Previous studies have shown that reinforcement-based motor learning requires the brain to process feedback-related information after movement execution. However, whether reinforcement feedback changes how the brain processes motor preparation before movement execution is unclear. By using electroencephalography (EEG), this study investigates whether reinforcement feedback changes cortical preparatory activity to modulate motor learning and memory. Human subjects were divided in three groups [reward, punishment, control] to perform a visuomotor rotation task under different conditions that assess adaptation (learning) and retention (memory) during the task. Reinforcement feedback was provided in the form of points after each trial that signaled monetary gains (reward) or losses (punishment). EEG was utilized to evaluate the amplitude of movement readiness potentials (MRPs) at the beginning of each trial for each group during the adaptation and retention conditions of the task. The results show that punishment feedback significantly decreased MRPs amplitude during both task conditions compared to Reward and Control groups. Moreover, the punishment-related decrease in MRPs amplitude paralleled decreases in motor performance during the retention but not the adaptation condition. No changes in MRPs or motor performance were observed in the Reward group. These results support the idea that reinforcement feedback modulates motor preparation and suggest that changes in cortical preparatory activity contribute to the visuomotor retention deficits observed after punishment feedback.