Electrophysiological measurement of information flow during visual search.

The temporal relationship between different stages of cognitive processing is long debated. This debate is ongoing, primarily because it is often difficult to measure the time course of multiple cognitive processes simultaneously. We employed a manipulation that allowed us to isolate ERP components related to perceptual processing, working memory, and response preparation, and then examined the temporal relationship between these components while observers performed a visual search task. We found that, when response speed and accuracy were equally stressed, our index of perceptual processing ended before both the transfer of information into working memory and response preparation began. However, when we stressed speed over accuracy, response preparation began before the completion of perceptual processing or transfer of information into working memory on trials with the fastest reaction times. These findings show that individuals can control the flow of information transmission between stages, either waiting for perceptual processing to be completed before preparing a response or configuring these stages to overlap in time.

Templates for rejection: configuring attention to ignore task-irrelevant features.

Theories of attention are compatible with the idea that we can bias attention to avoid selecting objects that have known nontarget features. Although this may underlie several existing phenomena, the explicit guidance of attention away from known nontargets has yet to be demonstrated. Here we show that observers can use feature cues (i.e., color) to bias attention away from nontarget items during visual search. These negative cues were used to quickly instantiate a template for rejection that reliably facilitated search across the cue-to-search stimulus onset asynchronies (SOAs), although negative cues were not as potent as cues that guide attention toward target features. Furthermore, by varying the search set size we show a template for rejection is increasingly effective in facilitating search as scene complexity increases. Our findings demonstrate that knowing what not to look for can be used to configure attention to avoid certain features, complimenting what is known about setting attention to select certain target features.

Attentional templates in visual working memory.

Most theories of attention propose that we maintain attentional templates in visual working memory to control what information is selected. In the present study, we directly tested this proposal by measuring the contralateral-delay activity (CDA) of human event-related potentials during visual search tasks in which the target is cued on each trial. Here we show that the CDA can be used to measure the maintenance of attentional templates in visual working memory while processing complex visual scenes. In addition, this method allowed us to directly observe the shift from working memory to long-term memory representations controlling attention as learning occurred and experience accrued searching for the same target object. Our findings provide definitive support for several critical proposals made in theories of attention, learning, and automaticity.

A cuing study of the N2pc component: an index of attentional deployment to objects rather than spatial locations.

Symbolic visual cues indicating the location of an upcoming target are believed to invoke endogenous shifts of attention to cued locations. In the present study, we investigated how visual attention is shifted during such cuing paradigms by recording event-related potentials (ERPs). We focused on a component known to index lateralized shifts of perceptual attention during visual search tasks, known as the N2pc component. The ERP data show that attention was shifted to a cued location in anticipation of a target shape when the location is marked by a placeholder object (Experiments 1 and 2). However, when the possible locations were not marked by placeholder objects, we found no evidence for an anticipatory shift of attention to the cued location (Experiment 3). These findings indicate that the perceptual attention mechanism indexed by the N2pc is deployed to objects and not simply locations in space devoid of object structure.

Staying in bounds: Contextual constraints on object-file coherence.

Coherent visual perception necessitates the ability to track distinct objects as the same entities over time and motion. Calculations of such object persistence appear to be fairly automatic and constrained by specific rules. We explore the nature of object persistence here within the object-file framework; object files are mid-level visual representations that track entities over time and motion as the same persisting objects and store and update information about the objects. We present three new findings. First, objects files are constrained by the principle of "boundedness"; persisting entities should maintain a single closed contour. Second, object files are constrained by the principle of "containment"; all the parts and properties of a persisting object should reside within, and be connected to, the object itself. Third, object files are sensitive to the context in which an object appears; the very same physical entity that can instantiate object-file formation in one experimental context cannot in another. This contextual influence demonstrates for the first time that object files are sensitive to more than just the physical properties contained within any given visual display.