Control of visual attention.

One way of perceptually organizing a complex visual scene is to attend selectively to information in a particular physical loca tion. Another way of reducing the complexity in the input is to attend selectively to an individual object in the scene and to process its elements preferentially. This latter, object-based attention process was examined, and the predicted superiority for reporting features from 1 relative to 2 objects was replicated in a series of experiments. This object-based process was robust even under conditions of occlusion, although there were some b oundary conditions on its operation. Finally, an account of the data is provided via simulations of the findings in a computational mod el. The claim is that object-based attention arises from a mechanism that groups together those features based on internal representations developed over perceptual experience and then preferentially gates these features for later, selective processing.

Object-Based Attention and Occlusion Evidence From Normal Participants and a Computational Model

We examined the allocation of attention in depth using a multi-element tracking paradigm. Observers were required to track a predefined subset of from two to eight elements in displays containing up to sixteen identical moving elements. We first show that depth cues, such as binocular disparity and occlusion through T-junctions, improve performance in a multi-element tracking task in the case where element boundaries are allowed to intersect in the depiction of motion in a single frontoparallel plane. We also show that the allocation of attention across two perceptually distinguishable planar surfaces, either frontoparallel or receding at a slanting angle and defined by coplanar elements, is easier than allocation of attention within a single surface. The same result was not found when attention was required to be deployed across items of two-color populations rather than across items of a single color. Our results suggest that, when surface information does not suffice to distinguish between targets and distractors that are embedded in these surfaces, division of attention across two surfaces aids in tracking moving targets. A final experiment with populations of elements moving within distinct volumes produced similar results, suggesting that spatial separation in three dimensions, rather than confinement to surfaces as such, may explain the improved performance for the two-surface case.

Dynamics of attention in depth: evidence from multi-element tracking.

Flexible visual processing of spatial relationships.

Two interspersed and differently colored sets of dots were rotated in opposite directions and were perceived as superimposed transparent surfaces. Probes consisting of brief changes in dot motion direction were reported. Two probes affecting the same surface were discriminated accurately. The 2nd probe was discriminated poorly if it affected a surface different from the 1st and if the time between probes was less than 600 ms. This reflects a difficulty in switching attention rapidly between surfaces, Spatial proximity increased the interference. Controls were incompatible with traditional spatial mechanisms (2- or 3-dimensional) or with simple sensory filters. Instead, probes were apparently selected by object files. The interference is not simply due to an inability to process 2 objects at once but requires close spatial proximity of incompatible motion signals.

Attention to object files defined by transparent motion.

This experiment explored whether attentional selection observed in a spatial cuing task is based on a representation that includes depth information or not. Targets were presented inside placeholders appearing at the samex,y location on a stereoscopic display, but on different depth planes, or at differentx,y locations on the same depth plane. A peripheral precue produced significant cuing effects in the latter but not in the former condition. In a control experiment, significant cuing effects were found for targets appearing at differentx,y coordinates within the fovea, confirming that the lack of cuing effects in the depth condition was not due to foveal presentation. Together, the results suggest that spatial selection in spatial cuing tasks operates on a representation that does not include depth information.

Thomas G. Ghirardelli

Papers

Spatial cuing in a stereoscopic display: Evidence for a “depth-blind” attentional spotlight