Consolidation of motor memories associated with skilled practice can occur both online, concurrent with practice, and offline, after practice has ended. The current study investigated the role of dorsal premotor cortex (PMd) in early offline motor memory consolidation of implicit sequence specific learning. Thirty-three participants were assigned to one of three groups of repetitive TMS over left PMd (5 Hz, 1 Hz or control) immediately following practice of a novel continuous tracking task. There was no additional practice following repetitive TMS. This procedure was repeated for 4 days. The continuous tracking task contained a repeated sequence that could be learned implicitly and random sequences that could not. On a separate fifth day, a retention test was performed to assess implicit sequence-specific motor learning of the task. Tracking error was decreased for the group who received 1 Hz repetitive TMS over the PMd during the early consolidation period immediately following practice compared to control or 5 Hz repetitive TMS. Enhanced sequence specific learning with 1 Hz repetitive TMS following practice was due to greater offline consolidation, not differences in online learning between the groups within practice days. A follow-up experiment revealed that stimulation of PMd following practice did not differentially change motor cortical excitability, suggesting that changes in offline consolidation can be largely attributed to stimulation induced changes in PMd. These findings support a differential role for the PMd in support of online and offline sequence specific learning of a visuomotor task and offer converging evidence for competing memory systems.

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Locomotor adaptations to a novel environment can be measured through changes in muscle activity patterns and lower limb kinematics. The location and mechanisms underlying these adaptive changes are unknown. The purposes of the current study were (1) to determine whether corticospinal tract (CST) excitability is altered by resisted walking and (2) to ascertain whether changes in cortical excitability are muscle specific. Forty healthy participants walked with a robotic gait device (Lokomat) that applied a velocity-dependent resistance against hip and knee movements during walking. CST excitability was assessed by quantifying motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation immediately before and after baseline and resisted walking. MEPs were measured in either the biceps femoris (BF) or the rectus femoris (RF). Recruitment curves were collected by stimulating in 5 % increments from 105 to 145 % of active motor threshold. Results demonstrated a significant increase in MEP amplitude in the BF following baseline walking in the Lokomat. The RF did not demonstrate these changes. There was no further change in MEP size following resisted walking in either muscle group. These results suggest that locomotion increases CST excitability in a muscle-specific fashion. As such, it may be important for determining how to enhance the central nervous system's ability to integrate adaptive strategies during walking.

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