A variable mapping task produces symmetrical interference between global information and local information.

When processing global and local aspects of compound visual figures, a robust finding is that global targets are detected faster and more accurately than local targets. Moreover, unidirectional interference is often observed. Despite the convincing evidence that global information and local information are available together, when attention is focused on the global level, items from the local level often have very little, if any, effect on behavior. If local information is available with global information, then why is global dominance so often observed under such a wide range of conditions? This paper is concerned with the fate of the ignored, and apparently ineffective, local distractors. In our experiments, at least one critical factor was stimulus-response (S-R) mapping. We compared a consistent S-R task, which facilitated a speed advantage for global, with a variable S-R task, which required a higher degree of semantic analysis for each stimulus. The two tasks produced large differences in behavior, showing unidirectional interference in the consistent S-R task, and strong bidirectional interference in the variable S-R task. Thus, the identity of ignored local distractors was available, even under conditions that favored focused attention to global information. The results provide support for a model in which global processing proceeds more quickly at early perceptual stages and in which local processing can catch up if processing demands are increased at later stages.

ERP time course of perceptual and post-perceptual mechanisms of spatial selection.

Event-related potentials (ERPs) were recorded from volunteers performing a task requiring simple judgements about the spatial location of a single target that could appear with equal probability to the left or right of fixation. A robust finding in the ERP literature is a dichotomy between attentional selection for spatial and non-spatial features. Visual spatial selection is manifest as a modulation of early components (P1, N1) that reveal exogenous processes, while non-spatial selection is revealed by the presence of longer latency endogenous components (N2). We present an analysis of several conditions that require different degrees of visual analysis to confirm the location of the single target, and show that spatial selection can be manifest at early (N1) or later (N2) stages. Observers identified the location of targets that were more salient (2D line drawings with abrupt onset) or less salient (2D line drawings without abrupt onset or 3D objects embedded in random-dot stereograms). We examined differences in amplitude, latency, and topography of early ERP components (P1, N1, P2, N2), and compared responses measured over the left and right hemispheres in response to left and right targets. The results support the hypothesis that the processes involved in spatial selection can be manifest at early or late stages, dependent on the quality of the incoming data. Moreover, the iterative process by which the percept is established benefits from a change in the visual input that is specific to the target.

The effect of variability of unattended information on global and local processing: evidence for lateralization at early stages of processing.

Visual objects can often be analyzed as hierarchical in structure, composed of local elements that are spatially arranged to form a global shape. The brain mechanisms involved in the analysis of hierarchical figures have been under considerable scrutiny in recent years, and one of the many interesting features that have emerged is that there is an asymmetry across the two hemispheres for global (right hemisphere) vs local (left hemisphere) processing. Event-related potentials (ERP) were used to examine selective attention to global or local levels of hierarchical figures to determine the stage of processing at which the asymmetry first emerges. Two conditions were tested, one in which unattended information was variable from trial to trial, and one in which it was not. The variability of unattended information influenced the lateralization of processing. Presentation of invariable, neutral distractors resulted in global/local processing asymmetries at early stages (P1). In contrast, presentation of variable, task-relevant distractors resulted in processing asymmetries that occurred at much later stages (N2). Our hypothesis is that lateralized enhancement of neural populations in extrastriate cortex results from both selective attention to locations in the visual field, as well as selective attention to global or local information. We suggest that unattended information that varies from trial to trial is processed in parallel with attended information, masking hemisphere biases for local vs global information at early stages of processing.

Attention switching in depth using random-dot autostereograms: attention gradient asymmetries.

Random-dot autostereograms (RDASs) were used to investigate attention shifts along the sagittal plane in distractor-free tasks of high perceptual load. In three experiments using a same/different comparison task, the shape of the gradient over five different depths was examined and the conditions under which the gradient is and is not observed were compared. When the target set consisted of five similar objects, a robust asymmetric depth gradient was observed. When the target set consisted of two dissimilar objects, no gradient was observed. The results support a hypothesis of a viewer-centered asymmetric attention gradient in the depth plane that is dependent on perceptual or attentional load defined by target-set discriminability.