Academic journal article Canadian Journal of Experimental Psychology

Multiple Parallel Access in Visual Attention

Academic journal article Canadian Journal of Experimental Psychology

Multiple Parallel Access in Visual Attention

Article excerpt

Abstract It is widely accepted that there exists a region or locus of maxim al resource allocation in visual perception - sometimes referred to as the spotlight of atte ntion. We have argued that even if there is a single locus of processing, there must be multipl e loci of parallel access - several places in the visual field must be indexed at once and these in dexes can be used to determine where attention is allocated. We have carried out a variety of stu dies to support these ideas, including experiments showing that subjects can track multiple inde pendent moving targets in a field of identical distractors, that the enhanced ability to detect changes occurring on these targets does not accrue to nontargets nor to items lying inside the con vex polygon that they form (so that a zoom - lens of attention does not fit the data). We have u sed a visual search paradigm to show that (serial or parallel) search can be confined to a su bset of indexed items and the layout of these items is of little importance. We have also studi ed the phenomenon known as subitizing and have shown that subitizing occurs only when i tems can be preattentively individuated and in those cases location precuing has little effe ct, compared with when counting occurs, which suggests that subitizing may be carried out by count ing active indexes rather than items in the visual field. And finally we have run studies showing that a certain motion effect that is sensitive to attention can occur at multiple precu ed loci. We believe that this evidence suggests that there is an early preattentive stage in vision where a small number of salient items in the visual field are indexed and thereby made readily accessible for a variety of visual tasks.

It has generally been assumed that there is a single focus of attention, represe nting the current locus of visual processing, that can be moved across the visual field independen t of eye movements. The precise details of this "spotlight" view vary among investigator s, although it is universally thought to apply to a single contiguous spatial region. There is some disagreement as to the possible shape of the attended region (whether circular or toroidal), whether the extent of the spotlight can be varied by "zooming", under what conditions - voluntary o r automatic - the spot can move, whether the spotlight must move continuously, and if so at wh at speed, or whether it can skip from place to place, and precisely what processing advantage accrues at the current locus. Despite these differences, the idea that a single locus is invol ved in allocation of processing resources seems widely accepted.(f.1) And yet there is good reason t o doubt that this is the entire story of how spatially local information is accessed in the visual field. In particular, there is reason to believe that even if attention is unitary and spatially focus sed, there is also a more primitive mechanism for simultaneously indexing several places in a visual field, thus individuating these places and making them directly accessible for further proce ssing. There are both general considerations and some direct experimental data that are relevant to this issue .

General Considerations Suggesting the Need for a Multiple - Locus Indexing Mechanism

In order to detect simple relational geometrical properties in a visual scene - properties like insidedness, collinearity, and so on - the visual system must be able to in some way simultaneously reference more than one place or feature or object in the scene,( f.2) since the relations in question apply synchronically over several places. Of course it co uld be that places are scanned in series and their coordinates (or distance & direction of scanning ) recorded for each of the places, and then the property is subsequently computed from the inte rnalized synchronic representation. But evidence concerning the speed and accuracy with which we can detect such properties as collinearity over a series of points seems to argue ag ainst an alternative that involves prior scanning to locate the elements of the relation. …

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