Determining the optimal pulse number for theta burst induced change in cortical excitability

Abstract

Theta-burst stimulation (TBS) is a form of non-invasive neuromodulation which is delivered in an intermittent (iTBS) or continuous (cTBS) manner. Although 600 pulses is the most common dose, the goal of these experiments was to evaluate the effect of higher per-dose pulse numbers on cortical excitability. Sixty individuals were recruited for 2 experiments. In Experiment 1, participants received 600, 1200, 1800, or sham (600) iTBS (4 visits, counterbalanced, left motor cortex, 80% active threshold). In Experiment 2, participants received 600, 1200, 1800, 3600, or sham (600) cTBS (5 visits, counterbalanced). Motor evoked potentials (MEP) were measured in 10-min increments for 60 min. For iTBS, there was a significant interaction between dose and time (F = 3.8296, p = 0.01), driven by iTBS (1200) which decreased excitability for up to 50 min (t = 3.1267, p = 0.001). For cTBS, there was no overall interaction between dose and time (F = 1.1513, p = 0.33). Relative to sham, cTBS (3600) increased excitability for up to 60 min (t = 2.0880, p = 0.04). There were no other significant effects of dose relative to sham in either experiment. Secondary analyses revealed high within and between subject variability. These results suggest that iTBS (1200) and cTBS (3600) are, respectively, the most effective doses for decreasing and increasing cortical excitability.

Repetitive transcranial magnetic stimulation for major depressive disorder: basic principles and future directions

“…Indeed, a group recently published a study where healthy subjects received various doses of TBS protocols. 25 In this study, the only TBS protocols which caused statistically significant changes in cortical excitability were 1200 pulses of iTBS, which paradoxically had an inhibitory effect, and 3600 pulses of cTBS, which paradoxically had an excitatory effect. These surprising results highlight the need to better understand and predict individual responses to TBS protocols to try and optimize clinical response to therapeutic protocols.…”

Facilitation of Motor Evoked Potentials in Response to a Modified 30 Hz Intermittent Theta-Burst Stimulation Protocol in Healthy Adults

“…To our knowledge, only one recent study has observed a similar rate of facilitatory responses (i.e., 68%; Guerra et al [ 32 ] following standard iTBS. Most studies using the standard 50 Hz iTBS protocol have reported much lower response rates, including McCalley et al [ 33 ], who recently reported only 33% of responders. It may be argued that high inter-individual response variability will persist regardless of the TBS protocol used in terms of bursting frequency and inter-burst intervals.…”

“…However, our goal was not to test overall efficacy but rather to investigate the effectiveness of the modified 30 Hz/6 Hz iTBS protocol in inducing lasting MEP facilitation. Nevertheless, adding a sham condition could provide critical information regarding the influence of expectations and anticipation on individual responses to the modified iTBS protocol [ 33 ].…”

Modulating Visuomotor Sequence Learning by Repetitive Transcranial Magnetic Stimulation: What Do We Know So Far?

“…TBS protocols consist of bursts that comprise three pulses at 50 Hz that are repeated at 5 Hz. The most common TBS protocol delivers 600 pulses in total, although shorter (i.e., 300 pulses) or longer (i.e., 1800 pulses) variants also exist (Gentner et al, 2008;McCalley et al, 2021). Based on the presence or absence of intermittent stimulation-free periods, we further distinguish between continuous TBS (cTBS) and intermittent TBS (iTBS) (Huang et al, 2005).…”