On the origin of the Helmholtz’s square illusion: An attentional account

Abstract

A square filled with parallel horizontal or vertical lines appears perceptually extended in the direction orthogonal to the lines. Here, we suggest that this Helmholtz illusion arises due to changes in spatial attention that entail changes at very early stages of perceptual processing. Three experiments are reported which tested this assumption. In Experiment1 and Experiment2, transient attentional cues were flashed in such a way that they either promoted (congruent condition) or hindered (incongruent condition) the attentional state presumably induced by the target objects. We predicted a decline of the illusion in the incongruent condition compared with the congruent condition. This prediction was confirmed in both experiments. However, the influence of (in)congruent attention cuing on the Helmholtz illusion depended on more sustained distributions of attention as well. An influence of sustained attention on the illusion was confirmed in Experiment 3, in which changes of attentional focus were induced by a secondary task. Overall, the results were consistent with our claim that the origin of the Helmholtz illusion is closely linked to the distribution of spatial attention.

An attentional approach to geometrical illusions

“…Illusion shown in Figure 6C seems to reflect a similar phenomenon – perceptual expansion of space containing stimuli of higher spatial frequency. In a recent study of the Helmholtz square illusion, we observed results that supported this attentional approach ( Kirsch and Kunde, 2023 ; see also see also Section 6.1).…”

“…Based on these (and similar) results, we reasoned that the origin of some geometrical illusions is closely related to spatial attention. So far, we found preliminary evidence for this claim for the Ebbinghaus, Helmholtz’ square and Ponzo figures ( Kirsch and Kunde, 2021b , 2023 , 2024 ). Encouraged by these results, we here aimed to take a closer look at whether and how attention could be responsible for geometrical illusions in general.…”

“…In case of the Helmholtz square illusion ( Figure 6B ), we assumed that the attentional field is compressed orthogonal to the direction of the lines ( Kirsch and Kunde, 2023 ). We varied the shape of the attentional field in an exogenous and an endogenous attentional task and tested how this affects the illusion.…”