The elaborative-processing and forgetting-reconstruction hypotheses are the 2 principal explanations for the contextual interference (CI) effect. The present authors' purpose was to identify which of these 2 hypotheses better accounts for the CI effect. They synchronized single transcranial magnetic stimulation (TMS) pulses to each intertrial interval to modulate information processing during Blocked and Random Practice conditions. Participants practiced 3 arm tasks with either a Blocked or Random Practice order. The 3 stimulation conditions (No TMS, TMS, Sham TMS) by 2-practice order (Blocked, Random) between-participant design resulted in 6 experimental groups. Without TMS, motor learning increased under Random Practice. With TMS, this learning benefit diminished. These results support the elaborative-processing hypothesis by showing that perturbing information processing, evoked by Random Practice, deteriorates the learning benefit. Unlike the prediction of the forgetting-reconstruction hypothesis, adding perturbation during Blocked Practice did not significantly enhance motor learning.