It is generally conceived that the amount of practice trials and the delivery of performance-contingent monetary rewards are key drivers of long-term retention of motor memories. While these two factors likely interact to promote retention, this possibility has never been tested.
Here, we tested this hypothesis by using a gradual visuomotor adaptation paradigm where the amount of practice trials (i.e., either 400 or 800) as well as the delivery of performance-contingent monetary rewards (i.e., either rewards or no reward) were manipulated.
Participants took part in two experimental sessions separated by a 24-hour interval. During initial acquisition, performance and short-term retention were assessed via vision and no-vision catch trials, respectively. Long-term retention was measured the following day by using no vision trials.
Expectedly, results revealed that performance-contingent rewards enhanced performance during the first 400 acquisition trials. Counter-intuitively, although performance remained similar between groups, levels of short-term retention were lower in the rewarded as compared to the unrewarded group during the last 400 acquisition trials.
Unexpectedly, practicing for 800 trials with performance-contingent rewards impaired long-term retention as compared to when rewards were not delivered. Anticipatedly, practicing for 800 trials led to better long-term retention than practicing for 400 trials.
The present results suggest that extrinsic rewards can undermine the formation of long-term motor memories, an idea that finds support in cellular/molecular studies, as well as work stemming from experimental psychology.
Overall, these findings challenge classic reinforcement learning theories by showing that long-term retention does not always benefit from reward delivery during acquisition.