Temporal dynamics of size constancy reveals an advantage for real-world stimuli

Size constancy is the perceptual ability to maintain a stable representation of an object’s 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 (Noviello et al., 2023). However, it remains unclear whether these findings are applicable to both three-dimensional (3D) objects and two-dimensional (2D) stimuli. Notably, the early cortical involvement in size constancy may be influenced by top-down mechanisms, such as affordance—the potential to physically interact with an object in a meaningful manner—which could enhance the perceptual processing of real 3D objects relative to 2D representations. In this study, participants were presented with both 3D and 2D stimuli placed at different viewing distances and tasked with performing a manual size estimation using their right thumb and index finger. The physical size of the stimuli was scaled according to distance to yield a constant retinal angle. Concurrently, electroencephalographic (EEG) data were recorded using a 64-channel scalp electrode array. The results revealed an advantage for real objects in size constancy, as indicated by an earlier disparity in neural responses to small versus large stimuli, observable in the P1 component. In contrast, size constancy for 2D stimuli emerged approximately 150 ms after stimulus onset. These findings provide electrophysiological evidence for a ‘real-object advantage’ in size constancy.