Paired associative stimulation increases motor cortex excitability more effectively than theta-burst stimulation.

OBJECTIVE: To examine the effects of theta burst stimulation (TBS) and paired associative stimulation (PAS) on excitability in the human motor cortex.

METHODS: Sixteen healthy young participants received intermittent TBS (iTBS) or PAS to the primary motor cortex on two testing occasions, at least a week apart. Ten of the participants also received iTBS or PAS after conditioning with continuous TBS on two other occasions. Cortical excitability was assessed with single TMS pulses to the motor cortex. Motor evoked potentials (MEPs) were measured from the first dorsal interosseus (FDI) muscle before TBS or PAS stimulation, and every 10min for 60min after stimulation. Changes in excitability were compared against the potential for motor learning, assessed with the rotor pursuit task.

RESULTS: After the PAS protocol MEP amplitudes were significantly increased. This increase was greater than after intermittent TBS, which did not change MEPs significantly. Conditioning with continuous TBS showed no significant effect. Participants' responses were not correlated across protocols and were not correlated with rotor pursuit learning.

CONCLUSIONS: PAS was the only protocol which induced significant increases in MEP amplitude.

SIGNIFICANCE: PAS is robust in inducing excitatory cortical change. This makes it a suitable protocol for testing plasticity in healthy and patient groups.