Mechanisms of the contextual interference effect 2 in individuals post - stroke 3 4

24 Although intermixing different motor learning tasks via random schedules enhances long-term 25 retention compared to “blocked” schedules, the mechanism underlying this contextual interference effect 26 has been unclear. Furthermore, previous studies have reported inconclusive results in individuals post27 stroke. 28 29 We instructed participants to learn to produce three grip force patterns in either random or 30 blocked schedules, and measured the contextual interference effect by long-term forgetting: the change in 31 performance between immediate and 24-hour post-tests. Non-disabled participants exhibited the 32 contextual interference effect: no forgetting in the random condition, but forgetting in the blocked 33 condition. Participants at least 3 months post-stroke exhibited no forgetting in the random condition, but 34 marginal forgetting in the blocked condition. However, in participants post-stroke, the integrity of visuo35 spatial working memory modulated long-term retention after blocked schedule training: participants with 36 poor visuo-spatial working memory exhibited little forgetting at 24 hours. 37 38 These counter-intuitive results were predicted by a computational model of motor memory that 39 contains a common fast process and multiple slow processes, which are competitively updated by motor 40 errors. In blocked schedules, the fast process quickly improved performance, therefore reducing error41 driven update of the slow processes, and thus poor long-term retention. In random schedules, 42 interferences in the fast process led to slower change in performance, therefore increasing error-driven 43 update of slow processes and thus good long-term retention. Increased forgetting rates in the fast process, 44 as would be expected in individuals with visuo-spatial working memory deficits, led to small updates of 45 the fast process during blocked schedules, and thus better long-term retention. 46