The gradient of the reinforcement landscape influences sensorimotor learning

Consideration of previous successes and failures is essential to mastering a motor skill. Much of what we know about how humans and animals learn from such reinforcement feedback comes from experiments that involve sampling from a small number of discrete actions. Yet, it is less understood how we learn through reinforcement feedback when sampling from a continuous set of possible actions. Navigating a continuous set of possible actions likely requires using gradient information to maximize success. Here we addressed how humans adapt the aim of their hand when experiencing reinforcement feedback that was associated with a continuous set of possible actions. Specifically, we manipulated the change in the probability of reward given a change in motor actionthe reinforcement gradientto study its influence on learning. We found that participants learned faster when exposed to a steep gradient compared to a shallow gradient. Further, when initially positioned between a steep and a shallow gradient that rose in opposite directions, participants were more likely to ascend the steep gradient. We introduce a model that captures our results and several features of motor learning. Taken together, our work suggests that the sensorimotor system relies on temporally recent and spatially local gradient information to drive learning. Author summary In recent years it has been shown that reinforcement feedback may also subserve our ability to acquire new motor skills. Here we address how the reinforcement gradient influences motor learning. We found that a steeper gradient increased both the rate and likelihood of learning. Moreover, while many mainstream theories posit that we build a full representation of the reinforcement landscape, both our data and model suggest that the sensorimotor system relies primarily on temporally recent and spatially local gradient information to drive learning. Our work provides new insights into how we sample from a continuous action-reward landscape to maximize success.