Mirko Tommasini¹, Sara Battisti¹, Romeo M. Minutolo¹, Giulia Tonielli¹, Simona Noviello¹, Juan Chen³, Melvyn A. Goodale², Irene Sperandio¹; ¹University of Trento, Trento, Italy, ²University of Western Ontario, ³South China Normal University, Guangzhou, China

Size constancy is the ability to maintain a stable percept of object size despite variations in the retinal image due to changes in viewing distance. Recent research using real-world objects at real distances has demonstrated that this phenomenon emerges at the earliest stages of cortical processing. It remains unclear, however, whether these findings are applicable to both 3D and 2D stimuli. Here, participants were presented with either 3D or 2D stimuli placed at different distances and asked to perform a manual size estimation using their right thumb and index finger. The stimulus physical size was scaled with respect to distance to yield a constant retinal angle. Concurrently, electroencephalographic (EEG) data were recorded using a 64-channel scalp electrode array. Results revealed an advantage for real objects in the computation of size constancy, as indicated by an earlier difference in neural responses to small versus large stimuli, observable in the first positive-going component, peaking at ~80 ms after stimulus onset. In contrast, size constancy for 2D stimuli emerged approximately 150 ms after stimulus onset. Furthermore, stimulus predictability played a role in enabling faster size-distance integration. These findings provide electrophysiological evidence for a ‘real-object advantage’ in size constancy. This advantage may be partially explained by top-down mechanisms, such as affordance—the potential to physically interact with an object—which could enhance the perceptual processing of real 3D objects relative to 2D representations. Additionally, 3D objects may provide visual cues to distance that are not available in 2D stimuli, further contributing to size constancy.