Object-based attention

Abstract: The two basic phenomena that define the problem of visual attention can be illustrated in a simple example. Consider the arrays shown in each panel of Figure 1. In a typical experiment, before the arrays were presented, subjects would be asked to report letters appearing in one color (targets, here black letters), and to disregard letters in the other color (nontargets, here white letters). The array would then be briefly flashed, and the subjects, without any opportunity for eye movements, would give their report. The display mimics our. usual cluttered visual environment: It contains one or more objects that are relevant to current behavior, along with others that are irrelevant. The first basic phenomenon is limited capacity for processing information. At any given time, only a small amount of the information available on the retina can be processed and used in the control of behavior. Subjectively, giving attention to any one target leaves less available for others. In Figure 1, the probability of reporting the target letter N is much lower with two accompa­ nying targets (Figure la) than with none (Figure Ib). The second basic phenomenon is selectivity-the ability to filter out un­ wanted information. Subjectively, one is aware of attended stimuli and largely unaware of unattended ones. Correspondingly, accuracy in identifying an attended stimulus may be independent of the number of nontargets in a display (Figure la vs Ie) (see Bundesen 1990, Duncan 1980).

Abstract: Detection of a visual signal requires information to reach a system capable of eliciting arbitrary responses required by the experimenter. Detection latencies are reduced when subjects receive a cue that indicates where in the visual field the signal will occur. This shift in efficiency appears to be due to an alignment (orienting) of the central attentional system with the pathways to be activated by the visual input. It would also be possible to describe these results as being due to a reduced criterion at the expected target position. However, this description ignores important constraints about the way in which expectancy improves performance. First, when subjects are cued on each trial, they show stronger expectancy effects than when a probable position is held constant for a block, indicating the active nature of the expectancy. Second, while information on spatial position improves performance, information on the form of the stimulus does not. Third, expectancy may lead to improvements in latency without a reduction in accuracy. Fourth, there appears to be little ability to lower the criterion at two positions that are not spatially contiguous. A framework involving the employment of a limited-capacity attentional mechanism seems to capture these constraints better than the more general language of criterion setting. Using this framework, we find that attention shifts are not closely related to the saccadic eye movement system. For luminance detection the retina appears to be equipotential with respect to attention shifts, since costs to unexpected stimuli are similar whether foveal or peripheral. These results appear to provide an important model system for the study of the relationship between attention and the structure of the visual system.

Abstract: The operation of attention in the visual field has often been compared to a spotlight. We propose that a more apt analogy is that of a zoom or variable-power lens. Two experiments focused upon the following questions: (1) Can the spatial extent of the attentional focus be made to vary in response to precues? (2) As the area of the attentional focus increases, is there a decrease in processing efficiency for stimuli within the focus? (3) Is the boundary of the focus sharply demarked from the residual field, or does it show a gradual dropoff in processing resources? Subjects were required to search eight-letter circular displays for one of two target letters and reaction times were recorded. One to four adjacent display positions were precued by underlines at various stimulus onset asynchronies before display presentation. A response competition paradigm was used, in which the “other target” was used as a noise letter in noncued as well as cued locations. The results were in good agreement with the zoom lens model.