An uninformative exogenous cue speeds target detection if cue and target appear in the same location separated by a brief temporal interval. This finding is usually ascribed to the orienting of spatial attention to the cued location. Here we examine the role of perceptual merging of the two trial events in speeded target detection. That is, the cue and target may be perceived as a single event when they appear in the same location. If so, cueing effects could reflect, in part, the binding of the perceived target onset to the earlier cue onset. We observed the traditional facilitation of cued over uncued targets and asked the same observers to judge target onset time by noting the time on a clock when the target appeared. Observers consistently judged the onset time of the target as being earlier than it appeared with cued targets judged as earlier than uncued targets. When the event order is reversed so that the target precedes the cue, perceived onset is accurate in both cued and uncued locations. This pattern of results suggests that perceptual merging does occur in exogenous cueing. A modified attention account is discussed that proposes reentrant processing, evident through perceptual merging, as the underlying mechanism of reflexive orienting of attention.
When storing multiple objects in visual working memory, observers sometimes misattribute perceived features to incorrect locations or objects. These "swaps" are usually explained by a failure to store object representations in a bound form. Swap errors have been demonstrated mostly in simple objects whose features (color, orientation, shape) are easy to encode independently. Here, we tested whether similar swaps can occur with real-world objects where the connections between features are meaningful. In Experiment 1, observers were simultaneously shown four items from two object categories (two exemplars per category). Within a category, the exemplars could be presented in either the same (two open boxes) or different states (one open, one closed box). After a delay, two exemplars drawn from one category were shown in both possible states. Participants had to recognize which exemplar went with which state. In a control task, they had to recognize two old vs. two new exemplars. Participants showed good memory for exemplars when no binding was required. However, when the tested objects were shown in the different states, participants were less accurate. Good memory for state information and for exemplar information on their own, with a significant memory decrement for exemplar-state combinations suggest that binding was difficult for observers and "swap" errors occurred even for real-world objects. In Experiment 2 we used the same tasks, but on half of trials the locations of the exemplars were swapped at test. We found that participants ascribed incorrect states to exemplars more frequently when the locations were swapped. We conclude that the internal features of real-world objects are not perfectly bound in VWM and can be attached to locations independently. Overall, we provide evidence that even real-world objects are not stored in an entirely bound representation in working memory.
Effects of display heterogeneity on visual search efficiency are well documented (Duncan & Humphreys, 1989). Even when searching a clearly distinguishable feature singleton, attentional salience falls down with heterogeneity of distractors (e.g., Santhi & Reeves, 2004). It is presumed that the visual system is able to preattentively separate heterogeneous features to homogenous subsets and attend each subset serially to find a singleton. The issue we addressed in our study was as follows: How does the visual system process heterogeneous sets that can’t be clearly distinguished? Theoretically, it should conjoin all heterogeneous items under the same subset representation and a singleton, therefore, would become more salient despite large heterogeneity. In our visual search task observers searched for an odd-sized target (either small, or large) among 13, 25, or 37 differently sized items. There were two homogenous conditions: (1) all distractors were of medium or (2) opposite size (e.g., large distractors with small targets and vice versa). Above, two heterogenous conditions were tested. In one such conditions all distractors were of (3) medium and opposite sizes (the difference between medium and each opposite size were clearly distinguishable ). Finally, in condition (4) four transition sizes filled the gap between medium and opposite distractors providing six heterogeneous sizes. We found in the result near parallel pattern of search performance in all positive conditions. The fastest detection was predictably found for homogenous displays with opposite sizes. The slowest detection was found for two distinct sizes of distractors. The intermediate efficiency was found for both medium homogenous and heterogeneous sets with transition sizes. RTs were substantially the same in these two conditions. This suggests that the visual system does fail to separate such transitional sets to subsets and treat them as a unitary perceptual entity opposing to a singleton (despite large heterogeneity and wide range of differences).
Four experiments were performed to examine the hypothesis that abstract, nonspatial, statistical representations of object numerosity can be used for attentional guidance in a feature search task.Participants searched for an odd-colored target among distractors of one, two, or three other colors. An enduring advantage of large over small sets (i.e., negative slopes of search functions) was found, and this advantage grew with the number of colored subsets among distractors. The results of Experiments 1 and 2 showed that the negative slopes cannot be ascribed to the spatial grouping between distractors but can be partially explained by the spatial density of the visual sets. Hence, it appears that observers relied on numerosity of subsets to guide attention. Experiments 3a and 3b tested the processes within and between color subsets of distractors more precisely. It was found that the visual system collects numerosity statistics that can be used for guidance within each subset independently. However, each subset representation should be serially selected by attention. As attention shifts from one subset to another, the “statistical power” effects from every single subset are accumulated to provide a more pronounced negative slope.