Dynamic interaction of object- and space-based attention in retinotopic visual areas

Abstract

We investigated the interaction between object- and space-based attention by measuring activity in early visual cortex. After central cueing, when subjects directed attention to a spatially defined part of an object, activity in early visual areas was enhanced at corresponding retinotopic representations but also at representations of other locations covered by the object. Different from the assumption of automatic attentional "spreading" within an object, however, activity was greater for representations of cued than of uncued locations on the same object. These findings support an interaction of object-based spatial selection with object-independent spatial mechanisms in directing attention. When the target stimulus did not appear at the expected location, we found higher activation in areas representing other locations on the same object than equidistant locations on other objects. Objects, hence, also guide spatial search, and this may account for the behaviorally observed delay in processing parts of an unattended object.

Figure 1.

Experimental design and conditions. a, Presentation scheme (stimuli not to scale). b, Experimental conditions during cueing. Arrows indicate for which locations activity in corresponding retinotopic representations was assessed. Depending on vertical or horizontal orientation of the objects, the bottom left and top right locations belonged to either the cued object or the uncued object, respectively. c, Experimental conditions during target processing. Trials in which the slit appeared at the cued top left corner are referred to as valid. Only trials in which the slit did not appear at the respective location were analyzed for the corresponding representations. This avoided confounding by sensory input processing.

Figure 2.

Neural activity in early visual areas during cueing and target search. ROI were defined as retinotopic representations of the locations depicted in Figure 1, a and b. a, Event-related activity in response to the spatial cue. Single-subject data are shown on a flattened cortical reconstruction. When the objects were oriented vertically, the bottom left representation coded the uncued/same object location and the top right representation coded the uncued/different object location, and vice versa with horizontal objects. Accordingly, a same-object advantage for uncued locations would manifest as higher amplitude for the blue curve at the bottom left representation and for the red curve at the top right representation. b, Reaction times for correct answers. c, Group-averaged peak BOLD responses to the cue at ROI representing the cued and the uncued locations. d, Group-averaged peak BOLD responses during target processing in valid and invalid trials at ROI representing the uncued locations.