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A Simultaneous EEG-fMRI Study of Thalamic Load-Dependent Working Memory Delay Period Activity

Working memory (WM) is an essential component of executive functions which depend on maintaining task-related information online for brief periods in both the presence and absence of interfering stimuli. Active maintenance occurs during the WM delay period, the time between stimulus encoding and subsequent retrieval. Previous studies have extensively documented prefrontal (PFC) and posterior parietal (PPC) cortex activity during the WM delay period, but the role of subcortical structures including the thalamus remains to be fully elucidated, especially in humans. Using simultaneous EEG-fMRI, we investigated the role of the thalamus during the WM delay period following low and high memory load encoding. During the delay, participants passively viewed scrambled images containing similar color and spatial frequency to serve as a perceptual baseline. Using individual fMRI-weighted source analyses centered around delay period onset, the effects of increased and decreased memory load on maintenance were observed bilaterally in thalamus with higher source activity evoked during low compared to high load maintenance. The main finding that thalamic activation was attenuated during high compared to low load maintenance suggesting a sensory filtering role for thalamus during consolidation of stimuli in WM where the highest evoked activity occurs when fewer stimuli need to be maintained in the presence of interfering perceptual stimuli during the delay. The results support the idea that the thalamus plays a role in short-term memory maintenance by regulating processing of interfering stimuli.