Limits and Research 

Limits:

Transcranial Magnetic Stimulation (TMS) faces several limitations that impact its precision and interpretation. Standardized protocols, such as using 80% of the motor threshold, are criticized for assuming uniform thresholds across brain regions, neglecting factors like skull thickness, cortical excitability, and anatomical variations. These factors affect stimulation efficacy and focality, reducing reliability across individuals.

Moreover, TMS is not entirely selective due to network connectivity, often activating neighbouring or connected areas, complicating attributions of behavioural changes. Its limited penetration depth restricts access to deeper brain structures.

Finally, individual variability, including the presence of non-responders, underscores the need for personalised parameters and supplementary imaging techniques to optimise its application.

How to improve? 

This section consists of my discussions with ChatGPT about whether it might be possible in the future to improve TMS and whether these ideas have already been tried.  

-> Why not design another shape for the coil, like a four-leaf clover shape?

“Adding more coils, such as arranging four circular coils in a clover-like pattern, could theoretically allow for more precise stimulation of specific regions. This might enhance spatial resolution and provide finer control over targeting. However, this design increases the complexity of magnetic field interactions. Overlapping or closely interacting fields could make the resultant magnetic field less predictable, reducing precision rather than improving it. Additionally, creating effective software to model and control these fields would require significant advancements.”  

Open Question: Could we use advanced AI modeling or machine learning to predict and optimize the interactions between such complex coil configurations?  

Related Research: Multi-locus TMS systems, which attempt to achieve similar effects by dynamically shaping magnetic fields using phased arrays, are being explored but remain in early stages. (more here)

-> What happens if we increase the magnetic field strength in TMS? Could it produce effects like those seen in Transcranial Electrical Stimulation (TES)?

Increasing the magnetic field in TMS might allow stimulation of deeper brain regions without needing special coil designs. However, stronger fields can also result in unintended activation of surrounding or interconnected areas, reducing specificity. Unlike TES, which primarily modulates neural activity using electric fields, TMS induces electrical currents through magnetic pulses.  

-> How can we improve TMS timing and focus to increase efficacy?

One possibility is using real-time neuroimaging, like EEG or fMRI, to guide TMS stimulation dynamically. For instance:  If EEG indicates the onset of a brainwave pattern associated with a task, TMS could target that area precisely at the right moment.  (See: closed loop stimulation)

- Open Question: Could ultra-precise neuromodulation (guided by machine learning) allow us to activate individual neurons or clusters selectively, improving spatial resolution beyond the current 1 cm limit?  

-> Could we build arrays of smaller coils for ultra-focused stimulation?

Using smaller coils in an array could allow for precise stimulation of adjacent regions with minimal overlap, but it introduces significant technical challenges. The first is field Interference: adjacent coils would need to be carefully synchronized to avoid overlapping or canceling each other's effects. Secondly, there is an increase in hardware Complexity: Designing compact arrays with sufficient power while keeping the system portable is difficult.  

- Open Question: Could quantum technologies (e.g., nitrogen-vacancy centers) improve the focus of magnetic fields in TMS by creating highly precise, small-scale fields?