Peripersonal and extrapersonal space encoding in virtual reality: Insights from an fMRI study

The brain processes objects in reachable peripersonal space and non-reachable extrapersonal space in different neural networks. In contrast to extrapersonal space, spatial processing in peripersonal space is linked to the activation of affordances in dorsal visual pathways. However, the literature on how object characteristics like size, graspability and stereoscopic presentation influence object processing in virtual environments is still unclear. In the current study, 44 healthy participants performed a visual discrimination task involving graspable objects presented in peripersonal space and extrapersonal space. The paradigm was presented via MRI-compatible goggles during fMRI scanning. The four sessions alternated between monoscopic and stereoscopic presentations and stimuli varied within the sessions in apparent distance, size, and orientation. To validate the effect of distance, the pixel size of objects was also controlled. Stereoscopic presentation enhanced dorsal stream activation, particularly in V5/MT, lateral occipital cortex and the posterior intraparietal sulcus, associated with depth processing, suggesting increased peripersonal space processing. In addition to that, analyses revealed characteristic bilateral activation patterns of primary to tertiary visual areas, extending dorsally from the lateral occipital cortex to the posterior intraparietal sulcus for stimuli in peripersonal space, while extrapersonal space activated mostly ventral regions of the tertiary visual cortex. Notably, as the first study to control for pixel object size, these patterns persist, indicating that stimuli in peripersonal space engage the dorsal visual stream, potentially reflecting action-oriented and grasping feature encoding linked to their interactive affordances, while stimuli in extrapersonal space engage ventral regions primarily mediating semantic aspects and scene analysis.