Electroencephalography reveals late-stage modulation of stereoscopic object processing by naturalness and real-world size.

Stereoscopic object categorization is influenced by both low-level visual features and high-level contextual attributes. Here, we investigated how object naturalness and real-world size affect behavioural and neural responses during object categorization when visual input is restricted to stereoscopic depth information. In Experiment 1, participants performed a behavioural task to judge (as quickly as possible) whether stereoscopic objects, defined solely by binocular disparity and without texture or colour cues, were 'natural' or 'man-made'. The stimuli presented varied in terms of their naturalness (natural vs. man-made) and real-world object size (small vs. large). Results showed that natural objects were categorized faster than man-made ones within the large-object grouping, while small man-made objects were categorized faster than larger man-made objects. In Experiment 2, neural responses were measured using EEG during concurrent behavioural judgments. While no significant differences were observed in early ERP components (P1, N2, P300), multivariate pattern analysis revealed distinct neural representations of object categories emerging at later stages of processing, particularly around 600 ms post onset. These findings suggest that stereoscopic object categorization is influenced by intrinsic stimulus attributes, including semantic category (naturalness) and real-world object size, and that these effects emerge at relatively late stages of visual processing. This late timing likely reflects higher-order integrative processes rather than early sensory encoding.