Spatial attention facilitates selection of illusory objects: evidence from event-related brain potentials

The relationship between spatial attention and object-based attention has long been debated. On the basis of behavioral evidence it has been hypothesized that these two forms of attention share a common mechanism, such that directing spatial attention to one part of an object facilitates the selection of the entire object. In a previous study (Martinez, A., Teder-Salejarvi, W., Vazquez, M., Molholm, S., Foxe, J.J., Javitt, D.C., Di Russo, F., Worden, M.S., Hillyard, S.A., 2006. "Objects are highlighted by spatial attention." J. Cogn. Neurosci. 18(2): 298-310) we used recordings of event-related potentials (ERPs) during a paradigm modeled after that of Egly et al. (Egly, R., Driver, J., Rafal, D.R., 1994. Shifting visual attention between objects and locations: evidence from normal and parietal lesion subjects. J. Exp. Psychol. Gen. 123(2) 161-77) to investigate this relationship. As reported in numerous studies of spatial attention, we found the typical pattern of enhanced neural activity in visual cortex elicited by attended stimuli. Unattended stimuli belonging to the same object as the attended stimuli elicited a very similar spatiotemporal pattern of enhanced neural activity that was localized to lateral occipital cortex (LOC). This similarity was taken as evidence that spatial- and object-selective attention share, at least in part, a common neural mechanism. In the present study we further investigate this relationship by examining whether this spread of spatial attention within attended objects can be guided by objects defined by illusory contours. Subjects viewed a display consisting of two illusory rectangular objects and directed attention to continuous sequences of stimuli (brief onsets) at one end of one of the objects. Stimuli occurring at irrelevant locations but belonging to the same attended object elicited larger posterior N1 amplitudes than that elicited by unattended objects forming part of a different object. This object-selective N1 enhancement was localized to lateral occipital cortex. The present data support the hypothesis that the allocation of spatial attention can be guided by illusory object boundaries and that this allocation strengthens the perceptual representations of attended objects at the level of visual area LOC.