Electrical Perspectives on Magnitude Processing: Investigating the Classical and Numerical Delboeuf Illusion

The human experience is inherently intertwined with the faculty of vision, a remarkable cognitive process that allows us to perceive, interpret, and interact with the world. As an integral aspect of daily life, vision forms the bedrock of our understanding and communication. Since there is little conclusive evidence, there is ongoing discussion regarding the existence of a single magnitude system that includes non-symbolic number estimates in addition to other magnitudes like time and space. To clarify this further, recent research used visual illusions such as the Delboeuf illusion to examine if biases impacting spatial decisions also affect assessments of numerosity. In this study, using innovative experimental paradigms and transcranial alternating current stimulation (tACS), we attempt to investigate the possible existence of a general order-of-magnitude system that assesses whether the same perceptual bias similarly affects the processing of different magnitudes. According to previous literature, we hypothesized that theta-frequency tACS over the parietal cortices would improve visual integration and enhance the illusion. In contrast, beta-frequency tACS reduces the strength of the illusion, increasing visual discrimination. Participants underwent three tACS sessions (7 Hz, 18 Hz, and control/no stimulation) while performing a series of discrimination tasks involving the classical and numerical Delboeuf Illusion.