Neural Mechanisms of Model-Based Planning in the Rat

Abstract

Imagine a game of chess. As the players think about their next moves, they consider

the outcome each action would have on the board, as well as the opponent’s likely reply.

The players’ knowledge of the board and the rules constitutes an internal model of chess,

a knowledge structure that links actions to their likely outcomes. The process of using

such an “action-outcome” model to inform behavior is defined as the act of planning.

While it has been known for many decades that humans and animals plan, we are only

beginning to understand the neural algorithms of planning, as well as the neural circuits

which instantiate those algorithms. Recently, a surge of research has been fueled by the

development of sophisticated planning tasks for human subjects. This research has

revealed important insights into the neural mechanisms of planning, but has been limited

by the experimental methods available for use with human subjects.

In my thesis work, I have adapted one of these tasks (the “two-step” task) for use with

rats. Analysis of behavior on this task reveals that rats overwhelmingly adopt a strategy

of planning. The rat two-step task therefore opens the door to investigations of the neural

mechanisms of planning which combine the sophisticated tasks and formal algorithmic

analysis traditional in the human literature with the precise tools for measuring and

perturbing neural function that are available using invasive experiments in rodents. In a

first experiment, I sought to test which regions of the brain are crucial for planning. I

found that silencing neural activity in either the orbitofrontal cortex (OFC) or the dorsal

hippocampus (dH) while rats perform the task results in a planning-specific impairment,

exposing these regions as promising targets for future research into planning. Follow-up

experiments in OFC are ongoing, and their results suggest that it may play a role in

making predictions about the future in the service of planned behavior. These

experiments, and the methods that they introduced, open the door to new and more

detailed investigations of the neural mechanisms of planning, in the orbitofrontal cortex,

in the hippocampus, and throughout the brain.