Time course of spatial and feature selective attention for partly-occluded objects

Abstract

Attention selects objects/groups as the most fundamental units, and this may be achieved by an attention-spreading mechanism. Previous event-related potential (ERP) studies have found that attention-spreading is reflected by a decrease in the N1 spatial attention effect.The present study tested whether the electrophysiological attention effect is associated with the perception of object unity or amodal completion through the use of partly-occluded objects. ERPs were recorded in 14 participants who were required to pay attention to their left or right visual field and to press a button for a target shape in the attended field. Bilateral stimuli were presented rapidly, and were separated, connected, or connected behind an occluder. Behavioral performance in the connected and occluded conditions was worse than that in the separated condition, indicating that attention spread over perceptual object representations after amodal completion. Consistently, the late N1 spatial attention effect (180-220 ms post-stimulus) and the early phase (230-280 ms) of feature selection effects (target N2) at contralateral sites decreased, equally for the occluded and connected conditions, while the attention effect in the early N1 latency (140-180 ms) shifted most positively for the occluded condition. These results suggest that perceptual organization processes for object recognition transiently modulate spatial and feature selection processes in the visual cortex.

Emergence of visual objects involves multiple stages of spatial selection

“…The N2 attention effect (300-350 ms) was shifted positively for connected objects in the easy condition. The present lack of N1 effect stands in contrast with our previous studies, which found attention-spreading effects in the N1 latency range (Kasai, 2010;Kasai et al, 2011;Kasai & Takeya, 2012;Takeya & Kasai, 2014). The general stimulus configurations and tasks in the current study were similar to those in the previous studies; the critical difference was that unfilled line stimuli, rather than geometric shapes with filled surfaces, were used.…”

“…However, it may be observed only when objects are clearly separated from each other. In our previous studies, the P2 attention effect may not have been found because filled objects can be grouped according to similarities in surface properties (Kasai, 2010;Kasai et al, 2011;Takeya & Kasai, 2014), whereas a P2 effect was observed when another object interrupted the grouping between bilateral stimuli (Kasai & Takeya, 2012). Luck et al (1990) also found a similar P2 attention effect for letters that were aligned in hemi-circles across the left and right hemi-fields, which raises the question of whether the existence of multiple elements within each field reduces grouping of bilateral stimuli.…”

“…However, later ERP lateralization effects were also sensitive to grouping manipulations. The attention effects in the N2 latency range at around 300-400 ms for connected or similar stimuli were more positive than those for separated or dissimilar stimuli, respectively (Kasai, 2010;Kasai et al, 2011), and those in the P2 latency range (200-250 ms) were more positive for separated stimuli, although this result was observed only when another object was located between them (Kasai & Takeya, 2012).…”

“…Such objects may require rapid identification and further stages of processing. Previously, we examined object-based spatial attention processes by using a sustained-focal attention task with bilateral stimuli (Kasai, 2010;Kasai, Moriya, & Hirano, 2011;Kasai & Takeya, 2012;Takeya & Kasai, 2014). While the participants attended to the left and right visual field, grouped or ungrouped bilateral objects were presented in random order, and the direction of the subject's attention was indexed by posterior ERP lateralization effects comparing recording sites that were contralateral versus ipsilateral with respect to the attended visual fields.…”

“…We have previously used simple, filled geometric shapes (Kasai, 2010;Kasai et al, 2011;Kasai & Takeya, 2012;Takeya & Kasai, 2014), which may involve neural activations within object-selective regions or figural enhancement in relatively late or intermediate visual cortical processing (Poort, Raudies, Wannig, Lamme, Neumann, & Roelfsema, 2012). In contrast, the letters or checkerboards used in previous studies may not involve the same type of enhancement.…”