Academic journal article Perception and Psychophysics

Top-Down Effects of Semantic Knowledge in Visual Search Are Modulated by Cognitive but Not Perceptual Load

Academic journal article Perception and Psychophysics

Top-Down Effects of Semantic Knowledge in Visual Search Are Modulated by Cognitive but Not Perceptual Load

Article excerpt

Moores, Laiti, and Chelazzi (2003) found semantic interference from associate competitors during visual object search, demonstrating the existence of top-down semantic influences on the deployment of attention to objects. We examined whether effects of semantically related competitors (same-category members or associates) interacted with the effects of perceptual or cognitive load. We failed to find any interaction between competitor effects and perceptual load. However, the competitor effects increased significantly when participants were asked to retain one or five digits in memory throughout the search task. Analyses of eye movements and viewing times showed that a cognitive load did not affect the initial allocation of attention but rather the time it took participants to accept or reject an object as the target. We discuss the implications of our findings for theories of conceptual short-term memory and visual attention.

Desimone and Duncan (1995) suggested that selective visual attention emerges from a biased competition among visual stimuli, moderated by both bottom-up saliency and top-down knowledge (e.g., stored object representations). Top-down control is assumed to be exerted from working memory (WM), where an object template is held that can guide the allocation of visual selective attention (see Chelazzi, Duncan, Miller, & Desimone, 1998; Chelazzi, Miller, Duncan, & Desimone, 1993; Duncan & Humphreys, 1989). Although numerous studies have focused on visual aspects of top-down influences on visual search (e.g., Downing, 2000; Soto, Heinke, Humphreys, & Blanco, 2005; Woodman, Vogel, & Luck, 2001), less is known about semantic influences on the deployment of visual selective attention.

Moores, Laiti, and Chelazzi (2003, Experiment 4) demonstrated the existence of top-down associative effects on the deployment of visual attention during visual search for objects. They gave participants a target prompt (e.g., "motorbike"), followed by a central fixation point and an object display of four objects. The display was flashed only briefly (73 msec on average; individually determined presentation times ranged from 47 to 97 msec). The authors compared participants' performance when an associate to the target (e.g., "motorbike helmet") was present with their performance when no such associate was present. They obtained no effect of the presence of an associate on target-present trials. However, on target-absent trials, participants were less accurate and responded more slowly when an associate was present (with 18% false "targetpresent" reports and a reaction time [RT] of 867 msec) compared with how they performed when no associate was present (with 10% false '"target-present" reports and an RT of 801 msec). The authors hypothesized that, following the establishment of a template for a target (Duncan & Humphreys, 1989), there is a spread of activation to templates of semantically related items. Related items then compete for selection and are more likely than are unrelated distractors present in the field to be selected.

The findings obtained by Moores et al. (2003) imply that participants were able to quickly activate and process conceptual information about objects in the visual field. This is in keeping with theories of early and rapid conceptual processing, such as Potter's (1976, 1993, 1999) conceptual short-term memory (CSTM) hypothesis. Potter proposed that the apparent effortlessness with which people process conceptual information when reading or attending a conversation requires a form of WM that allows for a rapid identification and initial conceptual processing of incoming information. CSTM differs from early stages of visual short-term memory in that it is associated with rapid access to conceptual information about fixated stimuli far beyond visual stimulus properties. With this information, semantic associations that exist in longterm memory (LTM) between momentarily active items become available as well. …

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