Unlocking Depression Treatment: Are Two Brain Stimulation Coils Better Than One?
"A new study reveals significant differences in motor threshold between figure-8 and double-cone coils for rTMS, offering potential for personalized depression therapy."
Transcranial Magnetic Stimulation (TMS) is emerging as a pivotal tool in the treatment of refractory depression, offering hope to those who haven't found relief through traditional methods. TMS involves using magnetic fields to stimulate nerve cells in the brain to alleviate depression symptoms. The figure-8 (Fo8) coil and the double cone (DC) coil are two common types of coils used in TMS therapy. Each coil has unique characteristics that affect how it stimulates the brain.
The figure-8 coil is known for its ability to deliver a superficially focal stimulation, concentrating the magnetic field on a specific area of the brain's surface. This precision can be particularly useful when targeting specific cortical regions believed to be involved in depression. In contrast, the double cone coil is designed to reach deeper brain structures. While it offers less focal stimulation, it can activate a larger volume of cortical tissue, making it a valuable alternative for patients who may benefit from broader stimulation.
Recent research conducted at the Centre Hospitalier de l'Université de Montréal (CHUM) has shed light on the differences between these two coils. The study revealed significant variations in motor thresholds (MTs) when using the Fo8 and DC coils in patients with refractory depression. This finding could have profound implications for how TMS therapy is administered and tailored to individual patients.
Clinical Use and Coil Safety
Transcranial magnetic stimulation (TMS) is used as an alternative treatment for several treatment-resistant psychiatric disorders, although information about routine daily practice has historically been limited. A 2024 study examined real-world TMS utilization and outcomes using data from a large, multisite psychiatric practice, while noting that general psychiatrists vary in when they implement TMS for major depressive disorder. Coil selection also has safety implications: the motor threshold determined with a figure-8 coil cannot be assumed to apply to a double-cone coil. These findings underscore both the expanding clinical use of TMS and the need for coil-specific treatment decisions.
From Faraday to FDA Clearance
TMS is a form of non-invasive brain stimulation based on electromagnetic induction, a principle first discovered by Michael Faraday in 1831. Early studies by Jacques-Arsène d’Arsonval explored the possibility of magnetic brain stimulation. Later historical accounts identify Anthony Barker’s 1985 breakthrough as a major step in the development of modern TMS. The therapy subsequently evolved from a research tool into an FDA-cleared treatment for depression, with development accelerating from the 1980s onward.
Decoding Motor Thresholds: Why Coil Choice Matters in rTMS
Motor threshold (MT) is a crucial parameter in TMS, representing the minimum intensity of stimulation required to produce a motor response. Determining MT accurately is essential for ensuring that patients receive the appropriate dose of stimulation during TMS therapy. Too little stimulation may be ineffective, while too much can lead to adverse effects.
- Significant MT Differences: The study found a significant difference in MTs between the two coils. The average MT with the Fo8 coil was 57.9%, while the average MT with the DC coil was 39.7%.
- Higher MTs with Fo8: On average, the MT was 31.5% higher for the Fo8 coil compared to the DC coil. This means that a greater intensity of stimulation was needed to elicit a motor response with the Fo8 coil.
- Correlation Between Coils: The researchers observed a positive correlation between the MTs of the two coils. As the MT with the Fo8 coil increased, the difference in cortical activation capacity between the coils also increased.
Evidence and Remaining Limits
The evidence base does not establish that using two stimulation sites is consistently superior to using one. In a pilot randomized controlled study of dual-site orbitofrontal cortex and dorsolateral prefrontal cortex accelerated repetitive TMS for depression, changes in HRSD-24 scores at one- and four-week follow-up favored active treatment over sham, but there was no difference between the dual-site and single-site groups. A review of TMS for depression also emphasizes that further research is needed to clarify therapeutic implementation and limitations. These findings support cautious interpretation of claims that adding a second target automatically improves outcomes.
Targeting the Left DLPFC
TMS over the left dorsolateral prefrontal cortex has been widely used for major depressive disorder for the past two decades. Researchers have compared different ways of locating this target, including the traditional 5-cm method, the F3 method, and neuro-navigational approaches. A 2025 evidence review selected randomized controlled trials comparing sham with active TMS using these targeting methods, as well as trials comparing the methods directly. Whether treatment efficacy differs according to the localization method remains unclear.
Personalizing Depression Therapy: The Future of rTMS
The CHUM study highlights the importance of individualizing TMS therapy based on coil type and patient characteristics. The significant differences in MTs between the Fo8 and DC coils suggest that a one-size-fits-all approach may not be optimal. By carefully considering the coil type and adjusting stimulation parameters accordingly, clinicians can potentially improve treatment outcomes and minimize adverse effects. Future research should focus on further elucidating the factors that influence MT and optimizing TMS protocols for different patient populations. This will pave the way for more effective and personalized depression therapy.
Toward Updated TMS Guidance
Recent work is moving beyond a single standard TMS approach by comparing and ranking the efficacy and acceptability of different repetitive TMS modalities. A 2025 consensus review updated the National Network of Depression Centers’ 2018 consensus statement on TMS for depression. The update incorporates recent research and clinical developments, reflecting the field’s continuing reassessment of how TMS should be used. Together, these sources support an evidence-based approach rather than assuming that a more complex stimulation arrangement is necessarily better.
When Medication Falls Short
TMS is being considered in the broader context of depression that does not improve with medication. Harvard Health describes TMS as targeting brain areas involved in mood regulation and reports that treatment may succeed when depression drugs fail. The same source says that updated protocols hold the promise of more rapid relief from depression. This positions TMS as a potential option within a treatment landscape shaped by incomplete responses to antidepressants, while not establishing that every patient will benefit.
Making Treatment More Practical
Conventional TMS regimens require weeks of clinic visits, creating a substantial practical burden for patients. A retrospective case series described a single-day optimized, neuroplasticity-enhanced depression regimen called ONE-D, delivering 600-pulse intermittent theta-burst stimulation at 120% of motor threshold to the left dorsolateral prefrontal cortex through scalp targeting. Separately, a case series examined once-weekly or once-fortnightly continuation TMS in a real-life setting. These reports illustrate efforts to make treatment schedules more workable, although they describe case-series evidence rather than definitive comparative proof.