The Ebbinghaus illusion involves two identical circles perceived as different in size based on the surrounding shapes/inducers' size and number. Larger inducers make the central circle appear smaller while smaller inducers have the opposite effect. Despite extensive research, a unified explanation remains elusive. While size contrast theories fail to explain stronger effects with shape-matched inducers, Kirsch et al. (2018) propose that the illusion stems from attentional allocation, with greater receptive field recruitment resulting in bigger perceived sizes. However, due to the complexity of the illusion's features, this theory lacks neural data.
We will address these challenges using Steady-State Visual Evoked Potentials (SSVEPs) as an attentional correlate (Norcia et al., 2015) and a dynamic Ebbinghaus variant (Mruczek et al., 2015) which amplifies the circle’s perceived size difference from 19.9% to 37.3%. In this version, the main circle moves steadily along the screen while the inducers dynamically change size, enhancing both perceptual effects and thus signal-to-noise ratio. Frequency tagging of the main circle with luminance-based SSVEPs at 24 Hz and two Rapid Invisible Frequency Tagging (RIFT) at 60 Hz with a white and gray background (rendering the main circle invisible), will allow us to isolate attentional mechanisms and provides a robust control condition.
The experiment includes 960 trials per frequency (4 spatial locations × 2 transition directions × 45 trials), with participants rating perceived size changes on a 3-point scale. This design explores the attentional allocation theory, temporal dynamics, grouping effects, and offers a novel EEG-based framework for studying size illusions.