a Participants were scanned while tapping a five-element sequence (S: 4–1–3–2–4) on a keypad using their left hand. The session consisted of 14 successive performance blocks with 60 keypresses, equivalent to 12 repetitions of the correctly performed and completed sequence (i.e., correct trials; S1, S2,…, S12), each block being separated by 15-s periods of rest. During performance blocks, participants were asked to look at a fixation cross and to tap the sequence repeatedly as fast and accurately as possible. In case of errors, they were instructed to continue with the task as smoothly as possible from the beginning of the sequence. A change in the color of the fixation cross from red to green and from green to red indicated the beginning (GO) and the end (STOP) of each performance block, respectively. Only isolated errors preceded and followed by at least three correct trials in a row were analyzed; the last error within the block followed by at least two correct trials in a row was also considered. b All wrong keypresses that violated the predetermined order (i.e., sequence) or incomplete sequences with missing keys, between two correct trials, were considered as a single error. Thus, the first wrong keypress that violated the sequential order (marked with red oval) can be the first, second, third, fourth, or fifth error key. S—correct trial, i.e., one correctly performed and completed sequence, E—erroneous trial, i.e., an error, Ek1 … Ekn—keys within the error, Eki—the first wrong key within the error, black rounded arrows—transitions within the sequence, red rounded arrows—error transitions.
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