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4.6 Top-Down Control in Declarative Memory Retrieval – Current
In the current dataset, we made three observations that are relevant to the discussions on the role of top-down control in memory retrieval. The first observation is that we identified a region in the left IFG pars triangularis exhibiting BOLD timecourse profiles consistent with a domain-general role in top-down processes guiding memory retrieval. The second finding was that even though a literature-derived region in the left ventral IFG was identified, the region was only partially consistent with a semantic retrieval hypothesis. Third, we identified two sets of bilaterally distributed regions previously implicated in task-control. These regions, when put in a clustering analysis with the rest of the task-evoked regions, organized into two distinct modules: one that closely resembles the cingulo-opercular (COP) control system implicated in sustained task set maintenance and one that includes frontal regions of the frontal-parietal control system implicated in adaptive task-control operations (Dosenbach et al., 2006, 2007). The three observations are discussed below.
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Upon examining the functional profile in left dorsal IFG pars triangularis across stimulus types, the most consistent observation was, as mentioned above, greater error-related BOLD activity, relative to correct trials, with no evidence of modulation by stimulus semantic content (e.g. meaning vs. perceptual training). Given its (stimulus-nonspecific) response to errors that is suggestive of a role in task-control, and its clustering-based assignment with regions resembling the frontoparietal adaptive task control system (Dosenbach et al., 2007; J. D. Power et al., 2011), we posit a role for dorsal IFG in domain-general task control in line with previous proposals (Badre et al., 2005).
As for the left ventral IFG (pars orbitalis), we had (an initially surprising) non-finding in that the region was not identified in any of the primary voxelwise contrasts conducted to test our a-priori expectations for regions involved in semantic memory retrieval. This (non) finding led us to consider that, consistent with other studies that failed to identify vIFG in a simple lexical decision task context (Fiebach et al., 2002, 2007; Henson et al., 2002), the task may not require controlled semantic retrieval, i.e. automatic semantic access may be sufficient for task performance. Subsequently, examination of literature-derived regions did identify vIFG
exhibiting some properties consistent with a role in semantic processing. The observed profiles were very similar to that observed in left MTG in that the region exhibited sensitivity to
previously known word meanings (based on word > PW and word repetition suppression effects) but not to the novel words acquired during the experiment. Unlike the proximal IFG triangularis discussed above, ventral IFG did not exhibit profiles suggestive of a role in domain general task control, such as the error > correct profile. Given the profiles described above for
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vIFG, we offer a similar hypothesis as was forwarded for MTG – it may be the case that the reason for vIFG failing to be engaged by the novel words is that the information has yet to be adequately consolidated into long-term semantic memory representations. The future
experiment, suggested in the previous section, can similarly test for the hypothesis that, given more training and consolidation opportunity, the novel words may indeed begin engaging the left vIFG. In addition, recall that in the potential follow-up experiment we also suggested conducting the recognition memory and semantic priming experiments in an fMRI setting. Those experiments, particularly the semantic priming paradigm, would be suited for testing the aforementioned hypothesis because that paradigm can be leveraged to target automatic vs. controlled semantic retrieval processes, for instance via SOA manipulations, as documented previously (Gold et al., 2006).
Finally, as mentioned above, clustering of regional task-evoked timecourses revealed two clusters with functional neuroanatomy resembling previously identified frontal parietal and cingulo-opercular task control systems. The frontal parietal cluster itself, as well as most of the constituent regions, exhibited BOLD timecourse profiles consistent with a domain-general role in task-control. Most of the regions exhibited a canonical error > correct trial response typical of control regions likely corresponding to a performance feedback signal (Badre & Wagner, 2004; Botvinick et al., 2004; Carter et al., 1998; Dosenbach et al., 2007). Similarly, in addition to anatomically corresponding to regions previously characterized as forming the core of a task-set maintenance system, the identified COP module, and most of the constituent regions, also showed a typical control profile of higher activity for errors than correct trials. Given that
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control demand was not explicitly tested as an experimental factor, we can only ascribe the two modules discussed above and the regions within, with a role in top-down control based on ancillary evidence, such as the error responses described above, and neuroanatomical correspondence with prior work (Dosenbach et al., 2006, 2007; J. D. Power et al., 2011).
To conclude this section, in the current lexical decision task context, we identified regions whose functional profiles, prior history in the literature, and clustering-based
characteristics were consistent with previously outlined roles in domain general task control. The evidence for a domain-specific brain region (i.e. left IFG pars orbitalis) dedicated to controlled semantic retrieval was less convincing, at least based on the region’s absence during memory retrieval of novel words. However, given that the left IFG pars orbitalis was recruited by the previously known words, and the potential caveat of inadequate consolidation for the novel words, a role in domain-specific controlled semantic retrieval cannot be ruled out based on our findings.