Elucidating the functional role of fronto-striatal-hippocampal networks

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Traditionally, the striatum and the hippocampus systems of the brain were conceived as independent. It was thought that the striatal system was involved in stimulus-response habit learning and shifting between rules. In particular, our group and others have shown that the dorsal part of the striatum is involved when a novel action need to be initiated, while the ventral part of the striatum is solicited when stimulus-response associations need to be reinforced. The hippocampus would contribute to episodic and associative memory in humans. Episodic memory has been defined as the memory of autobiographical events that can be explicitly stated. It is responsible for the recollection of past personal experiences that are linked with a specific time and place. Associative memory is a form of episodic memory which is defined as the ability to learn and remember the relationship between different items.******More recently, it has been proposed that the striatum and the hippocampus may actually interact and possibly complement each other. Current studies suggest that the prefrontal cortex (PFC), striatum and hippocampus co-operate in the context of decision-making dependent on long-term memory. However, the precise role of the PFC, striatum and hippocampus in associative memory processes and context-dependent decision making have yet to be determined. In the present proposal we will use a combination of functional Magnetic Resonance Imaging, together with two methods of non-invasive brain stimulation, including transcranial magnetic stimulation (TMS) and high-definition transcranial direct stimulation (HD-TDCS), to study functional contributions of these regions as well as their interactions in healthy human adults. ******The proposed combination of methods for the study of fronto-striatal-hippocampal interactions in healthy individuals is unique with broad future implications. It will provide invaluable information regarding the causal mechanisms occurring between different regions of the PFC, the striatum and their interaction with the hippocampus. Our project will increase knowledge on the possibility that the striatum serves similar functions in seemingly dissimilar cognitive processes. It will put Canada at the forefront of research regarding how different networks of the brain interact when both episodic memory and decision making are required. Finally, it will also greatly inform us on the potential modulatory effects of TMS and HD-TDCS, which are increasingly being used without extensive knowledge regarding their mechanism of action.**