Dissertations, Theses, and Capstone Projects

Date of Degree

9-2020

Document Type

Thesis

Degree Name

M.S.

Program

Cognitive Neuroscience

Advisor

Timothy Ellmore

Subject Categories

Cognition and Perception | Cognitive Neuroscience | Cognitive Psychology

Keywords

Simultaneous EEG-fMRI, Source Analysis, Working Memory, Thalamus, Scene Memory, Delay Activity

Abstract

Working Memory represents a limited-capacity store for maintaining information and manipulating the store's contents over a short period for the guidance of goal-directed behavior. Working Memory is an essential component of executive functions that are intricately associated with the prefrontal cortex (PFC). The PFC has been implicated in maintaining task-related information online for brief periods in the absence of relevant information. This active maintenance phase is called the delay period that occurs between encoding and retrieval of the stimulus. Previous studies have attempted to understand the relationship between working memory and the PFC, especially during the delay or maintenance phase of memory. However, subcortical structures like the thalamus have not been extensively studied in humans. Using simultaneous Electroencephalography (EEG) and functional Magnetic Resonance Imaging (fMRI), we explored the relative roles of thalamic regions during the delay period of a working memory task under different memory-load conditions. During the delay, participants passively viewed scrambled images containing similar spatial frequency to serve as a perceptual baseline and an interfering environmental stimulus. An additional aim of the study was to investigate whether there is a working memory load effect during encoding in the parahippocampal regions.

In a group source analysis, effects of increased and decreased memory load were observed bilaterally in the thalamus in the delay period. fMRI analysis revealed thalamic activity was lateralized to the left hemisphere. It was observed that thalamic activation was increased during the low-load condition when compared to the high-load condition. While no working memory load effects were observed in group source analysis during encoding, fMRI analysis did show significant differences in the posterior cortical regions.

The main finding was that during high load delay condition, the thalamus activation was attenuated compared to low load condition, suggesting its sensory filtering role. This study supports the idea that the thalamus plays an essential role in cognition, especially during memory maintenance, by regulating the processing of interfering disruptive stimuli during different load conditions.

Significance Statement

Thalamic activity has been implicated in memory disorders like Korsakoff syndrome, where the critical symptom is dense amnesia (Wolff & Vann, 2019). Recently, the thalamic activity during WM has been implicated in schizophrenia research as well. PFC communication with both the hippocampus and thalamus is essential for the normal functioning of spatial and non-spatial working memory, and any miscommunication between these structures underlies deficits in schizophrenia (Kupferschmidt & Gordon, 2018). Our study results imply that the thalamus could be tuning irrelevant sensory inputs required by the ongoing behavioral demand, supporting its sensory mechanism theories in cognition.

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