Conclusions: No overall problems in visual processing were found in ASD. Instead, we tapped into some subtle aspects of the (a)typical visual processing style of individuals with ASD and showed the differential effect of task demands and stimulus characteristics on perceptual organisation. This set of paradigms where participants are required to make sense of  changing visual input with variable levels of noise and structure, suggests that the peculiarities in ASD are situated in the interplay between bottom-up and top-down processes. Thus far, the brain imaging data did not provide evidence for atypical neural processing in ASD, but the subtraction-based analyses will be complemented with structural and functional connectivity analyses, evaluating the efficiency of communication between brain regions.