Interference from mere thinking : mental rehearsal 1 temporarily disrupts recall of motor memory

11 Interference between successively learned tasks is widely investigated to study motor 12 memory. However, how simultaneously-learned motor memories interact with each 13 other has been rarely studied despite its prevalence in daily life. Assuming that motor 14 memory shares common neural mechanisms with declarative memory system, we 15 made unintuitive predictions that mental rehearsal, as opposed to further practice, of 16 one motor memory will temporarily impair the recall of another 17 simultaneously-learned memory. Subjects simultaneously learned two sensorimotor 18 tasks, i.e., visuomotor rotation and gain. They retrieved one memory by either 19 practice or mental rehearsal and then had their memory evaluated. We found that 20 mental rehearsal, instead of execution, impaired the recall of un-retrieved memory. 21 This impairment was content-independent, i.e., retrieving either gain or rotation 22 impaired the other memory. Hence, conscious recollection of one motor memory 23 interferes the recall of another memory. This is analogous to retrieval-induced 24 forgetting in declarative memory, suggesting a common neural process across 25 memory systems. Our findings indicate that motor imagery is sufficient to induce 26 interference between motor memories. Mental rehearsal, currently widely regarded as 27 beneficial for motor performance, negatively affects memory recall when it is 28 exercised for a subset of memorized items. 29