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Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2023)

As time passes after learning, connections between the hippocampus and neocortex gradually change. The exact time course of these changes is not well established and competing theories of memory consolidation debate if the role of the hippocampus in memory retrieval changes with the age of the memory. Functional magnetic resonance imaging (fMRI) can measure changes in retrieval-related brain function associated with the age of the memory to characterize the roles of the hippocampus and neocortex in memory consolidation. This dissertation aims to identify common functional brain networks associated with long-term memory consolidation by examining how these networks change with the age of the memory. Three fMRI studies in healthy adults were conducted to examine changes in retrieval-related brain function associated with different memory ages and types of memoranda. Each study measured whole-brain changes in retrieval-related brain activity, hippocampal functional connectivity, and ventromedial prefrontal cortex connectivity. In Chapter 1, I examine changes in brain function during retrieval of natural scenes learned 1 month to 1 hour prior to test. I demonstrate changes consistent with systems consolidation theory can be detected across short time periods. In Chapter 2, I examine changes in brain function during retrieval of three-word sentences learned 1 month to 1 hour prior to test. I similarly demonstrate changes consistent with systems consolidation theory can be detected across short time periods. In Chapter 3, I examine changes in brain function during retrieval of news events learned 1 year to 30 years prior to test. I demonstrate changes both consistent and inconsistent with memory consolidation theories are observed across long time periods. In the discussion, I compare commonalities between the brain networks identified in Chapters 1 and 2 which similarly change across short time periods but for different types of memoranda. I also compare the commonalities between the brain networks identified in Chapters 2 and 3 which change across both short and long time periods for similar memoranda. In sum, common changes in the functional roles of the hippocampus and neocortex consistent with memory consolidation can be detected across different types of memoranda and memory ages.

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Cover page: Functional Brain Networks Associated with Long-term Memory Consolidation: Changes in the Roles of the Hippocampus and Neocortex with the Age of the Memory
  • Article
  • Peer Reviewed
UC San Diego Previously Published Works (2018)
When individuals select the recently studied (and familiar) item in a multiple-choice memory test, they direct a greater proportion of viewing time toward the to-be-selected item when their choice is correct than when their choice is incorrect. Thus, for both correct and incorrect choices, individuals indicate that the chosen item is old, but viewing time nevertheless distinguishes between old and new items. What kind of memory supports this preferential viewing effect? We recorded eye movements while participants made three-alternative, forced-choice recognition memory judgments for scenes. In experiment 1 (n = 30), the magnitude of the preferential viewing effect was strongly correlated with measures of conscious, declarative memory: recognition accuracy as well as the difference in confidence ratings and in response times for correct and incorrect choices. In four analyses that minimized the contribution of declarative memory in order to detect a possible contribution from other processes, the preferential viewing effect was absent. In experiment 2, five memory-impaired patients with medial temporal lobe lesions exhibited a diminished preferential viewing effect. These patients also exhibited poor recognition accuracy and reduced differences in confidence ratings and response times for correct and incorrect choices. We propose that the preferential viewing effect is a phenomenon of conscious, declarative memory and is dependent on the medial temporal lobe. The findings support the link between medial temporal lobe function and declarative memory. When the effects of experience depend on the medial temporal lobe, the effects reflect conscious memory.
    Cover page: Eye movements support the link between conscious memory and medial temporal lobe function
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2023)

    Objectives

    Physical activity (PA) may help maintain brain structure and function in aging. Since the intensity of PA needed to effect cognition and cerebrovascular health remains unknown, we examined associations between PA and cognition, regional white matter hyperintensities (WMH), and regional cerebral blood flow (CBF) in older adults.

    Method

    Forty-three older adults without cognitive impairment underwent magnetic resonance imaging (MRI) and comprehensive neuropsychological assessment. Waist-worn accelerometers objectively measured PA for approximately one week.

    Results

    Higher time spent in moderate to vigorous PA (MVPA) was uniquely associated with better memory and executive functioning after adjusting for all light PA. Higher MVPA was also uniquely associated with lower frontal WMH volume although the finding was no longer significant after additionally adjusting for age and accelerometer wear time. MVPA was not associated with CBF. Higher time spent in all light PA was uniquely associated with higher CBF but not with cognitive performance or WMH volume.

    Conclusions

    Engaging in PA may be beneficial for cerebrovascular health, and MVPA in particular may help preserve memory and executive function in otherwise cognitively healthy older adults. There may be differential effects of engaging in lighter PA and MVPA on MRI markers of cerebrovascular health although this needs to be confirmed in future studies with larger samples. Future randomized controlled trials that increase PA are needed to elucidate cause-effect associations between PA and cerebrovascular health.
      Creative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2022)

      Objective

      Episodic memory impairment and hippocampal pathology are hallmark features of both temporal lobe epilepsy (TLE) and amnestic mild cognitive impairment (aMCI). Pattern separation (PS), which enables the distinction between similar but unique experiences, is thought to contribute to successful encoding and retrieval of episodic memories. Impaired PS has been proposed as a potential mechanism underling episodic memory impairment in aMCI, but this association is less established in TLE. In this study, we examined behavioral PS in patients with TLE and explored whether profiles of performance in TLE are similar to aMCI.

      Method

      Patients with TLE, aMCI, and age-matched, healthy controls (HCs) completed a modified recognition task that relies on PS for the discrimination of highly similar lure items, the Mnemonic Similarity Task (MST). Group differences were evaluated and relationships between clinical characteristics, California Verbal Learning Test-Second Edition scores, and MST performance were tested in the TLE group.

      Results

      Patients with TLE and aMCI demonstrated poorer PS performance relative to the HCs, but performance did not differ between the two patient groups. Neither the side of seizure focus nor having hippocampal sclerosis affected performance in TLE. However, TLE patients with clinically defined memory impairment showed the poorest performance.

      Conclusion

      Memory performance on a task that relies on PS was disrupted to a similar extent in TLE and aMCI. The MST could provide a clinically useful tool for measuring hippocampus-dependent memory impairments in TLE and other neurological disorders associated with hippocampal damage.
        Cover page: Impaired Behavioral Pattern Separation in Refractory Temporal Lobe Epilepsy and Mild Cognitive Impairment
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