Spatial attention in vision. Evidence for early selection

Automatic control of visual selection

This paper seeks out to reduce the role of the homunculus, the 'little man in the head' that is still prominent in most psychological theories regarding the control our behaviour. We argue that once engaged in a task (which is a volitional act), visual selection run off more or less in an automatic fashion. We argue that the salience map that drives automatic selection is not only determined by raw physical salience of the objects in the environment but also by the way these objects appear to the person. We provide evidence that priming (feature priming, priming by working memory and reward priming) sharpens the cortical representation of these objects such that these objects appear to be more salient above and beyond their physical salience. We demonstrate that this type of priming is not under volitional control: it occurs even if observers try to volitionally prepare for something else. In other words, looking at red prepares our brain for things that are red even if we volitionally try to prepare for green.

Spatiotemporal Segregation in Visual Search: Evidence From Parietal Lesions

The mechanisms underlying segmentation and selection of visual stimuli over time were investigated in patients with posterior parietal damage. In a modified visual search task, a preview of old objects preceded search of a new set for a target while the old items remained. In Experiment 1, control participants ignored old and prioritized new items, but patients had severe difficulties finding the target (especially on the contralesional side). In Experiment 2, simplified displays yielded analogous results, ruling out search ease as a crucial factor in poor preview search. In Experiment 3, outlines around distractor groups (to aid segmentation) improved conjunction but not preview search, suggesting a specific deficit in spatiotemporal segmentation. Experiment 4 ruled out spatial disengagement problems as a factor. The data emphasize the role of spatiotemporal segmentation cues in preview search and the parietal lobe in the role of these cues to prioritize search of new stimuli.

Inattentional blindness as a function of proximity to the focus of attention

The effect of the distance between the center of the focus of attention and an unexpected stimulus on detection was examined in two experiments with the use of the inattentional-blindness paradigm [Mack and Rock, 1998 Inattentional Blindness (Cambridge, MA: MIT Press)]. In experiment 1, the closer a stimulus was to the center of attention, the more likely it was to be detected. Experiment 2 replicated this finding and controlled for retinal eccentricity. These results suggest that low-level stimulus characteristics (e.g. location) may play an important role in the detection of unexpected stimuli. The data are consistent with previous research on the spatial aspects of attention demonstrating that the distance to the focus of attention is a critical variable. This study demonstrates that the effect of distance is similar under conditions of inattention. Theories put forward to explain inattentional blindness should include 'early' attentional factors, as well as factors resulting from later stages of processing.

Visual selective attention: a theoretical analysis

The present paper outlines a framework which allows a consistent interpretation of data regarding visual selection in visual search tasks. It organizes and reviews visual search tasks in which the target is defined by primitive features, by conjunctions of features and when the target is categorically different from non-targets. The special role of spatial attention is reviewed and different theoretical accounts are discussed. Because visual selection depends principally on the outcome of the early parallel preattentive stage of processing, the main focus will be on this stage. It is concluded that visual selection is to a large extent determined by the physical characteristics of the stimuli present in the visual field. The early preattentive parallel process computes how different each object is from each of the other objects within a particular stimulus dimension. Attention is automatically drawn to the location having the highest activation, implying that the object at that location is automatically selected irrespective of the intentions of the subject. The model also assumes some top-down control. It is well known that attention can be voluntarily directed to nonfixated locations in visual space, varying from a uniform distribution over the visual field to a highly focused concentration. The model assumes that the endogenous direction of attention to an area in the visual field is the only top-down manner of affecting visual selection. Within the area of directed attention, no top-down control is possible: selection is completely determined by the physical properties of the stimuli.

The flanker compatibility effect as a function of visual angle, attentional focus, visual transients, and perceptual load: a search for boundary conditions

When subjects must respond to a relevant center letter and ignore irrelevant flanking letters, the identities of the flankers produce a response compatibility effect, indicating that they are processed semantically at least to some extent. Because this effect decreases as the separation between target and flankers increases, the effect appears to result from imperfect early selection (attenuation). In the present experiments, several features of the focused attention paradigm were examined, in order to determine whether they might produce the flanker compatibility effect by interfering with the operation of an early selective mechanism. Specifically, the effect might be produced because the paradigm requires subjects to (1) attend exclusively to stimuli within a very small visual angle, (2) maintain a long-term attentional focus on a constant display location, (3) focus attention on an empty display location, (4) exclude onset-transient flankers from semantic processing, or (5) ignore some of the few stimuli in an impoverished visual field. The results indicate that none of these task features is required for semantic processing of unattended stimuli to occur. In fact, visual angle is the only one of the task features that clearly has a strong influence on the size of the flanker compatibility effect. The invariance of the flanker compatibility effect across these conditions suggests that the mechanism for early selection rarely, if ever, completely excludes unattended stimuli from semantic analysis. In addition, it shows that selective mechanisms are relatively insensitive to several factors that might be expected to influence them, thereby supporting the view that spatial separation has a special status for visual selective attention.

Categorization and identification of simultaneous targets

Early and late selection theories of visual attention disagree about whether identification occurs before or after selection. Studies showing the category effect, i.e., the time to detect a letter is hardly affected by the number of digits present in the display, are taken as evidence for late selection theories since these studies suggest parallel identification of all items in the display. As an extension of previous studies, in the present study two categorically different targets were presented simultaneously among a variable number of nontargets. Subjects were shown brief displays of two target letters among either 2, 4 or 6 nontarget digits. Subjects responded 'same' when the two letters were identical and 'different' otherwise. Since the 'same-different' response reflects the combined outcome of the simultaneous targets, late-selection theory predicts that the time to match the target letters is independent of the number of nontarget digits. Alternatively, early-selection theory predicts a linear increase of reaction time with display size since the presence of more than one target disrupts parallel preattentive processing, leading to a serial search through all items in the display. The results provide evidence for the early-selection view since reaction time increased linearly with the number of categorically different nontargets. A control experiment revealed that none of the alternative explanations could account for the display size effect.

Perceptual selectivity is task dependent: evidence from selective search

Four visual search experiments examined the allocation of attention in a multi-item display with a salient stimulus difference. If, irrespective of its relevance to performance, an item with a unique property attracts attention, it can be said that the unique property automatically captures attention. Alternatively, if capturing attention depends on the relevance of the unique property it would suggest that subjects are capable of engaging in active filtering. In the experiments, subjects searched visually for a horizontal line segment positioned in either one of the 4, 8 or 16 surrounds, arranged in an imaginary circle around the fixation point. In each trial, there was always one surround that differed from the others. The data showed that attention was not automatically attracted to the exceptional form (experiment 1) or exceptional color (experiment 2). The control conditions showed that form and color differences could serve as the basis for active allocation of attention to the unique item. Attention capturing is occasionally observed when the surround changes abruptly in form (experiment 3) but not when it changes abruptly in color (experiment 4). Implications of these findings are discussed in relation to theories of attention that distinguish between a first, parallel stage of perceptual processing followed by a second stage of limited capacity. The results are compatible with the notion that pre-attentive processing is under strategic control and can be stopped by filtering on a spatial basis. The results are related to the specific properties of the transient channels in the visual system.

The processing of non-target words: semantic or not?

It is known that people reacting to visual words may be affected by the meaning of accompanying non-target words. On the approach to perception developed by Treisman (e.g. 1986), this is surprising, because meaning might be thought to require analysis of conjunctions of physical features and so should remain uncomputed for non-target words. Treisman's approach does, however, assert that analysis of the target may unleash further processes that would prime the system for detection of related words. If this were so, then presentation of the target earlier than the distractors would increase the effect of the latter; whereas if analysis of non-targets were independent of priming, they might be expected to have a smaller effect when delayed. Further, if the sets of words involved are small and familiar, then individual features of primed non-targets, rather than conjunctions of features, might trigger interference. They might especially do so when spatial separation of target and non-target is small. Five experiments using a paradigm developed by Shaffer and LaBerge confirm that the meaning of non-target words affects response to targets; but (1) this is more true for early than for late arrival of the target; (2) it is affected by target/non-target separation in space; (3) it is true for familiar sets of repeated words but not, in these data, for words used once only in the experiment. It is therefore concluded that the results are more consistent with a Treisman type of explanation than with a theory of universal and automatic full analysis.