The role of strategic scanning in object-based attention

Abstract

Evidence has accumulated over the past decade suggesting that visual attention is often object-based (e.g., Egly et al., 1994). However, some recent studies have suggested that strategic effects such as attentive scanning can contribute to object-based effects. In the present study, we investigated the strategic contribution to object-based selection. We used a modified Egly paradigm in which we manipulated (1) the probability that a target would appear in each of the two uncued locations and (2) the cue to target SOA. Of the invalidly cued trials, the target appeared in the high probability location (absolute spatial location, i.e. upper right) 83% of the time and in the low probability location (i.e., lower left) 17% of the time. In both conditions, uncued targets appeared in the cued object half the time and in the uncued object half the time. At short SOAs, the object-based and probability effects were roughly additive. However, at long SOAs (400 and 600 ms), the object-based effects disappeared, and response times depended exclusively on the target probability. These results suggest that observers may adopt an implicit scanning strategy (in which unattended locations within an attended object have high priority) or an explicit scanning strategy (in which objectively high-probability locations have high priority) depending on task contingencies and the amount of time that is available to deploy attention. This provides further constraints on the mechanisms of object-based selection.