Feedforward Processing in Early Visual Cortex Underlies the Ebbinghaus Illusion

Low-level occipital regions, including the primary visual cortex (V1), are known to play a pivotal role in generating the Ebbinghaus size illusion, in which a circle surrounded by small inducers looks larger than an identical circle surrounded by large inducers. However, a definitive answer is still lacking as to whether the low-level visual cortex plays its role at the feedforward stage of processing. To resolve this issue, we measured event-related potentials (ERPs) evoked by two equal-size circles flashed on the left and right sides of human participants, with one side constantly displaying large surrounding inducers and the other side small inducers. Experiment 1 unveiled a significant lateralization in the evoked C1 component, a well-established ERP index of afferent V1 activity, only when the circle within small inducers was perceived as larger but not when the circle within large inducers was perceived as larger. This illusion-specific C1 lateralization, which closely resembled the C1 modulation evoked by physical differences in size, was further replicated in Experiment 2 even when the attentional bias toward small inducers was invalidated as an alternative account. Furthermore, Granger causality analysis revealed a one-way transfer of information from V1-representative electrodes to extrastriate-representative electrodes during the C1 lateralization. Together, these findings offer the strongest evidence to date that the Ebbinghaus illusion begins at the feedforward stage of cortical visual processing, consistent with the prediction of the contour interaction theory.