Los compromisos para incorporar variables demográficas en planificación de desarrollo . 4

There are a number of observations that emerged from the current project that are relevant to the preceding discussion. First, left medial and lateral parietal regions previously implicated in episodic memory retrieval show many properties similar to the two regions (PHG and mSFC) that were top candidates for semantic memory retrieval, particularly for novel words. These properties shared by the two sets of regions include a bigger effect of meaning relative to perceptual training as well as a training type X retrieval accuracy interaction. These observations may suggest a role for the parietal regions in semantic retrieval of novel words, consistent with the a-priori criteria. However, as noted below, this may be a premature interpretation.

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The second observation is that, despite the similar profiles discussed above for PHG/mSFC vs. medial/lateral parietal regions implicated in episodic retrieval with respect to memory retrieval for novel words, the response for real words resulted in a functional

distinction between PHG/mSFC and medial/lateral parietal regions. The first distinction is the finding that while PHG/mSFC showed a pattern of word repetition suppression across days, the parietal regions showed an effect resembling the old > new episodic retrieval success effect (i.e. repetition enhancement) for words. The second distinction, admittedly more subtle, was the finding that while both sets of regions showed a ‘word > novel PW’ effect prior to training, the effect was driven by a less negative timecourse for words relative to PWs. It is clear that despite the similar properties recapped above, the PHG/mSFC also exhibit properties different enough from the parietal regions to suggest distinct functional roles. The noted functional distinction is in support of distinct semantic and episodic contributions made by left PHG/mSFC and the left parietal regions, respectively, to the memory retrieval of the novel words.

A third observation is that, unlike PHG/mSFC which were engaged during retrieval of both novel and previously known words, regions in the lateral temporal cortex, which we expected to show similar properties, were only engaged by previously known words. At this early stage following novel word learning, the current data suggest that the PHG and mSFC are central to the memory retrieval process. Both the Complementary Learning Systems

(McClelland et al., 1995; McClelland, 2013) and the Multiple Traces Model (Nadel & Moscovitch, 1997) discussed above would predict the involvement of the MTL in retrieving

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recently acquired memory for novel word meanings. However, the fact that the PHG also demonstrates sensitivity to remote memories (i.e. the previously known words) is perhaps more consistent with the MTM model.

Again, with regard to timing, the novel words likely have yet to undergo further exposure and consolidation to engage classic neocortical regions, such as the lateral temporal cortex, typically recruited for word-level semantic memory retrieval. The lack of engagement of the left lateral temporal lobe by the novel word stimuli is surprising, given the novel word semantic priming effects (Chapter 2) that may lead to a contrary expectation. In addition, one may expect learning synonyms to existing words, categorized as living vs. nonliving entities, may mirror schema-based learning. If so, one may then expect the functional neuroanatomy of novel synonyms to be less reliant on MTL, but recruit regions in the MPFC and ATL, as per the schema-based learning literature. The latter expectation is partially satisfied in that we did identify a region in mSFC that potentially corresponds to the MPFC machinery proposed in the schema literature. However, as mentioned above, lateral temporal regions were not recruited during memory retrieval for the novel words.

As for the MTL, the schema-based learning literature had deemphasized its importance by suggesting that when damaged, learning can proceed via direct neocortical support. This suggestion does not disqualify an undamaged and functional MTL from being recruited during retrieval and reconsolidation of the novel words. Hence, we do not consider our identification of the PHG as an important region for retrieval of novel word meanings as inconsistent with findings from the schema-based learning literature. Taken together, these observations suggest

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that (A) the novel words are still undergoing full neocortical semantic integration, and will potentially engage the lateral temporal lobe given more exposure; and (B) behavioral semantic priming effects can apparently still be observed despite the potential caveat in (A) that may suggest an alternative expectation; (C) at this early stage, the medial temporal lobe (i.e., PHG) and the medial superior frontal cortex appear important in the retrieval of newly learned semantic memories; and (D) while potential distinctions between the functional neuroanatomy of episodic vs. semantic memory retrieval await clearer evidence, a role for left medial and lateral parietal regions in semantic processing should not be excluded as a working hypothesis.

The above observations clearly beg a future experiment to test the hypothesis that, given more time and exposure, contextually acquired novel words would indeed recruit the lateral temporal regions, as did the real word stimuli. To allow further fine-grained

characterization of the memory retrieval machinery, we would recommend implementing certain changes to the current experimental setup. The first and obvious change would be to include additional behavioral training sessions as well as an additional imaging session following training. These additions would allow investigating a focused hypothesis that the novel word stimuli would engage lateral temporal regions following the added training. The additional imaging session would also allow for observation of repetition priming effects for the novel word stimuli, a property that, if identified in a manner dissociative of meaning vs. perceptual training, would be an important observation for a semantic hypothesis. Another proposed change would be to run the post-training recognition memory and semantic priming experiments in an fMRI setting, as opposed to our current out-of-scanner approach.

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Conducting the two behavioral tests in the scanner would still enable an item-level investigation, with the added value of being able to examine item-level functional

neuroanatomy in different task contexts that potentially differentially engage episodic and semantic processes. Having direct fMRI data across the multiple task settings (i.e. simple LDT, item recognition memory, and semantic priming) could potentially shed more light on potential episodic/semantic functional neuroanatomical distinctions.