Does the identification of simple features require serial processing?

Effect of attention on the detection and identification of masked spatial patterns

The effect of attention on the detection and identification of vertically and horizontally oriented Gabor patterns in the condition of simultaneous masking with obliquely oriented Gabors was studied. Attention was manipulated by varying the set size in a visual-search experiment. In the first experiment, small target Gabors were presented on the background of larger masking Gabors. In the detection task, the effect of set size was as predicted by unlimited-capacity signal detection theory. In the orientation identification task, increasing the set size from 1 to 8 resulted in a much larger decline in performance. The results of the additional experiments suggest that attention can reduce the crowding effect of maskers.

Bottom-up and top-down control in visual search

Previous research suggests that the allocation of attention is largely controlled either in a stimulus-driven or in a goal-driven manner. To date, few studies have systematically manipulated variables affecting stimulus-driven and goal-driven selection independently in order to investigate how both manners of control interrelate and affect performance in visual search. In the present study observers were presented with search displays consisting of an array of line segments rotated at various orientations. The task of observers was to indicate the presence or absence of a vertical line segment (the target) presented amongst a series of nontargets and possibly one distractor. By varying the absolute differences in orientation between the target, nontargets, and distractors, relative target-distractor salience and target-distractor similarity were independently manipulated to investigate the contribution of stimulus-driven and goal-driven control. The major result was that relative target-distractor salience and target-distractor similarity affected search performance independently. Performance was better in cases where the irrelevant distractor was not a salient item in the search display and did not look similar to the target. The results are discussed in terms of models of attentional control.

Attentional capture by auto- and allo-cues

In a host of studies, the ability of various types of cues to capture attention has been examined. This article reviews a number of these studies by organizing them into a classification scheme based on the relationship between the putative attention-capturing item (the cue) and the item used to assess the distribution of attention (the probe). The second dimension of this taxonomy divides paradigms of attentional capture into those in which capture is indexed by performance benefits and those in which capture is indexed by performance costs. The relative methodological merits and disadvantages of the paradigms that occupy each of the cells of the resulting two-by-two matrix are discussed. A final section offers a new interpretation of the finding that dynamic cues capture attention.

Stimulus segmentation in the visual search task

Four experiments were conducted in order to study the segmentation process in a visual search task with relevant stimuli (target and distractors) randomly distributed among textural elements. The basic idea was that a parallel segmentation process of the relevant stimuli would contribute to the overall reaction time independently of the contribution of the number of relevant stimuli. In the first experiment, with relevant stimuli and textural elements that differed in the orientation of their component lines, texture presence interacted with number of relevant stimuli and with target presence. These results were not favorable to the parallel segmentation hypothesis. In the second and third experiments, in which the relevant and the textural stimuli differed in orientation and in the luminance contrast of their component lines, the results support a parallel segmentation process for the higher contrast conditions. In these experiments, the effect of texture presence was greater on target-absent than on target-present trials. Experiment 4 shows that the search can be restricted to the high-contrast relevant stimuli when the number of these stimuli is constant and the number of textural stimuli changes from trial to trial. The present results suggest that the relevant stimuli can be segmented in parallel and then submitted to a restricted analysis, even when they are scattered among textural stimuli.

Do locally defined feature discontinuities capture attention?

Results from previous visual search studies have suggested that abrupt onsets produce involuntary shifts of attention (i.e., attentional capture), but discontinuities in simple features such as color and brightness do not (Jonides & Yantis, 1988). In the present study we tested whether feature discontinuities (i.e., "singletons") can produce attentional capture in a visual search task if defined "locally" or over a small spatial range. On each trial, a variable number of letters appeared, one of which differed from the others in color or intensity. The location of this singleton was uncorrelated with target location. Local discontinuities were created by embedding the letters in a dot texture. In Experiment 1, display size effects for singleton targets were not reduced with the addition of a background dot texture. Similar results were obtained in Experiment 2, regardless of variations in texture density. Experiment 3 confirmed that when targets are defined by a color or intensity singleton, they are detected preattentively, and that increasing texture density yields faster detection. We conclude that the spatial range over which feature discontinuities are defined may influence the guidance of spatial attention, but it has no influence on their ability to capture attention.

Visual search: detection, identification, and localization

In two studies, observers searched for a single oblique target in a field of vertical distractors. In one experiment, target detection and identification (left versus right tilt) were compared. In another experiment, detection and localization were compared for the left versus the right half of the display. Performance on all three tasks was virtually identical: if a target could be detected, it could also be identified and localized. A review of previous studies generally supports the conclusion that performance on the three tasks is similar. This argues against current search theories, which rest heavily on data showing differences in identification and localization.

Cross-dimensional perceptual selectivity

Three visual search experiments tested whether top-down selectivity toward particular stimulus dimensions is possible during preattentive parallel search. Subjects viewed multielement displays in which two salient items, each unique in a different dimension--that is, color and intensity (Experiment 1) or color and form (Experiments 2 and 3)--were simultaneously present. One of the dimensions defined the target; the other dimension served as distractor. The results indicate that when search is performed in parallel, top-down selectivity is not possible. These findings suggest that preattentive parallel search is strongly automatic, because it satisfies both the load-insensitivity and the unintentionality criteria of automaticity.

Effects of spatially directed attention on visual encoding

The purpose in this study was to distinguish among possible mechanisms by which focused attention facilitates visual perceptual processing in a cued discrimination task. In two experiments, subjects verified the presence of an X in masked, briefly presented, four-letter arrays. On most trials, subjects were precued to the location of the stimulus array (valid-cue condition); however, sometimes a nonstimulus location was cued (invalid-cue condition). The exposure duration of the stimulus array was varied. In Experiment 1, there was a large effect of cue condition on hit probability, but no effect of cue condition on false alarm probability. In Experiment 2, there was a large effect of cue condition on d'. In both experiments, the stimulus duration needed to reach any given performance level was greater by a constant factor for stimuli in the invalid-cue than it was in the valid-cue condition. This suggests that visual information is acquired (or utilized) more rapidly from attended than from unattended locations.