Are objects the same as groups? ERP correlates of spatial attentional guidance by irrelevant feature similarity

Abstract

It has been proposed that the most fundamental units of attentional selection are "objects" that are grouped according to Gestalt factors such as similarity or connectedness.Previous studies using event-related potentials (ERPs) have shown that object-based attention is associated with modulations of the visual-evoked N1 component, which reflects an early cortical mechanism that is shared with spatial attention. However, these studies only examined the case of perceptually continuous objects. The present study examined the case of separate objects that are grouped according to feature similarity (color, shape), by indexing lateralized potentials at posterior sites in a sustained-attention task that involved bilateral stimulus arrays. A behavioral object effect was found only for task-relevant shape similarity. Electrophysiological results indicated that attention was guided to the task-irrelevant side of the visual field due to achromatic-color similarity in N1 (155-205 ms post-stimulus) and early N2 (210-260 ms) and due to shape similarity in early N2 and late N2 (280-400 ms) latency ranges. These results are discussed in terms of selection mechanisms and object/group representations.

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, 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).…”

“…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.…”

“…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.…”

“…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.…”