Control of movement vigor and decision making during foraging

Significance How long should one stay and accumulate reward, and then, how fast should one travel to the next reward site? Marginal value theorem describes the decision-making process: the brain compares the immediate rate of harvest with its global history of capture, deciding to leave when the immediate rate falls below the average. Here, we extended the theory, showing that the same principle can be used to control speed of movements: the brain should compare the immediate rate of energy expenditure during movement with the global capture rate, planning to arrive at the destination when the two become equal. Experimental results confirmed many of the predictions, suggesting that a shared principle may underlie decision making and control of movement vigor. During foraging, animals decide how long to stay at a patch and harvest reward, and then, they move with certain vigor to another location. How does the brain decide when to leave, and how does it determine the speed of the ensuing movement? Here, we considered the possibility that both the decision-making and the motor control problems aimed to maximize a single normative utility: the sum of all rewards acquired minus all efforts expended divided by total time. This optimization could be achieved if the brain compared a local measure of utility with its history. To test the theory, we examined behavior of people as they gazed at images: they chose how long to look at the image (harvesting information) and then moved their eyes to another image, controlling saccade speed. We varied reward via image content and effort via image eccentricity, and then, we measured how these changes affected decision making (gaze duration) and motor control (saccade speed). After a history of low rewards, people increased gaze duration and decreased saccade speed. In anticipation of future effort, they lowered saccade speed and increased gaze duration. After a history of high effort, they elevated their saccade speed and increased gaze duration. Therefore, the theory presented a principled way with which the brain may control two aspects of behavior: movement speed and harvest duration. Our experiments confirmed many (but not all) of the predictions, suggesting that harvest duration and movement speed, fundamental aspects of behavior during foraging, may be governed by a shared principle of control.