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Dissociating the Components of Working Memory: An Event-Related fMRI Study of Memory Load and Distraction

Working memory comprises executive functions responsible for the manipulation of information and maintenance functions responsible for keeping information active over short intervals (Baddeley, 1996). Physiological studies in humans and animals have attempted to identify brain regions involved with working memory and the component processes they subserve (e.g., Cohen et al., 1997; Goldman Rakic, 1987). Neuroimaging studies in humans have demonstrated a consistent pattern of activation across a variety of working memory tasks, with prominent activation of dorsolateral prefrontal regions (dPFC) (Cohen et al., 1994; D'Esposito et al., 1995; McCarthy et al., 1994). Some studies have supported the thesis that prefrontal activity reflects the operation of executive processes (D'Esposito et al., 1995) while others have argued that prefrontal activation reflects maintenance processes (Braver et al., 1997; Cohen et al., 1997). Other studies have shown that attention tasks that require no memory for stimulus sequence evoke similar (although lesser) activation in these same dPFC regions as working memory tasks (McCarthy et al., 1997). Finally, McCarthy and colleagues (Kirino et al., in submission) have demonstrated that infrequent and irregularly presented target stimuli also evoke similar dPFC activation, and have suggested that response categorization may be a relevant factor in the activation of this region. Thus while dPFC is reliably activated during working memory tasks, the precise function or functions reflected by this activation is as yet uncertain.

Posterior brain regions are also activated in working memory tasks (e.g., Belger et al., 1998; Haxby et al., 1995; Jonides et al., 1993). Of particular interest is the degree to which extrastriate regions activated by complex visual stimuli such as faces participate in the maintenance of those stimuli during delay intervals. To date the issue is undecided with some studies concluding they do not (Belger et al., 1998; Haxby et al., 1995), while a recent study has concluded that they do (Courtney et al., 1997). One factor potentially relevant to extrastriate activation during working memory tasks is the degree to which sustained activation, when observed, is maintained in the presence of task irrelevant visual distractors.

Here we propose to use behavioral analysis and functional MRI to investigate the degree to which prefrontal and extrastriate regions participate in stimulus maintenance. There are two specific aims: 1. To investigate the degree to which sustained activation of dPFC and face specific regions of fusiform gyrus is influenced by memory load for faces during a delay interval. 2. To investigate the degree to which visual distractors presented during the delay interval influence memory performance and affect sustained activation of dPFC and face specific regions of the fusiform gyrus.