Selective attention and asymmetry in the Müller-Lyer illusion

Predebon, John · 2004 · OpenAlex-citations

DOI: 10.3758/bf03196721

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Summary

This paper investigates how selective spatial attention modulates the magnitude of the Müller-Lyer (M-L) illusion, specifically addressing the asymmetry observed in superimposed (XX) versions of the figure. The M-L illusion, consisting of wings-in (underestimated) and wings-out (overestimated) forms, is often used to debate whether perceptual distortions arise from low-level visual mechanisms or high-level cognitive processes. Previous research using the XX figure—where both wing types are superimposed—showed that ignoring outer wings significantly reduced illusion magnitude, while ignoring inner wings had little effect. This asymmetry was previously attributed to difficulties in perceptually segregating inner wings from the shaft (an object-based account). The current study tests this segmentation hypothesis against space-based theories of attention, which suggest that the spatial extent of the attentive field makes it easier to ignore peripheral (outer) elements than central (inner) ones. The study employed two experiments with undergraduate participants who judged the length of a central shaft by adjusting a comparison line. Experiment 1 used colored stimuli to distinguish wing components, allowing subjects to attend to one color while ignoring another. Conditions included neutral viewing, selective attention to the shaft while ignoring wings in standard M-L figures, and selective attention to either inner or outer wings in the XX figure. Experiment 2 served as a control to ensure that color differences alone did not account for the reduction in illusion magnitude observed in Experiment 1. Results from Experiment 1 confirmed the asymmetry in the XX figure: ignoring outer wings produced significant underestimation of shaft length, whereas ignoring inner wings had no significant effect. For standard M-L figures, ignoring wings attenuated the illusion, but the effect was significantly stronger for the wings-out form than the wings-in form. Experiment 2 demonstrated that using colored stimuli did not inherently reduce illusion magnitudes compared to black-and-white stimuli, ruling out color as a confounding variable. The findings support space-based approaches to visual attention, such as Pressey’s attentive-field theory, over object-based segmentation accounts. The authors argue that the asymmetry arises because the shaft defines the minimum diameter of the attentive field, making it easier to exclude outer wings from processing than inner wings. Consequently, the study concludes that attentional modulation of the M-L illusion can be explained by external factors related to the sampling of visual input and the spatial deployment of attention, rather than necessitating high-level cognitive explanations. This suggests that the illusion’s formation involves interactions between spatial attention mechanisms and low-level visual processing.

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