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In the episodic memory domain, similar control processes and corresponding functional neuroanatomy have been identified. In this context, results from studies using item recognition

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memory tasks and a comparison of item vs. source-memory retrieval tasks are relevant to the current discussion. Wagner et al. (A. D. Wagner et al., 1998) contrasted encoding and retrieval related activity using a high-frequency words (verbal) and chromatic visual textures (non- verbal) stimuli in an fMRI study. Their results revealed that regardless of the mnemonic operation (encoding vs. retrieval) greater BOLD activity was identified for words relative to textures in posterior left dorsal inferior frontal gyrus (BA 44) and anterior left inferior frontal gyrus (BA 45/47). The opposite contrast revealed greater activity for textures relative to words in right precentral and posterior inferior frontal gyri (BA 44/6) and anterior right inferior frontal gyrus (BA 45). The results from the above study imply that the inferior frontal regions implicated in top-down control are recruited in stimulus-content-dependent manner, i.e. retrieval of (meaningful) verbal vs. (non-meaningful) nonverbal information was associated with activity in left and right ventral IFG, respectively. The content-dependent hemispheric lateralization of regions implicated in top-down control, was recapitulated in multiple studies from Dobbins and colleagues, as summarized below.

Dobbins et al. (I. G. Dobbins & Wagner, 2005) conducted a study using a source memory retrieval task that held the retrieval cues (pictures of common animals and artifacts) constant while experimentally varying the nature of the information to be recollected. The items were initially encoded via a perceptual judgment task (a 1-back size rating task) or one of two conceptual judgment tasks (living/non-living or pleasant/unpleasant). Three types of retrieval tasks were conducted during a post-encoding fMRI session, two of which were source retrieval tasks (conceptual vs. perceptual) and the third was a novelty-detection task in which

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subjects identified a novel picture from among two additional previously seen probes. During conceptual source retrieval, subjects had to remember whether they made living/non-living or pleasant/unpleasant judgments whereas during perceptual source retrieval, they had to

remember whether the object appeared in large or small size. Two relevant findings are discussed below.

First, left vIFG pars orbitalis and left MTG were selectively engaged during conceptual source retrieval, whereas right vIFG (BA 44/45) and right inferior temporal cortex (ITC) were engaged during perceptual source retrieval. Consistent with prior work ,(Gold et al., 2006) the former finding was interpreted as supporting a domain-specific role for left IFG pars orbitalis and left MTG in controlled semantic retrieval. To test a functional coupling hypothesis between left MTG and vIFG, the authors conducted a correlational analysis between the difference in peak task-evoked activity in the aforementioned regions during perceptual vs. conceptual source retrieval trials. The analysis revealed a positive correlation between the left vIFG pars orbitalis and the left MTG, and a marginally significant positive correlation between the right vIFG and right ITC, confirming the expected context-specific functional coupling.

Second, Dobbins et al., (I. G. Dobbins & Wagner, 2005) identified activity in left IFG pars triangularis interpreted as supporting a domain-general role in post-retrieval selection. The supporting evidence was such that relative to novelty detection, the two relatively more

demanding source retrieval conditions (confirmed behaviorally), resulted in greater BOLD activity with no concurrent difference due to source content (perceptual vs. conceptual). Note that despite the conceptual vs. perceptual processing distinction during encoding, the study

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used the same retrieval probes, all of which were meaningful pictures. As stated above, the proposed interpretation of the greater recruitment of left vIFG pars orbitalis, during conceptual relative to perceptual source memory retrieval was that it is reflective of domain specific role in controlled semantic retrieval. An alternative interpretation, although less likely given

converging evidence implicating left vIFG pars orbitalis in controlled semantic retrieval (Badre et al., 2005; Roskies et al., 2001; A D Wagner et al., 2001), is that perceptual vs. conceptual source retrieval difference is driven by differential demand in selection from competing episodic representations. To adjudicate between the two interpretations, the Dobbins group conducted a follow-up study, which, as summarized below, supported a role for left vIFG pars orbitalis in controlled semantic retrieval.

The follow up experiment (Han et al., 2011) used a similar setup as the prior study discussed above (I. G. Dobbins & Wagner, 2005). In contrast to the earlier study that used a single set of meaningful picture probes, the follow up study employed two sets of picture stimuli. The first was a set of photographs of common scenes (e.g. kitchen) designed to evoke meaningful associations. The second set was based on (meaningless) fractal patterns generated from the same photographs of scenes using kaleidoscopic rendering software, which helped match the two sets of stimuli in basic color characteristics. On both the meaningful scenes and meaningless fractal probes, subjects performed pleasant/unpleasant or simple/complex

judgments. Two findings emerged, one of which was consistent with those from the prior study while the second led to a slightly different interpretation. In support of a role in controlled semantic retrieval, left vIFG pars orbitalis and left MTG were recruited to a greater extent

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during source memory retrieval relative to item recognition exclusively for the meaningful scenic pictures. Recapitulating prior findings (Bokde, Tagamets, Friedman, & Horwitz, 2001), a subsequent resting state functional connectivity analysis revealed a correlational coupling between left vIFG and left MTG, supporting their hypothesized interactive functional roles in controlled semantic retrieval task settings. Taken together, the aforementioned results were interpreted as supporting the role of left vIFG pars orbitalis in controlled semantic retrieval.

The second finding suggested that the role of left vIFG pars triangularis in post- retrieval selection is more specific to linguistic stimuli (i.e. less domain-general). Unlike the prior study that revealed higher BOLD activity during both perceptual and conceptual source retrieval relative to novelty-detection, the subsequent study only revealed differences between novelty-detection and source retrieval for meaningful stimuli. The absence of a difference between novelty-detection and source retrieval for non-meaningful stimuli, was suggestive that the putative post-retrieval selection processes supported by left vIFG pars triangularis may be specific to linguistic representations.

To summarize, studies conducted in the domains of episodic and semantic memory retrieval, specifically targeting top-down control processes, have revealed similar functional neuroanatomy commonly recruited during episodic and semantic memory retrieval. In the episodic memory domain, left vIFG pars orbitalis has demonstrated recruitment to a greater extent when control demands are high, e.g. conceptual source retrieval vs. item memory retrieval. In the semantic domain, the region is commonly recruited in semantic tasks that likely require controlled retrieval (e.g verb generation, semantic judgment/classification (J A

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Fiez, Raichle, & Petersen, 1996; Petersen et al., 1988; Roskies et al., 2001; A D Wagner et al., 2001)), while being absent in simple word vs. nonword lexical decision tasks in which

automatic semantic access may be sufficient for task performance (Fiebach et al., 2002, 2007; Henson et al., 2002). This interpretation is also supported by studies that directly dissociated automatic vs. controlled retrieval in left vIFG pars orbitalis in a targeted fashion (Gold et al., 2006). The aforementioned summary of common top-down control related functional

neuroanatomy further highlights the similarities between episodic and semantic memory retrieval processes.

4.6 Top-Down Control in Declarative Memory Retrieval –