Individuation of parts of a single object and multiple distinct objects relies on a common neural mechanism in inferior intraparietal sulcus

Abstract

Object identification and enumeration rely on the ability to distinguish, or individuate, objects from the background. Does multiple object individuation operate only over bounded, separable objects or does it operate equally over connected features within a single object? While previous fMRI experiments suggest that connectedness affects the processing and enumeration of objects, recent behavioral and EEG studies demonstrated that parallel individuation occurs over both object parts and distinct objects. However, it is unclear whether individuation of object parts and distinct objects relies on a common or independent neural mechanisms. Using fMRI-based multivariate pattern analyses, we here demonstrate that activity patterns in inferior and superior intraparietal sulci (IPS) encode numerosity independently of whether the individuated items are connected parts of a single object or distinct objects. Lateral occipital cortex is more sensitive to perceptual aspects of the two stimulus types and the targets of the stimuli, suggesting a dissociation between ventral and dorsal areas in representing perceptual object properties and more general information about numerosity, respectively. Our results suggest that objecthood is not a necessary prerequisite for parallel individuation in IPS. Rather, our results point toward a common individuation mechanism that selects targets over a flexible object hierarchy, independently of whether the targets are distinct separable objects or parts of a single object.