Chapter 1: Introduction
1.7 Brief Summary of Data Chapters
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stores” (Nadel & Moscovitch, 1997). Hence, in the MTT, only decontextualized neocortical semantic memory could potentially be retrieved independently of the hippocampus.
The aforementioned models of memory consolidation, despite their differences, jointly highlight an important point directly relevant to this investigation. Memory representations are dynamic in nature and the functional neuroanatomy related to retrieving memory for old, previously known words, could potentially differ from that related to retrieving memory for recently learned words. Hence, in an effort to leverage semantic memory retrieval processes using both remote and recently acquired memory, our stimulus list constituted both previously known words and the novel words acquired over multiple days. Below, we provide a brief overview of the behavioral and functional imaging findings documented in Chapter 2 and Chapter 3, respectively.
1.7 Brief Summary of Data Chapters
Forthcoming in the remainder of this thesis are three chapters. Chapter 2 documents findings from our behavioral characterizations of memory for newly acquired meaningful words relative to that of perceptually trained word forms based on performance in tasks targeting implicit and explicit memory. Chapter 3 documents findings from functional
neuroanatomical investigations of word-level semantic processing using previously known and newly learned words, as well as novel and familiar pseudowords (PW) to leverage putative semantic processing regions. Also reported in Chapter 3 are results from a clustering-analysis of regional task-evoked timecourses primarily aimed at identifying a semantic brain system.
Finally, Chapter 4 integrates the findings from the two preceding data chapters with prior
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literature to contextualize our characterizations of the functional neuroanatomy underlying retrieval of memory for word meanings. The primary findings from data chapters 2 and 3 are summarized below.
As outlined in Chapter 2, our behavioral goals were two-fold. The first was to establish that behavioral training successfully resulted in meaning learning and form familiarity assessed using old/new item recognition and 2-alternative sentence-completion semantic memory tests targeting explicit memory. Based on recognition memory performance, we demonstrate that subjects were proficient at discriminating trained items (both meaning and perceptually trained) from novel foils. In addition, resembling level-of-processing effects documented for real words (Craik & Lockhart, 1972), we demonstrate that the deeper encoding afforded by meaning training results in faster and more accurate recognition for meaning relative to form trained items. Based on performance in the sentence-completion semantic memory test, we demonstrate that subjects successfully learned new words as evidenced by their proficiency in choosing appropriate newly trained words to complete novel sentences.
The second behavioral goal was to demonstrate using a semantic priming paradigm that newly learned words would act as semantic primes to previously known synonymous words that had no prior episodic association with the primes. We show that, relative to perceptually trained primes, meaning-trained primes significantly facilitated lexical decision latencies for synonymous word targets. We interpret the latter finding as evidence indicating that novel words acquired in young adulthood are representationally integrated with common previously known words in the lexicon likely acquired early in the lifespan.
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Chapter 3 documents the findings from the fMRI experiment used to investigate the functional neuroanatomy of word-level semantic processing using the PWs used in meaning and form training as well as actual English nouns (artifacts/animals) in a lexical decision task.
A widely spaced event-related fMRI experiment, administered before and after the multi-day behavioral training, allowed for isolation of item-level hemodynamic responses, and a subsequent cross-session within-item analysis where each item effectively served as its own control. A set of a-priori hypotheses described characteristics expected from regions with a role in retrieving/processing meanings at the single word level. We expected a putative semantic processing region to exhibit: a) higher BOLD activity for real words relative to novel PWs; b) cross-day repetition-related BOLD suppression for real words; c) higher BOLD activity for meaning-trained PWs relative to novel PWs; d) higher BOLD activity for meaning-trained PWs relative to perceptually trained PWs, and e) higher BOLD activity for correctly identified meaning-trained PWs (hits) relative to their incorrect counterparts (misses). A series of
individual ANOVA contrasts were computed at the voxelwise level, and subsequently entered in a fixed effects analysis to generate individual brain regions that were probed for the above effects. Finally, in an effort to identify sets of regions that may constitute potential brain systems, region-level cross-condition timecourses were extracted and subjected to clustering analyses using multiple clustering algorithms. Four primary observations were made, which, along with our interpretation, are outlined below.
First, we demonstrate that two regions, one in left parahippocampal gyrus (PHG) and the other in the left medial superior frontal cortex (mSFC), stood out from the other examined
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regions, satisfying essentially all of the expected semantic properties. Juxtaposed with prior findings (Frankland & Bontempi, 2005; McClelland, 2013; L. R. Squire, Genzel, Wixted, &
Morris, 2015; Takashima et al., 2006; van Kesteren et al., 2012), we posit that left PHG and mSFC constitute regions that are critical in the retrieval of memory for recently acquired words, while also continuing to be recruited during retrieval of remote well-established meanings.
Second, despite our expectations based on the ample literature implicating left MTG and left vIFG in semantic processing, the two regions were not recruited during retrieval of novel word meanings. However, the two regions were recruited during retrieval of remote previously known word meanings, although in the case of vIFG, identifying the said effects required using literature-derived regions due to its absence in the study-driven contrasts. We propose that, despite the behavioral semantic priming effects argued to support semantic integration, the novel words require additional exposure and consolidation to engage
neocortical regions such as the left MTG. While we do not discount a similar interpretation as above for the lack of engagement of left vIFG by the novel words, we propose an additional explanation. Consistent with prior work implicating left vIFG in controlled semantic retrieval (Badre et al., 2005; Gold et al., 2006; Roskies et al., 2001; A D Wagner et al., 2001), and its absence from multiple studies using lexical decision tasks for which automatic semantic access may be sufficient (Fiebach et al., 2002, 2007; Henson et al., 2002), our task may similarly lack the strategic demand to reliably engage the region.
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Third, barring two crucial distinctions (discussed below) between the two proposed semantic regions (i.e. PHG/mSFC) and regions in the left medial/lateral parietal lobe
implicated in episodic memory retrieval, the two sets of regions exhibited many similarities, particularly during memory retrieval for novel words. The first distinction is that while PHG/mSFC show BOLD suppression for repeated real words, the parietal regions showed an effect resembling the old > new episodic retrieval success effect (i.e. repetition enhancement) for words. The second more nuanced distinction was that while both sets of regions showed higher BOLD activity for real words relative to novel PWs, the effect was driven by a relatively less negative timecourse for words. The noted distinctions, particularly the word repetition priming effect, provide evidence for a semantic role in PHG/mSFC, and an episodic contribution to the memory retrieval of novel words for the parietal regions.
Fourth, despite identifying several regions exhibiting semantic properties, notably the two top candidates PHG and mSFC, none of the conducted clustering analyses revealed groupings containing the two regions. Similarly, there was no cluster containing canonical semantic regions such as left MTG and vIFG. Hence, we find no evidence for a group of regions that, based on the conducted clustering analyses, could be said to correspond to a semantic brain system. Clustering analysis identified region clusters with good functional neuroanatomical correspondence with well-established systems, such as task-control
(Dosenbach et al., 2006, 2007; J. D. Power et al., 2011) and dorsal attention systems (Corbetta
& Shulman, 2002). Hence, we believe it is unlikely that the lack of a putative semantic system is a feature specific to this particular dataset. Instead, we propose that semantics may be an
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emergent process built on interactions between multiple cognitive processes and corresponding brain systems such as task-control, memory retrieval and sensorimotor/perceptual processing systems.
Taken together, these findings suggest that a relatively small amount of word learning training is sufficient to create novel words that, in young adults, behaviorally resemble the semantic characteristics of well-known words. The functional observations, particularly the lack of engagement of left lateral temporal regions during novel word retrieval, suggests that the novels words require additional time/exposure for adequate consolidation. That said, the fMRI findings identified a parahippocampal and a medial superior frontal region recruited during memory retrieval of both old and novel words. The latter medial prefrontal region in particular has been implicated in integrating new memory with consolidated neocortical memory representations. Overall, we take the fMRI observations as reflective of novel word memory representations that are transitioning to their eventual functional neuroanatomical destination, which would presumably additionally include regions in the temporal neocortex.
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