Neural Bases of Focusing Attention in Working Memory: An fMRI Study Based on Group Differences

Article excerpt

Using fMRI, we investigated neural substrates for focusing attention in working memory. To explore this focusing effect, two kinds of reading span test (RST), focused and nonfocused, were performed. In the focused RST (F-RST), the target word to be maintained was the focus word in the sentence. In the nonfocused RST (NF-RST), the target word was not the focus word in the sentence. In both RST conditions, significant activations were found in three main regions: left dorsolateral prefrontal cortex (DLPFC), anterior cingulate cortex (ACC), and left superior parietal lobule (SPL). In addition, fMRI signal changes increased in the left SPL in the NF-RST condition. These findings suggest that the neural substrates of focusing attention are based on SPL and ACC-DLPFC networks. Furthermore, there were group differences in the focusing effect between high-span and low-span subjects: High-span subjects showed focusing effects (supported by SPL control) greater than those of low-span subjects.

Working memory supports immediate brain involved in the storage and processing of information plays a role in higher cognitive brain functions, such language comprehension, learning, and reasoning deley, 1986; Just & Carpenter, 1992). Working plays a particularly critical role in comprehension cesses during text reading. Incoming information, for ample, is decoded perceptually, recognized, and stored short periods while being integrated into a textual pretation (Daneman & Carpenter, 1980). In this working memory plays an important role in storing intermediate or final products, allowing readers or ers to integrate the contents of a text into context (Just & Carpenter, 1992).

Recent neuroimaging studies have explored the basis of two types of working memory system by Baddeley (1986). It was found that the two types working memory processes are subserved by distinct tical structures (Smith & Jonides, 1999). The retention verbal information in the phonological loop is with activation in the left ventrolateral prefrontal (VLPFC), and the retention of visuospatial information witii activation in the right homologues (Awh et al., 1 996; Jonides et al., 1993; Owen et al., 1998; Paulesu, Frith, & Frackowiak, 1993; Smith & Jonides, 1999; Smith, Jonides, & Koeppe, 1996).

It has been suggested that the executive control system is located in the dorsolateral prefrontal cortex (DLPFC). Increased activation in DLPFC was found during a dual task (D'Esposito et al., 1995), an n-back task (Cohen et al., 1993; Smith et al., 1996), and other working memory tasks that required executive control (Braver et al., 1997; Bunge, Klingberg, Jacobsen, & Gabrieli, 2000; D'Esposito, Postle, Ballard, & Lease, 1999; Rypma, Prabhakaran, Desmond, Glover, & Gabrieli, 1999; Smith & Jonides, 1999).

The executive control system especially serves as an attention controller that allocates and coordinates attentional resources for cognitive tasks (Baddeley, 1996; Engle, Tuholski, Laughlin, & Conway, 1999). According to this theory, it is conceivable that the DLPFC plays a role within the attention control system that is required during dual-task performance or when the maintenance function exceeds an individual's short-term memory span. Brain-imaging studies have also suggested that the central executive system is located in both DLPFC and anterior cingulate cortex (ACC) (Bunge et al., 2000; D'Esposito et al., 1995; M. Osaka et al., 2003; N. Osaka et al., 2004; Smith et al., 2001; Smith & Jonides, 1999).

Moreover, dissociation of ACC and DLPFC in attention control for cognitive task performance has also been discussed. MacDonald, Cohen, Stenger, and Carter (2000) demonstrated that DLPFC provides top-down support for attention maintenance during task-appropriate behaviors, whereas ACC plays a role in controlling attention that needs to be strongly engaged, such as for monitoring performance in the Stroop task. …