EXECUTIVE FUNCTION AND FRONTO-STRIATAL CIRCUITRY: INSIGHTS FROM ANTISACCADES, TASK SWITCHING, AND PARKINSON’S DISEASE
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Many studies of ‘executive control’ have focused on the prefrontal cortex (PFC), which contains the neuronal functional properties, modulatory neurotransmitters, and network connections with sensory and motor regions to make this large brain area a candidate region to provide all the necessary elements to voluntarily control behavior. However, like the motor and premotor cortex, the PFC is integrated with the basal ganglia (BG) in such a similar fashion, that it is impossible not to consider that the PFC might depend on the BG to implement executive control effectively. This thesis draws on knowledge of PFC and BG function, and combines studies that require the instantaneous top-down control over motor behavior with a neurological patient group with primarily BG dysfunction (Parkinson’s disease), to provide for a new understanding of prefrontal-BG networks sub-serving executive control. The tasks performed by subjects consist of antisaccades (generate a voluntary eye-movement away from a visual stimulus) and those dealing with task switching (change behavior after an alternate was previously required). Numerous neural and functional imaging studies have identified key areas of the prefrontal cortex and BG that are critical to antisaccade generation, and studies in task switching have implicated similar neural mechanisms that are involved in overriding one behavior with another. By combining task switching with antisaccades, this thesis specifically examines the neural mechanisms related to suddenly changing behavior, under conditions where one behavior is easier to perform than the other. The methods utilize on-line eye-tracking in healthy young adults and older adults with, and without, Parkinson’s disease, to develop theories of a role of the BG in executive control, and to search for specific neural correlates of executive control signals in the PFC, premotor cortex and BG using functional magnetic resonance imaging (fMRI). Together, the conclusions drawn from this thesis point to an important role of the BG in overriding more automatic behavior with behavior that is more difficult to perform. This thesis also suggests that this overriding mechanism occurs through the boosting of cortical executive control signals via net excitatory feedback from the BG.