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Prefrontal Mechanisms Underlying Sequential Tasks

  • Author(s): Chiang, Feng-Kuei
  • Advisor(s): Wallis, Joni
  • et al.

For decades, mechanisms of cognitive behaviors have been studied in a simple form of stimulus- and action-outcome associations. Those seminal studies serve as fundamental frameworks and enable us to explore how neural activities in brain represent the increasingly complex and temporarily-extended associations in sequential tasks. The prefrontal cortex (PFC) has long been suspected to play an important role in cognitive control, in the ability to orchestrate thought and action in accordance with internal representation. In particular, PFC “top-down” processes serve as an internal signal to guide high-level cognitive functions, such as working memory (WM), abstract rules, or goal-directed decision-making. Several models have described how task information, including supra- and super-ordinate information, is organized in prefrontal cortex, but it remains unclear precisely how this cognitive information maps onto neurophysiological functions.

To explore these issues, we devised primate versions of two tasks that tax sequential behavior: a spatial self-ordered search task and a hierarchical reinforcement learning (HRL) task. These tasks examine how sequential behavior interacts with WM and reinforcement learning (RL), respectively. We examined how prefrontal neurons encoded task-related information across these two cognitive tasks. Our results show that prefrontal neurons are capable of adaptively regulating the precision with which information is encoded. In the spatial self-ordered search task, lateral prefrontal neurons have spatiotemporal mnemonic fields, in that their firing rates are modulated both by the spatial location of future selection behaviors and the temporal organization of that behavior. Furthermore, the precision of this tuning can be dynamically modulated by the demands of the task. In the HRL task, prefrontal neurons are involved in integrating the abstract subject values. Especially, we found that the firing rate of a small population of neurons encoded pseudo-reward prediction errors and these neurons were restricted to anterior cingulate cortex. Taken together, our findings suggest that prefrontal neurons encode not only basic information associated with external stimuli, but also high-level information that is used to organize task relevant behaviors.

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