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P.C. and F.M. contributed equally to the study.
INTRODUCTION
Although traditional models have long considered that medial temporal lobe (MTL) structures are crucial for the formation of long-term memories (Eichenbaum, 1997), but not for working memory (WM) (Cave & Squire, 1992; Gabrieli, 1998; Squire et al., 2004), there is an emerging view supporting the essential contribution of MTL in WM processes (for a review, see Gazzaley et al., 2004; Ranganath & Blumenfeld, 2005). Data from animal lesion and neurophysiological experiments (Davachi & Goldman-Rakic, 2001; Holscher & Rolls, 2002; Suzuki et al., 1997; Young et al., 1997), as well as findings from human neuropsychological studies (Haettig et al., 2004; Lancelot et al., 2003; Nichols et al., 2006; Olson et al., 2006; Owen et al., 1996; Wagner et al., 2008), provide evidence for the critical involvement of MTL in WM. Recent human neuroimaging studies provide further support for the relevant contribution of MTL activation during WM (Axmacher et al., 2007, 2008; Cabeza et al., 2002; Glabus et al., 2003; Karlsgodt et al., 2005; Lee et al., 2006; Manoach et al., 2003; Monk et al., 2002; Petersson et al., 2006; Ranganath & D'Esposito, 2001; Schon et al., 2004; Stern et al., 2001; Tesche & Karhu, 2000).
While most of the neuroscientific research has focused on the study of brain activity during WM maintenance, "the neural mechanisms for WM encoding have not been extensively investigated" (Ranganath et al., 2004). Several authors (Cowan, 1995, 1999; Miyake & Shah, 1999) have considered an issue of central importance to understanding WM, the precise characterization of how the encoding of task-relevant information takes place in the context of WM tasks. In a previous study (Campo et al., 2005b), we used magnetoencephalography (MEG), a noninvasive technique with high temporal resolution and good spatial resolution, to investigate the encoding phase of a verbal task and a spatial WM task. The temporal pattern of neural activity provides valuable information to understand the cognitive functions supported by different cortical areas (Axmacher et al., 2006; Helenius et al., 1998). Thus, using MEG, we could specify not only which brain structures were involved but also a detailed time course of their activation. We observed a material-specific modulation of the activity of MTL, consisting...