The Effect of Objecthood on Processing Efficiency

The Effect of Objecthood on Processing Efficiency Luiza Shahbazyan (l.shahbazian@gmail.com) Central and East European Center for Cognitive Science, Department of Cognitive Science and Psychology, New Bulgarian University, 21 Montevideo Street Sofia 1618, Bulgaria Boicho Kokinov (bkokinov@nbu.bg) Central and East European Center for Cognitive Science, Department of Cognitive Science and Psychology, New Bulgarian University, 21 Montevideo Street Sofia 1618, Bulgaria Abstract The term objecthood is used to convey the degree of image regularity in terms of properties such as closure, symmetry and parallelism. The notion of objecthood is evaluated in the light of the existing psychological data on sensory-driven biases of attentional selection. Extending the biased competition for selection framework, we tested if higher objecthood of the image modulates its salience and exerts influence on the efficiency of its processing. In a series of three experiments, it is demonstrated that the presence of closure significantly impacts processing efficiency of the presented visual stimuli. Keywords: vision; selective attention; saliency. Introduction Visual image regularities are important both for the interpretation of the ambiguous visual flow and for the guidance of selective attention resources for extensive analysis. Current understanding of bottom-up saliency underestimates the influence of these structural properties on the selection process. Most theories and models propose that regions compete for resources on the basis of their visual conspicuity. The aim of this paper is to demonstrate that it is the regularity of an image in terms of properties such as closure, referred to as its degree of objecthood, which determines its efficiency of processing and consequent selection, than merely its visual distinctiveness. Due to the limited capacity of the visual system, only part of the information is selected for in-depth processing. It is important to direct resources towards regions that are potential figures because, first, usually they contain relevant information, and second, figures have invariant characteristics like shape, crucial for recognition, while the shape of the background varies depending on the figure and is meaningless (Rubin, 2001). It is generally accepted (Niebur & Koch, 1996; Itti & Koch, 2001; Desimone & Duncan, 1995) that selection operates on single features such as color, size and orientation, but they are not bound in any structure. The factor that guides attentional deployment and bias the selection of a region is its distinctiveness, defined by unique color, orientation or contrast, in comparison to the entire scene. Although probably there is strong correlation between clusters of features and objects, we argue that what differentiates figures and backgrounds are qualitative differences. It has been extensively documented that the visual system makes use of various data regularities to constrain the possible combinations of elements and speed up the segmentation of the visual field into objects. These are the figural principles extensively investigated by Gestalt psychologists: size, closure, orientation, and contrast, symmetry, parallelism, convexity, meaningfulness (see Palmer, 1999 for a review), and lower region (Vecera, Vogel, & Woodman, 2002). They are highly unlikely to occur by chance (Lowe & Binford, 1982), so they are diagnostic for the presence of a figure in this location of the visual field. However, the issue of whether such figural principles provide selection advantage and what is the mechanism that underlies it, has been left unexplored. Desimone and Duncan (1995) proposed that attentional selection is best understood as competition between all objects in the scene for cortical representation. Stronger, more salient stimuli have competitive advantage over weaker ones because they evoke more robust neural response. Although their account is mainly concerned with the physical distinctiveness of the stimuli, there is evidence that the degree of objecthood is still another factor that determines competitive strength. In the following section we will review behavioral data from visual search and object-based attention paradigms that one figural principle, closure, is associated with prioritized processing. Then, in a series of three experiments, we will examine further the nature and mechanisms of this competitive advantage. Visual search task is a kind of task that requires serial scanning of a number of items for a particular feature. When the search is efficient, the target feature captures attention, the so-called pop-up, by virtue of its salience in comparison to the other items and the time to report it does not depend on the number of distractors (Treisman & Gelade, 1982). There is evidence that targets with particular non-accidental properties such as closure, pop out. Conci, Muller and Elliott (2007) reported that when participants were presented with search display that contains a target consisting of four corner junctions grouped by closure, and distractors containing the same four corner junctions but arranged to be open or semi-closed, the search performance for the target was unaffected by the number of distractors.