Hermann J. Müller

In visual search for singleton feature targets, detection RTs are faster when the target-defining dimension is constant across trials rather than variable; in the latter case, RTs are faster when the target dimension on a trial is the same, rather than different, relative to preceding trial (with little effect of a feature change within a repeated dimension); and RTs are expedited when the target dimension is validly indicated by a symbolic precue on a given trial (e.g., Found & Müller, 1996; Müller, Heller, & Ziegler, 1995; Müller, Reimann, & Krummenacher, 2003). Müller and his colleagues have explained these effects in terms of a “dimension-weighting” account, according to which these effects arise at a preattentive, perceptual stage of saliency coding. In contrast, Cohen (e.g., Cohen & Magen, 1999) and Theeuwes (e.g., Theeuwes et al., 2006 this issue) have recently argued that these effects are postselective, response-related in nature. The present paper examines these challenges and puts forward counterarguments in support of the view that dimensional weighting operates at a preattentive stage of processing.

Visual search is a key paradigm in attention research that has proved to be a test bed for competing theories of selective attention. The starting point for most current theories of visual search has been Treisman's “feature integration theory” of visual attention (e.g., Treisman & Gelade, 1980 Treisman, A. and Gelade, G. 1980. A feature-integration theory of attention. Cognitive Psychology, 12: 97–136. [Crossref], [PubMed], [Web of Science ®] [Google Scholar] ). A number of key issues that have been raised in attempts to test this theory are still pertinent questions of research today: (1) The role and (mode of) function of bottom-up and top-down mechanisms in controlling or “guiding” visual search; (2) in particular, the role and function of implicit and explicit memory mechanisms; (3) the implementation of these mechanisms in the brain; and (4) the simulation of visual search processes in computational or, respectively, neurocomputational (network) models. This paper provides a review of the experimental work and the—often conflicting—theoretical positions on these thematic issues, and goes on to introduce a set of papers by distinguished experts in fields designed to provide solutions to these issues.

Five experiments were carried out to investigate under which conditions the apparent size of objects is computed and exploited optimally in visual search for size-defined targets. Observers searched for a target test circle that was retinally larger than the distractor test circles, with both types of circles surrounded by context circles modulating the apparent size of the test circles (Ebbinghaus illusion). RTs were the faster the better test circles could be differentiated from the context circles, ie with smaller numbers of context circles, larger distances, and higher lightness (or colour) contrast between test and context circles. Apparent-size modulation had a strong influence on search RTs, resulting in faster RTs with smaller, and slower RTs with larger, context circles. A model assuming overall facilitatory effects of the apparent-size modulation and interference effects arising from decreasing test-context circle discriminability can explain the present results.

Four pop-out search experiments investigated whether dimension-based visual attention is top-down modulable. Observers searched for singleton feature targets defined, variably across trials, by a color or an orientation difference to nontargets. Observers were precued to the most probable target-defining dimension (e.g., by the word color) or feature (red) on a given trial. Results revealed expedited reaction times (RTs) for valid-dimension targets relative to neutral-cue conditions, and slowed RTs for invalid-dimension targets. Cue information as to precise target feature yielded some extra effect only for color targets. The dimensional cuing significantly reduced, but did not abolish, the dimension-specific influence of the previous target on detection of the current target (same-dimension RT < different-dimension RT). These findings confirm that top-down dimensional set modulates stimulus-driven dimension processes in the detection of pop-out signals. ((c) 2003 APA, all rights reserved)

Six visual search experiments were carried out to investigate the processing of size information in early vision. The apparent size of display items was manipulated independently of their retinal size by placing items on a textured surface which altered the perceived distance in depth of the items. Overall, these experiments demonstrate that a target item differing from non-target items in terms of apparent size can be detected efficiently. However, the pattern of results indicates that, rather than deriving apparent-size information, target detection is guided by discontinuities in the ‘retinal-size gradient’ of items, in particular between items at the same ‘depth’. Although the arrangement of items on the texture surface strongly influenced search, this was largely due to the retinal size of items and the retinal separation between items. The implications of these experiments for the nature of the pre-attentive representation of size are discussed.