Non-invasive neuromodulation: revision 1 to 3

Revision 1

Revision 3

trl: 8

categories: ["cybernetics", "enhancement"]

tags: ["tms", "tdcs", "ultrasound", "vagus nerve", "cognitive enhancement", "replication"]

summary: "Techniques that alter neural excitability through the intact skull or peripheral nerves — magnetic, electrical, and ultrasonic — without surgery or implanted hardware."

summary: "Magnetic, electrical, and ultrasonic techniques that alter neural excitability through the intact skull or peripheral nerves, without surgery or implanted hardware."

updated: "2026-07-27"

updated: "2026-10-29"

humanEvidence: "Repetitive TMS for depression is supported by sham-controlled trials in patients and is cleared and reimbursed; tDCS enhancement studies in healthy adults are small and showed no reliable effect when pooled."

humanEvidence: "Repetitive TMS for depression is supported by sham-controlled trials in patients and is cleared and reimbursed; tDCS enhancement studies in healthy adults are small and showed no reliable effect when pooled."

access: "TMS is delivered in clinics and covered by many health systems; tDCS has no approved US indication yet is sold as a consumer wellness device and built at home; ultrasound is research-only."

access: "TMS is delivered in clinics and covered by many health systems; tDCS has no approved US indication yet is sold as a consumer wellness device and built at home; ultrasound is research-only."

reversibility: "reversible"

{ "label": "TMS introduced", "value": "1985, Sheffield" },

{ "label": "First depression clearance", "value": "2008 (rTMS, United States)" },

{ "label": "tDCS regulatory status", "value": "No US approval for any indication" },

{ "label": "Typical tDCS current", "value": "1–2 mA" },

{ "label": "Typical tDCS current", "value": "1-2 mA" },

{ "label": "Depth reachable", "value": "Cortex (TMS/tDCS); deep nuclei (ultrasound)" },

{ "label": "Readiness", "value": "TRL 9 (rTMS) to TRL 4 (ultrasonic)" }

]

**Non-invasive neuromodulation** covers the techniques that change nervous-system activity from

outside the body: magnetic pulses that induce currents in cortex, weak direct or alternating

currents applied to the scalp, focused ultrasound aimed through the skull, and electrical

stimulation of peripheral nerves reachable at the skin. They avoid the surgery, cost, and

stimulation of peripheral nerves reachable at the skin. These methods avoid the surgery, cost, and

hardware dependence of [[deep-brain-stimulation]] and of the implanted devices grouped under

[[neuroprosthetics]] — whose stimulation is adjustable and can be switched off, but whose electrodes

[[neuroprosthetics]], whose stimulation can be adjusted or switched off but whose electrodes come

come out only with another operation — and they pay for that with far less spatial precision,

out only with another operation. The price is far less spatial precision and shallower reach, and

shallower reach, and — for several of the most popular methods — an evidence base that has not

several of the most popular methods rest on an evidence base that has not survived rigorous

survived rigorous replication.

replication.

## Modalities

**Transcranial magnetic stimulation** (TMS) discharges a capacitor through a coil held against the

scalp, producing a rapidly changing magnetic field that induces an electric field in the underlying

cortex strong enough to depolarize axons. A single pulse over motor cortex produces a measurable

twitch, which gives TMS something no other non-invasive method has: a direct, objective readout that

twitch. That gives TMS something no other non-invasive method has: a direct, objective readout that

the stimulation reached its target — a form of ground truth that recording-based systems such as a

the stimulation reached its target, a form of ground truth that recording-based systems such as a

[[brain-computer-interface]] have to establish statistically. Repetitive TMS delivered in trains over days is the therapeutic

[[brain-computer-interface]] have to establish statistically. The therapeutic form is repetitive TMS

form, and patterned protocols such as intermittent theta burst compress a session from forty minutes

delivered in trains over days, and patterned protocols such as intermittent theta burst compress a

to a few.

session from forty minutes to a few.

**Transcranial electrical stimulation** passes 1–2 mA between scalp electrodes. Direct current

**Transcranial electrical stimulation** passes 1-2 mA between scalp electrodes. Direct current

(tDCS) shifts resting membrane potentials, making neurons under the anode marginally more likely to

fire and those under the cathode marginally less; it does not itself trigger action potentials.

Alternating current (tACS) aims to entrain endogenous oscillations at a chosen frequency. Both are

cheap, portable, and easy to build, which explains both their popularity in research and their

cheap, portable, and easy to build, which accounts for their popularity in research and for their

regulatory problems.

**Transcranial focused ultrasound** (tFUS) at low intensity modulates neural activity, most likely

through mechanical effects on ion channels and membranes rather than heating. Its distinguishing

through mechanical effects on ion channels and membranes rather than heating. What sets it apart is

property is depth with focus: an ultrasound beam can be steered to a target a few millimetres across

depth with focus: an ultrasound beam can be steered to a target a few millimetres across in the

in the thalamus or amygdala, structures unreachable by magnetic or electrical scalp stimulation. The

thalamus or amygdala, structures that magnetic or electrical scalp stimulation cannot reach. The

first human sensory-cortex demonstrations date from the 2010s.[^legon2014] High-intensity focused

ultrasound is a different technology, used to ablate the thalamic tremor target under magnetic

ultrasound is a different technology. Under magnetic resonance guidance it ablates the thalamic

resonance guidance, and is approved for essential tremor.

tremor target, and it is approved for essential tremor.

**Peripheral approaches** stimulate nerves that project into the brain. Vagus nerve stimulation is

long established in implanted form for epilepsy and treatment-resistant depression; transcutaneous

long established in implanted form for epilepsy and treatment-resistant depression. Transcutaneous

variants stimulate the auricular branch at the ear or the cervical vagus at the neck, and are

cleared for headache indications.

The physics sets the limits. The skull is a poor conductor and a strong acoustic barrier, and the

scalp is a low-resistance shunt. Direct measurements in human cadaver preparations found that the

great majority of current applied at conventional tDCS intensities is shunted through the scalp,

leaving field strengths in cortex of roughly a quarter of a volt per metre — well below the

leaving field strengths in cortex of roughly a quarter of a volt per metre. That is well below the

threshold for driving spikes, and at the low end of what modulates firing probability in animal

recordings.[^voroslakos2018] That finding did not show tDCS does nothing; it showed that whatever it

recordings.[^voroslakos2018] The finding did not show that tDCS does nothing. It showed that

does must be a subtle modulation of ongoing activity, and it rationalized both the small effect

whatever tDCS does must be a subtle modulation of ongoing activity, and it offered an explanation

sizes and the high inter-individual variability the literature reports.

for both the small effect sizes and the high inter-individual variability the literature reports.

Spatial focality is worse than most figures suggest. A conventional two-electrode tDCS montage

spreads current across large regions of cortex; TMS achieves perhaps a centimetre of focality at the

spreads current across large regions of cortex. TMS achieves perhaps a centimetre of focality at the

gyral crown and cannot reach the depths where the targets of implanted therapy sit. Ultrasound is

the exception, and its focality is why interest has moved that way.

the exception, and its focality explains why interest has moved toward it.

Individual variability is the other structural problem. Skull thickness, cortical folding, baseline

neurotransmitter state, and prior activity all change the response, and studies of standard

plasticity protocols find that a substantial fraction of healthy participants show no effect or the

opposite of the expected one. Group averages are therefore a poor guide to any individual's

opposite of the expected one. Group averages are therefore a poor guide to how any one person will

response.

respond.

## Clinical evidence

Repetitive TMS for major depression is the best-supported application. It was cleared in the United

States in 2008 after a sham-controlled trial, is reimbursed in many health systems, and produces

States in 2008 after a sham-controlled trial and is reimbursed in many health systems. In

response rates in medication-resistant patients that are clinically meaningful without approaching

medication-resistant patients it produces response rates that are clinically meaningful, though they

the remission rates of electroconvulsive therapy. An accelerated protocol delivering many theta-burst

do not approach the remission rates of electroconvulsive therapy. An accelerated protocol, which

sessions over five days with individualized functional-connectivity targeting reported high

delivers many theta-burst sessions over five days with individualized functional-connectivity

remission rates in a small randomised sham-controlled trial and was cleared in 2022, though

targeting, reported high remission rates in a small randomised sham-controlled trial and was cleared

replication at scale is still in progress.[^cole2022] TMS is also cleared for obsessive-compulsive

in 2022; replication at scale is still in progress.[^cole2022] TMS is also cleared for

disorder and as an aid to smoking cessation.

obsessive-compulsive disorder and as an aid to smoking cessation.

Transcranial electrical stimulation has no approved US indication. European expert guidelines

Transcranial electrical stimulation has no approved US indication. European expert guidelines assign

assign tDCS probable efficacy for depression and for some pain conditions and note insufficient

tDCS probable efficacy for depression and for some pain conditions, and find insufficient evidence

evidence for most other proposed uses.[^fregni2021] A tDCS headset is regulated as a medical device

for most other proposed uses.[^fregni2021] A tDCS headset is regulated as a medical device for

for depression in some European jurisdictions. The clinical case is real but modest, and it is

depression in some European jurisdictions. The clinical case is real but modest, and weaker than the

weaker than the volume of published literature implies.

volume of published literature implies.

Vagus nerve stimulation for treatment-resistant depression has been studied for two decades with

persistently ambiguous results, and a large sham-controlled trial reported mixed outcomes in the

acquisition, typically with fewer than twenty participants per group, one session, and one outcome

measure among several collected. A meta-analysis pooling single-session tDCS studies in healthy

adults found no reliable effect on any cognitive measure once the whole literature was

considered.[^horvath2015] The analysis was itself contested on methodological grounds, and the

considered.[^horvath2015] That analysis was itself contested on methodological grounds and the

argument has not fully resolved, but the field's own subsequent work has moved toward larger samples,

argument has not fully resolved. Still, the field's own subsequent work has moved toward larger

preregistration, and individualized dosing precisely because the earlier literature did not hold up.

samples, preregistration, and individualized dosing because the earlier literature did not hold up.

The structural causes are familiar: small samples, flexible analysis, publication bias toward

positive results, and sham conditions that participants can often detect from the scalp sensation.

A detectable sham is not peculiar to stimulation — it is the central objection to trials of

Detectable shams are not peculiar to stimulation. They are the central objection to trials of

[[psychedelic-therapy]], where an active dose is unmistakable within the hour — but a tingle under an

[[psychedelic-therapy]], where an active dose is unmistakable within the hour. A tingle under an

electrode is a far weaker cue than that, so the blind here is degraded rather than absent.

electrode is a far weaker cue, so the blind in stimulation trials is degraded rather than absent. A

A specific complication is that tDCS effects are frequently state-dependent, helping poor performers

further complication is that tDCS effects are frequently state-dependent, helping poor performers

and impairing good ones, so an average across a group can be near zero while real effects exist in

and impairing good ones, so a group average can be near zero while real effects exist in subgroups.

subgroups. That is a reason for careful design, not a defence of the existing literature.

That calls for careful design, but it does not defend the existing literature.

The same caution applies to claims made for [[nootropics]] and to any proposal that

[[intelligence-amplification]] can be achieved by stimulating cortex. Nothing in the current

evidence supports durable, general cognitive gain in healthy adults from any non-invasive

stimulation method, and the implanted approach to the same goal — the hippocampal

stimulation method. The implanted approach to the same goal, the hippocampal [[memory-prosthesis]],

[[memory-prosthesis]] — has only within-session results in patients. Claims that stimulation can

has only within-session results in patients. Claims that stimulation can reduce sleep need,

reduce sleep need, discussed under [[sleep-reduction]], rest on still weaker foundations.

discussed under [[sleep-reduction]], rest on still weaker foundations.

> [!caution] What a positive result usually means

> A typical enhancement finding is a within-session improvement of a few percent on one task, in

## The consumer market and its regulation

Direct-to-consumer stimulation devices occupy a regulatory gap. Marketed for relaxation, sleep,

Direct-to-consumer stimulation devices occupy a regulatory gap. Because they are marketed for

focus, or athletic training rather than for treating disease, many are sold under general-wellness

relaxation, sleep, focus, or athletic training rather than for treating disease, many are sold under

provisions that do not require efficacy evidence. A sports-oriented tDCS headset attracted

general-wellness provisions that do not require efficacy evidence. A sports-oriented tDCS headset

significant attention in the 2010s before its maker ceased operations; cranial electrotherapy

attracted significant attention in the 2010s before its maker ceased operations. Cranial

devices continue to be sold for anxiety and insomnia; and an active do-it-yourself community builds

electrotherapy devices continue to be sold for anxiety and insomnia, and an active do-it-yourself

its own units, an activity that sits alongside the practices described under

community builds its own units, an activity that sits alongside the practices described under

[[biohacking]].

Consumer electroencephalography headsets are increasingly sold alongside these devices, and scalp

recordings support more inference than their crudeness suggests — the concern taken up under

recordings support more inference than their crudeness suggests. That concern is taken up under

[[mental-privacy]] and legislated under [[neurorights]].

[[mental-privacy]], and the legislation addressing it under [[neurorights]].

Harms are mostly minor — skin irritation and burns under electrodes, headache, transient mood

Harms are mostly minor: skin irritation and burns under electrodes, headache, and transient mood

change — with the notable exception of TMS, where seizure is a rare but documented risk that keeps

change. TMS is the notable exception, since seizure is a rare but documented risk that keeps it in

it in clinical settings. The more consequential problem is opportunity cost and misinformation:

clinical settings. The more consequential problem is opportunity cost and misinformation, with

devices sold on the strength of a literature that has not replicated. Military interest in

stimulation for sustained vigilance has produced similar claims with similar evidentiary weakness,

stimulation for sustained vigilance has produced similar claims with similar evidentiary weakness.

and the question of whether such devices belong in competitive settings is unresolved, as discussed

Whether such devices belong in competitive settings is unresolved, as discussed under

under [[enhancement-in-sport]] and [[bioethics-of-enhancement]].

[[enhancement-in-sport]] and [[bioethics-of-enhancement]].

## Outlook

Focused ultrasound is the modality most likely to change what non-invasive stimulation can do,

because it is the only one that combines depth with a focal spot. If low-intensity ultrasound can

reliably modulate specific deep nuclei in humans, it would offer a reversible, repeatable

alternative to implanted electrodes for some psychiatric and pain indications, and would let

alternative to implanted electrodes for some psychiatric and pain indications. It would also let

investigators run causal experiments in deep human structures that currently require surgery. As of

2026 the human evidence consists of small studies with heterogeneous protocols, and the dose-response

2026 the human evidence consists of small studies with heterogeneous protocols, and the

relationship is not well characterized.

dose-response relationship is not well characterized.

The second trend is closing the loop. Stimulation timed to a measured brain state — an oscillation

A separate line of animal work aims stimulation at the brain's fluid clearance rather than at a

phase, a sleep spindle, a decoded attentional lapse — should outperform stimulation delivered on a

symptom. In October 2026 a group in Beijing reported that a single ten-minute exposure to 40.68 MHz

ultrashort-wave radio-frequency energy, a physiotherapy modality, raised delta-band power and sped

[[glymphatic-system|glymphatic]] tracer clearance in mice. A week of daily sessions lowered amyloid

staining and improved memory tasks in an Alzheimer's mouse model.[^liu2026] The work is a single

cohort of mice with no human data, and the clearance mechanism it targets is itself disputed.

The second trend is closing the loop. Stimulation timed to a measured brain state (an oscillation

phase, a sleep spindle, a decoded attentional lapse) should outperform stimulation delivered on a

fixed schedule, and it borrows directly from the methods of [[neural-decoding]] and from the

adaptive control now used in implanted devices. Whether that gain materializes in practice will

determine if non-invasive methods remain a weaker substitute for implanted devices or become a

distinct clinical tool with their own indications. The techniques that achieve genuine cell-type

specificity, [[optogenetics]] above all, work in animal brains and have reached people only in the

eye; a brain target would need gene delivery to a defined population and a light source implanted

eye. A brain target would need gene delivery to a defined population and a light source implanted

next to it, which leaves ultrasound as the only non-invasive candidate combining depth with focus.

## See also

[^fregni2021]: `paper` Fregni, F. et al. "Evidence-based guidelines and secondary meta-analysis for the use of transcranial direct current stimulation in neurological and psychiatric disorders." *International Journal of Neuropsychopharmacology*, 2021.

[^fregni2021]: `paper` Fregni, F. et al. "Evidence-based guidelines and secondary meta-analysis for the use of transcranial direct current stimulation in neurological and psychiatric disorders." *International Journal of Neuropsychopharmacology*, 2021.

[^horvath2015]: `paper` Horvath, J. C., Forte, J. D., Carter, O. "Quantitative review finds no evidence of cognitive effects in healthy populations from single-session transcranial direct current stimulation (tDCS)." *Brain Stimulation*, 2015. {Covers single-session studies in healthy adults only; its exclusion criteria were contested, and it does not bear on multi-session clinical protocols in patients.}

[^horvath2015]: `paper` Horvath, J. C., Forte, J. D., Carter, O. "Quantitative review finds no evidence of cognitive effects in healthy populations from single-session transcranial direct current stimulation (tDCS)." *Brain Stimulation*, 2015. {Covers single-session studies in healthy adults only; its exclusion criteria were contested, and it does not bear on multi-session clinical protocols in patients.}

[^liu2026]: `paper` Liu, J. et al. "Noninvasive ultrashort wave neuromodulation enhances glymphatic clearance to alleviate traumatic brain edema and Alzheimer's pathology." *Science Advances*, 2026. {Mice only, run as one cohort without independent replication; the amyloid result is a fall in immunofluorescence intensity after seven daily sessions.}