Abstract
The brain rapidly adapts reaching movements to changing circumstances by using visual feedback about errors. Providing reward in addition to error feedback facilitates the adaptation but the underlying mechanism is unknown. Here, we investigate whether the proportion of trials rewarded (the ‘reward abundance’) influences how much participants adapt to their errors. We used a 3D multi-target pointing task in which reward alone is insufficient for motor adaptation. Participants (N = 423) performed the pointing task with feedback based on a shifted hand-position. On a proportion of trials we gave them rewarding feedback that their hand hit the target. Half of the participants only received this reward feedback. The other half also received feedback about endpoint errors. In different groups, we varied the proportion of trials that was rewarded. As expected, participants who received feedback about their errors did adapt, but participants who only received reward-feedback did not. Critically, participants who received abundant rewards adapted less to their errors than participants who received less reward. Thus, reward abundance negatively influences how much participants learn from their errors. Probably participants used a mechanism that relied more on the reward feedback when the reward was abundant. Because participants could not adapt to the reward, this interfered with adaptation to errors.
Highlights
Anne’s first beach ball game of the holiday was a frustrating experience: somehow all her balls ended far right of Andy where he could not reach for them
Post-hoc comparisons with a Bonferroni-corrected p-value of 0.008 showed that for the reward only condition (Fig 2A) there were no differences between the reward groups, whereas for the reward + error condition (Fig 2B) the asymptotic error in the high reward group was larger than the asymptotic error in the low reward group (t(99) = -4.303, p < 0.001), the medium reward group (t(99) = -3.190, p = 0.002) and the chance reward group (t(92) = -3.339, p = 0.001)
The influence of reward on motor adaptation and motivation did not depend on the conflict between the reward and error-based feedback: motor adaptation and motivation were equal for a group that received performance-dependent rewards and a group that was rewarded at random
Summary
Anne’s first beach ball game of the holiday was a frustrating experience: somehow all her balls ended far right of Andy where he could not reach for them. When there are consistent errors in the movements, the brain needs to ‘adapt’ to these biases. This process of bias reduction has been widely studied in visuomotor adaptation paradigms in which participants are exposed to a (rotational) perturbation of visual feedback about their movement. These studies have shown that several processes contribute to the adaptation: use-dependent plasticity, implicit adaptation to ‘sensory prediction errors’, reward-based reinforcement learning and explicit updating of aiming strategy to name a few (for reviews see [1,2,3]). We focus on the combination of two classes of information involved in adaptation: error and reward.
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