Spatial working memory load impairs signal enhancement , but not attentional orienting

916 The visual system cannot process all the sensory inputs that it receives, because our capacity for processing visual information is limited. When faced with a huge amount of information, our selective attention allocates limited resources to the most critical and relevant information, thus preventing information overload. By attending to a specific component of a visual scene, selective attention allows certain information to be processed in finer detail or given greater processing priority. It has been suggested that visual or spatial information is maintained in working memory via allocations of attention to memory representations, activated online, of relevant objects or locations. In response, numerous studies have investigated selective attention together with working memory (Awh, Jonides, & Reuter-Lorenz, 1998; Cowan, 2001; Engle, 2002). A number of studies have proposed that spatial selective attention and spatial working memory functionally overlap and that spatial attention plays a critical role in the maintenance of spatial information in working memory (Awh & Jonides, 2001; Awh et al., 1998; Jonides, Lacey, & Nee, 2005). Awh et al. (1998) reported impaired spatial working memory accuracy with shifts of attention from a memorized location. They concluded that memorizing locations requires selective attention to be shifted to those locations. In addition, a recent brain imaging study has demonstrated considerable overlap in areas of the brain active during selective attention and spatial working memory tasks (Corbetta, Kincade, & Shulman, 2002). Oh and Kim (2004) and Woodman and Luck (2004) noted that the maintenance of spatial information in working memory impairs the efficiency of concurrent visual search tasks. Their studies imply that spatial working memory and visual search share a common resource. This close interaction between spatial working memory and selective attention needs to be specified further, because attentional effects on visual processing are heterogeneous. Prinzmetal and colleagues suggested that attention affects visual processing via two distinct mechanisms (Prinzmetal, McCool, & Park, 2005). First, attention can enhance the perceptual representation of an attended location (Luck, Hillyard, Mouloua, & Hawkins, 1996; Luck & Thomas, 1999; Moore & Egeth, 1998). In data-limited conditions (Norman & Bobrow, 1975), in which stimuli are presented briefly, observers respond faster and more accurately when a spatial cue is given. In this case, the enhanced performance afforded by the cue is mediated by sensory modulation on the attended location. That is, the primary factor that improves behavioral performance under data-limited conditions is the quality of the perceptual representations of stimuli. Prinzmetal et al. have labeled this processing channel enhancement and suggest that it corresponds with what other researchers have termed signal enhancement (Carrasco, Williams, & Yeshurun, 2002; Lu & Dosher, 1998; Yeshurun & Carrasco, 1999). Generally, in data-limited conditions, response accuracy has been used as a primary dependent variable for measuring behavioral performance, because the quality of visual information is degraded and observers are required to take enough time to respond as accurately as possible. The point to be noted is that an attentional cuing effect is found in accuracy only if the perceptual representation is enhanced. Second, atSpatial working memory load impairs signal enhancement, but not attentional orienting