SCIENCE
For more than a decade, transcranial direct current stimulation carried an open secret: the same dose, delivered the same way, produced very different results in different people.
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FREQUENTLY ASKED
Closed-loop tDCS means the system reads a live signal and uses it to decide when, where, or how strongly to stimulate, rather than running a fixed current on a preset schedule. In practice, it covers two distinct jobs: modulating current during a session, which requires a cortical signal such as EEG, and gating whether a session should begin at all, which requires a reliable index of physiological readiness. Different devices solve different halves.
Individual differences in skull thickness, scalp volume, cerebrospinal fluid, and other head anatomy strongly influence how much applied current reaches the target region. Modelling studies have found that a fixed dose can produce over 100% variation in delivered electric field intensity at the same target across individuals.
Early evidence suggests yes, to a meaningful degree. A machine learning study using MRI-derived electric field models classified responders versus non-responders with 86% accuracy but in a sample of only fourteen older adults, and the authors explicitly framed it as proof of concept rather than a validated clinical tool.
Electric field modelling personalises stimulation based on fixed anatomy, typically calculated once from an MRI before treatment. Closed-loop gating personalises stimulation based on changing brain or physiological state in real time. They address different halves of the same problem and are increasingly treated as complementary rather than competing.
Partially, and inconsistently. In one ten-person pilot, adaptive tDCS produced significantly faster reaction times than fixed-current stimulation, though the accuracy difference was not statistically significant. A systematic review found that only two studies have directly compared fixed and adaptive stimulation, and they reached opposite conclusions.
No. A study in 42 adults found individualised dosing reduced electric field variability but produced no significant behavioural benefit over fixed dosing or sham. A separate study individualising intensity by corticospinal excitability found no effect either. One 2025 trial in 16 participants did find individualised dosing superior. The evidence is genuinely mixed.
No. PPG measures blood volume changes at the skin surface, and HRV derived from it indexes autonomic nervous system balance. It's a well-researched readiness and arousal marker, but it is not a measure of neural activity, cortical excitability, or electric field at a brain target.
Clinical research relies on MRI-derived electric field models and EEG systems impractical outside a lab, and it mostly tests mid-session modulation. Consumer wearables generally target a different and more tractable decision: whether a person is in a physiological state where stimulation is worth starting. PPG-derived HRV is well suited to that question, since heart rate variability is regulated by prefrontal activity. What it cannot do is measure cortical activity directly or account for individual skull and scalp anatomy, so it addresses the timing question, not the anatomical dosing one.
Because tDCS effects are state-dependent. Research has shown that stimulation to the left DLPFC modulates executive function differently depending on the brain state it lands in. Whether formally gating stimulation to physiological state improves clinical outcomes is an active research question, not a settled one.
A real measured anatomical or physiological input rather than a manual preference setting; a way to verify the intended target is actually being reached; adaptation on a timescale matched to the variability being addressed; and evidence that the adaptation improves an actual outcome, not just a cleaner dose map.
THE INSTRUMENT
Sychedelic combines everything described in this article into one 20-minute protocol.