Mapping object space dimensions: New insights from temporal dynamics

How is object information organized in high-level visual cortex? A recent comprehensive model of object space in macaques defines object space via orthogonal axes of animacy and aspect ratio (i.e., stubby vs. spiky) (Bao et al., 2020). However, when using object stimuli that dissociated category, animacy, and aspect ratio in human fMRI, object space appeared to be principally defined by category and animacy, with limited tuning of aspect ratio in object-selective regions (Yargholi & Op de Beeck, 2023). Here, we aimed to further clarify the contribution of aspect ratio during object processing by using whole-brain electroencephalography (EEG) to systematically investigate the time course underlying aspect ratio, animacy, and category information during visual object processing. Participants (N = 20) viewed the stimulus set used by Yargholi and Op de Beeck (2023), as well as silhouette versions of the stimuli that lacked internal object details (thus increasing reliance on shape information). Stimuli appeared in 5 Hz rapid serial visual presentation streams, with intact and silhouette stimuli sets shown in separate streams. Using standard multivariate decoding pipelines and representational similarity analysis, we found that information about aspect ratio, category, and animacy was represented during visual object processing. The dominant dimension was modulated by stimulus type, demonstrating that the observable dimensions of object space depend on the nature of the stimuli presented. Taken together, these findings demonstrate that aspect ratio is indeed represented during object processing, but earlier and more transiently than categorical dimensions, such as animacy. By focusing on underlying temporal dynamics, our results provide a more nuanced understanding of how object space evolves over time that can speak of how extant findings on this topic might be reconciled.