| Visual attention has been likened to a spotlight that enhances both attended and nearby objects. However, experiments employing overlapping stimuli have shown that individuals are able to attend selectively to one object while suppressing the other. These data are problematic for spotlight theories of selection, suggesting that under some circumstances, selection may also be made with respect to objects rather than spatial location. It has been suggested that these two types of selection are achieved by a single mechanism by which the attentional spotlight conforms to the boundary of an object. Alternatively, space- and object-based selection may be independent, but mutually supporting systems. The present set of experiments examined the relationship between object-based and space-based selection using event-related potentials (ERPs). Previous research has shown that attention to spatial location results in an enhancement of the P1 and N1 components of the ERP. This pattern is associated with spatial attention and is not seen when observers are attending to non-spatial attributes, such as color or shape. In the object-based attention condition, ERPs were collected in response to flashes on attended objects, unattended overlapping objects, and unattended objects at far locations. In the space-based attention condition, ERPs were elicited by the same flashes occurring in empty locations. Both space-based and object-based ERPs produced P1 and N1 components, but only the space-based ERPs showed an attentional effect on P1 amplitude. The attentional effect on the parietal N1 was nearly identical for both object- and space-based ERPS. These data suggest that space and object-based attention may be separate, but interactive systems. |
