High-density theta burst stimulation (hdTBS) at 100 Hz triples the aftereffects of the conventional intermittent TBS

Slice electrophysiological studies have experimentally demonstrated that theta burst stimulation, consisting of electrical pulses delivered at 10 ms (100 Hz) inter-pulse intervals, optimally induces long-term potentiation in the hippocampus. Inspired by this observation, a novel transcranial magnetic stimulation (TMS) paradigm, 100 Hz high-density theta burst stimulation (100 Hz hdTBS), is presented. This paradigm delivers 6 pulses per burst with an inter-pulse interval of 10 ms - doubling the pulse frequency and total pulse count of the conventional intermittent TBS (iTBS). The effect of this new paradigm was studied in the motor cortex of awake rats using a rat-specific focal TMS coil and a hdTBS stimulator developed in house. Results reveal that 100 Hz hdTBS triples the after-effects of conventional iTBS. In a separate group of animals that received two consecutive iTBS session back-to-back (prolonged iTBS), we observed an inhibitory effect. Since that prolonged iTBS matches the total pulse count of 100 Hz hdTBS but produced opposite after-effects, our results underscore the critical roles of the temporal structure of TMS pulses--not merely the total number of pulses--in driving neuroplasticity. This new paradigm has the potential to significantly enhance therapeutic efficacy if confirmed to be safe and effective in humans.