Investigating cognitive functions mediated by ventral and dorsal striatum

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The aim of the proposed research is to better understand how learning, memory, and decision making are accomplished and the brain regions that produce these complex cognitive processes. This project is unique in focusing on the contributions of a subcortical structure called the striatum to these higher-order functions. Subcortical brain structures are formed by clusters of cells deep to the outer cell layers that form the cerebral cortex. These subcortical brain structures evolved earlier than the cortex, and are generally implicated in more primitive or automated functions. The striatum is widely understood to regulate movements. Increasingly, however, the striatum is ascribed a role in thinking processes as well. In fact, the striatum separates into ventral (i.e., toward the belly) and dorsal (i.e., toward the back) parts, with each appearing to underlie a different set of cognitive processes. These lists are far from fully defined, however. Using functional magnetic resonance imaging (fMRI), which allows measurement of blood flow as an index of regional brain activity, and various tests of learning, memory, and decision making, the aim is to better define the specific process(es) performed by ventral and dorsal striatum. The notion is that by comparing a number of skills, tasks, or conditions that preferentially engage each of these regions, and understanding what these scenarios have in common, we can deduce the specific computation(s) performed by ventral and dorsal striatum. From reviewing the existing literature, as well as my own recent research, the expectation is that ventral striatum will be implicated in general learning processes, not limited to the specific case of learning from reward, as had been thought. In contrast, dorsal striatum is not predicted to mediate early learning, though it has previously been linked to learning motor responses or skills. Rather, the expectation is that the dorsal striatum underlies decision making processes, particularly in ambiguous contexts. A further aim of this research will be to clarify how these subcortical structures interact with regions of cortex, and to understand the specific functions accomplished by ventral and dorsal striatum. Toward this goal, we will employ different fMRI analyses that focus on regions that are connected by their function or anatomy (e.g., connectivity analyses, diffusion tensor imaging) or that segregate from one another in terms of function (e.g., independent components analyses). An additional means for dissociating the specific functions accomplished by ventral and dorsal striatum relative to cortical regions to which they are connected, is to temporarily disable activity in cortical regions that are known to partner with striatum and to observe how behaviour and fMRI activity changes. In the proposed research, temporarily and reversibly depressed function of small and well-localized regions of cortex will be accomplished using a technique called repetitive transcranial magnetic stimulation, before participants perform specific tasks in the fMRI scanner. Taken together, this research programme aims to clarify the particular cognitive operations performed by the ventral and dorsal striatum, independent of their cortical partners. This research has the potential to augment current theories of attention, learning, and decision making. These theories ultimately have broad implications, informing public safety practices (e.g., traffic/driver safety), guiding educational policies and practices, and can be applied to understand and improve decision making in contexts such as industry or law.