Limitations and future directions
CORTEX
One important limitation of the work is the small sample size. The small sample limits our ability to make statistically robust and generalizable conclusions from the work. These findings should be tested in a larger sample of subjects for statistical robustness and generalizability. Additionally in future work, any robust findings should also be examined at the individual subject level to determine relevance for individualized patient treatments.Other possibilities for the null effects from single or multiple trains on TEP amplitudes are related to our methodological decisions. First, the exact probe latencies of 500 and 2000 ms may not have been ideal to capture neural effects. In a subsample of subjects, we found that jittering the early and late probe latencies from 500 to 700 ms and from 1900 to 2100 ms, respectively, did not influence the early local TEP amplitude (Fig. S4). Secondly, it is possible that the acute changes from the TMS train resolve by 500 ms and thus the TMS probe pulse was not applied close enough to the train to capture these effects. Unfortunately, applying probes earlier than 500 ms would likely result in TEPs that are confounded by sensory artifacts from the pulses within the train. Unless active and sham TMS trains are perfectly matched perceptually, the neural effects when placing the early probe closer to the TMS train would be difficult to interpret. It is worth noting that the timing of the single probe pulse was placed outside of the range of the sensory effects of the TMS train for two reasons: (1) because of the lack of perfectly perceptually matched sham train control, and (2) with an eye towards exploring the neural effects of other train frequencies, amplitudes, and patterns. Even if sham can be perfectly matched perceptually, the train differences between active conditions varying in frequency, intensity, or patterns will undoubtedly result in different perceptual effects. Hence, we feel that too many confounds exist between 0–300 ms after the TMS train to probe neural effects of perceptually un-matched train parameters using existing methods. Thirdly, rTMS dosing and coil orientation both were adjusted in some participants due to tolerability issues. These parameters were held constant within subject across all conditions, critically important for the within-subjects design. However, it is known that these parameters are related to the magnitude of brain response and to which neurons are most affectedAlthough our primary results were null, exploratory analyses indicate that the study design may be sufficient to capture changes in the TEP in non-local brain regions at later latencies, in phase coordination dynamics, and in distributed source estimates. These analyses provide the basis for motivating hypotheses for future testing that train effects may be examined as synchronization of excitatory and inhibitory neural activity in prefrontal cortex and connected networks. Future work should explore the EEG signal as a dynamical system with phase shifting behavior, and as such to characterize changes in this behavior following TMS trains to better understand how TMS modulates complex neuronal population synchronization. Phase coordination of EEG may provide the basis for a more detailed understanding of TMS-induced brain changes than by quantifying changes in the size of the TEP. Ongoing work in our lab will further explore neural complexity that may arise from application of TMS trains. As mentioned above, this train-probe approach described here can also be used to optimize the neural effects of different TMS train parameters for the next generation of treatment protocols. Finally, future work should explore whether this experimental approach can be used for real-time measurement and monitoring of the neural effects of TMS trains. If successful, this real-time approach would have important implications in developing closed-loop adaptive TMS treatmentsFinally, given our choice of stimulation parameters (5 s 10 Hz train) is different from the FDA-cleared 10 Hz protocol (4 s train) and the recently cleared theta burst stimulation (TBS) protocol, it is possible that stimulation with these other parameters may elicit different neural effects. Thus, our choice of stimulation parameters in this study limits generalizability. To better understand the intricate interplay between stimulation parameters and acute neural effects, future investigations need to systematically explore the neural effect of other stimulation parameters.