LAUSR

Theta burst stimulation (TBS), delivers bursts of TMS pulses at gamma frequency (e.g. 50 Hz) which are repeated at theta frequency intervals (e.g. 5 Hz). In animals, TBS can induce longlasting neural changes in neuronal behaviour [1e3]. Similar observations have been reported when applying TBS in humans [4,5] using TMS. Continuous TBS (cTBS) applies three pulses at 50 Hz frequency (the burst) every 200 ms (5 Hz) [4,5]. There is considerable interand intra-individual variability in response to TBS, despite comparable stimulation parameters and methodologies across studies [6e10]. Here we measured the consistency of magnetic fields produced by commercial stimulators when delivering a cTBS protocol, to assess whether variation in magnetic fields within and across bursts, and across a range of intensities, may provide a source of variability. We measured the intensity of magnetic pulses delivered by MagVenture MagPro 100 (MagVenture, Lucernemarken, Denmark) and by Magstim Rapid2 (Magstim Co., Whitland Dyfed, UK) when a cTBS protocol is applied during 40 sec (600 pulses) [4]. The intensity of the stimulation was set at 40, 50, 60, 70, 80, 90 and 100% of the maximum stimulator output (MSO) for MagProX100 and 50% MSO for Magstim-Rapid2 (maximal intensity allowed at 50 Hz). We used a figure-of-eight coil for both stimulators, MagVenture MCF-B65 coil and Magstim D70-Alpha-Flat coil. We measured the modulus of the time-varying magnetic field produced by the figure-of-eight coil by means of a custom-made isotropic triaxial probe consisting of single-turn shielded coils of 1 cm in diameter. The probe was placed in air at one cm from the surface of the figure-of-eight coil, and it measured simultaneously the three components of magnetic field with temporal resolution. We analysed the cTBS protocols in two different ways. First, we evaluated the intensity of the three pulses of the first burst to check if stimulators/coils are able to deliver the same intensity in the three burst components. Second analysis consisted of analysing the three pulses of the burst throughout the whole protocol (600 pulses/200 bursts) to check if stimulators/coils are able to maintain the same intensity over time. The magnetic field produced by the first pulse at 100% MSO was 1.96 T for MagProX100.