To isolate multiple coherent objects from their surrounds, each object must be represented as a stable perceptual entity across both time and space. Recent theoretical and empirical work has proposed that this process of object individuation is a mid-level operation that emerges around 200-300 ms post-stimulus onset. However, this hypothesis is based on paradigms that have potentially obscured earlier effects. Further, no study to date has directly assessed whether object individuation occurs for task-irrelevant objects. Here we used electroencephalography (EEG) to measure the timecourse of individuation, both for stimuli within and outside the focus of attention, to assess the information processing stage at which object individuation arises for both types of objects. We developed a novel paradigm involving items defined by illusory contours, which allowed us to vary the number of to-be-individuated objects while holding the physical elements of the display constant (a design characteristic not present in earlier work). As early as 100 ms post-stimulus onset, event-related potentials tracked the number of objects in the attended hemifield, but not those in the unattended hemifield. By contrast, both attended and unattended objects could be individuated at a later stage. Our findings challenge recent conceptualizations of the timecourse of object individuation, and suggest that this process arises earlier for attended than unattended items, implying that voluntary spatial attention influences the timecourse of this operation.
- object individuation
- selective attention
- illusory contours
- Copyright © 2016, Journal of Neurophysiology