From visual masking to ASD P A van der Helm |
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Laboratoratory of Experimental Psychology, University of Leuven (KU Leuven), Netherlands
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| Visual masking of features in a stimulus may occur when another stimulus is presented just before or after it. Post-hoc, forms of masking are defined in spatio-temporal terms - such as forward masking (or paracontrast masking if without spatial overlap, or priming in case of a negative masking effect) and backward masking (or metacontrast masking if without spatial overlap). Processing mechanisms in the visual hierarchy in the brain suggest, however, that structural relationships between both stimuli determine whether masking occurs, and if so, in which form. These mechanisms suggest (a) that global structures are represented at higher levels in the visual hierarchy, (b) that they emerge via bottom-up integration of local features represented at lower levels, and (c) that, consequently, top-down attention has to pass through global structures to arrive at local features. This mechanistic view is argued to provide a promising framework to explain masking phenomena in normal vision. Furthermore, it suggests that the "local advantage" phenomenon in ASD can be explained by (individually varying) impairments in the perceptual integration mechanism: such impairments hamper the emergence of global structures so that top-down attention has less trouble arriving at local features. |
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