Neuromodulation, mechanisms and evidence
Deep brain stimulation, spinal cord stimulation, vagus nerve stimulation, cochlear and retinal implants, focused ultrasound, TMS, and tDCS. What each does to tissue, what the evidence says, and the honest state of "we do not fully know why it works."
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Neuromodulation is the largest commercial part of this field and the least discussed by students, who find recording more interesting than stimulating. It is also the part with the most patients helped: over a hundred thousand people with deep brain stimulators, a million with cochlear implantsCochlear implant The most successful neural prosthesis: an electrode array in the inner ear that stimulates the auditory nerve in up to 22 bands. Related: Vocoder (noise vocoder)Vocoder (noise vocoder) A simulation of what a cochlear implant delivers: the loudness envelope of each frequency band, applied to noise in that band. . Glossary entry, many more with spinal cord stimulators. For most of these, the mechanism is only partly understood, which is a strange and important fact about medicine. Here is each modality: what it stimulates, what it treats, what the evidence looks like, and what we think it does.
Deep brain stimulation
ElectrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry in a deep nucleus (the subthalamic nucleus or globus pallidus for Parkinson’s, the thalamus for tremor), pulsed continuously at around 130 Hz, 60 to 90 µs, 2 to 4 mA, from a pacemaker-like generator in the chest. Approved for Parkinson’s disease, essential tremor, dystonia, epilepsyEpilepsy A tendency to recurrent seizures; its monitoring units are where seizure-detection algorithms get validated and where ECoG data comes from. Related: Epilepsy monitoring unit (EMU)Epilepsy monitoring unit (EMU) The hospital ward where patients are recorded on video-EEG for days to capture seizures before surgery. , Seizure detectionSeizure detection Automatically flagging seizures in EEG, a real clinical need and a good first machine-learning project on the TUH corpus. . Glossary entry, and obsessive-compulsive disorder; trials in depression and others. The effect on tremor is dramatic and immediate: turn it on and the shaking stops.
Mechanism: it was thought to silence the target, mimicking a lesion. It is now understood to do several things at once: drive axons near the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry at the stimulation frequency, overwriting the pathological rhythm (the excessive beta synchrony of the parkinsonian basal ganglia) with a regular one the downstream circuits can ignore; alter the firing of the nucleus’s neurons; and, over time, induce plasticity. Adaptive DBSDeep brain stimulation (DBS) Electrodes implanted deep in the brain and pulsed continuously, the standard treatment for advanced Parkinson's disease and essential tremor. Related: Charge densityCharge density The charge injected per pulse divided by electrode area; keeping it below safe limits prevents electrode damage and tissue injury.. Glossary entry, which senses beta bursts and stimulates only then, is in clinical use and supports the rhythm-disruption account. The frontier is closed-loop control and steering current between segmented electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry contacts to shape the field.
Spinal cord stimulation
ElectrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry in the epidural space over the dorsal columns, from a chest or abdominal generator. Long used for chronic pain (gate-control theory: stimulating large sensory fibres suppresses pain transmission; the evidence for benefit is real but modest and placebo effects are substantial, as sham-controlled trials show). Since about 2018, targeted SCSSpinal cord stimulation (SCS) Electrodes over the spinal cord, long used for pain and now being used to restore stepping after spinal cord injury. Glossary entry with patterns timed to intended movement has restored stepping in people with spinal cord injury previously thought complete, by amplifying residual signals across the lesion and engaging spinal circuits. This is the most exciting current application and it is early.
Vagus nerve stimulation
A cuff on the left vagus nerve in the neck. Approved for epilepsyEpilepsy A tendency to recurrent seizures; its monitoring units are where seizure-detection algorithms get validated and where ECoG data comes from. Related: Epilepsy monitoring unit (EMU)Epilepsy monitoring unit (EMU) The hospital ward where patients are recorded on video-EEG for days to capture seizures before surgery. , Seizure detectionSeizure detection Automatically flagging seizures in EEG, a real clinical need and a good first machine-learning project on the TUH corpus. . Glossary entry and depression, and, paired with rehabilitation, for strokeStroke Loss of blood supply to part of the brain; rehabilitation after it is the largest clinical use of neurotechnology today. Glossary entry recovery of arm function. Mechanism for epilepsyEpilepsy A tendency to recurrent seizures; its monitoring units are where seizure-detection algorithms get validated and where ECoG data comes from. Related: Epilepsy monitoring unit (EMU)Epilepsy monitoring unit (EMU) The hospital ward where patients are recorded on video-EEG for days to capture seizures before surgery. , Seizure detectionSeizure detection Automatically flagging seizures in EEG, a real clinical need and a good first machine-learning project on the TUH corpus. . Glossary entry: modulation of brainstem and thalamic activity that raises seizure threshold, incompletely understood. For strokeStroke Loss of blood supply to part of the brain; rehabilitation after it is the largest clinical use of neurotechnology today. Glossary entry: timed release of noradrenaline and acetylcholine that enhances plasticity during the movements being practised, which the plasticity explainer describes and which is well supported by animal work. Non-invasive ear-clip and neck versions exist with weaker evidence.
Cochlear and retinal implants
The sensory systems explainer covers them. The cochlear implantCochlear implant The most successful neural prosthesis: an electrode array in the inner ear that stimulates the auditory nerve in up to 22 bands. Related: Vocoder (noise vocoder)Vocoder (noise vocoder) A simulation of what a cochlear implant delivers: the loudness envelope of each frequency band, applied to noise in that band. . Glossary entry is the field’s clearest success: mechanism understood, evidence overwhelming, a million users. The retinal implant is its cautionary counterpart: mechanism understood, evidence for modest benefit, the leading company gone.
Responsive neurostimulation for epilepsy
ElectrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry on or in the seizure focus, a generator in the skull that records continuously, detects the electrographic start of a seizure, and stimulates to abort it. Approved since 2013; seizure reduction improves over years of use, suggesting plasticity, and the device’s recordings have taught the field about seizure cycles over days and weeks that nobody knew existed. It is the first closed-loop brain implant in routine clinical use.
Transcranial magnetic stimulation
A coil on the scalp discharges a brief, strong magnetic pulse that induces current in the cortex beneath, depolarizing neurons. Single pulses probe motor cortex excitability (the motor evoked potential). Repetitive protocols over the left prefrontal cortex are approved for depression, with response rates around a third to a half in medication-resistant patients and an accelerated protocol (many sessions a day for a week) recently approved. Mechanism: induced plasticity in prefrontal-limbic circuits, incompletely understood. Requires a trained operator; seizure risk is low but real.
Transcranial direct current stimulation
A milliamp or two through sponge electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry on the scalp, for twenty minutes. The current that reaches the cortex is a fraction of a volt per metre, enough to shift membrane potentialsMembrane potential The voltage across a cell's membrane, about −70 millivolts at rest, set by ion concentrations and which channels are open. Related: Resting potentialResting potential The steady membrane voltage of a neuron that is not firing, typically −60 to −70 millivolts. , Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Ion channelIon channel A protein pore in the membrane that lets specific ions through, often only when the voltage or a chemical tells it to open. . Glossary entry slightly and bias firing, not to drive it. The literature is large and inconsistent: modest effects on motor learning and some cognitive tasks in some studies, failures to replicate in others, large variability between people, and a strong placebo component. Not approved for any indication in the United States. It is the modality whose circuit is simplest and whose evidence is weakest, which is exactly why the safety charter treats it the way it does.
Focused ultrasound
Sound energy concentrated deep in the brain through the skull. At high intensity it ablates tissue and is approved for essential tremor (a non-invasive alternative to DBSDeep brain stimulation (DBS) Electrodes implanted deep in the brain and pulsed continuously, the standard treatment for advanced Parkinson's disease and essential tremor. Related: Charge densityCharge density The charge injected per pulse divided by electrode area; keeping it below safe limits prevents electrode damage and tissue injury. . Glossary entry, with a lesion instead of a stimulator). At low intensity it modulates neural activity reversibly, by mechanisms (mechanical effects on channels, possibly thermal) under active study. The only technique that can reach deep structures non-invasively with millimetre focus, which makes it the most watched experimental modality.
Deep brain stimulation stops a Parkinson's tremor within seconds of being switched on. Does that mean its mechanism is understood?
No. DBSDeep brain stimulation (DBS) Electrodes implanted deep in the brain and pulsed continuously, the standard treatment for advanced Parkinson's disease and essential tremor. Related: Charge densityCharge density The charge injected per pulse divided by electrode area; keeping it below safe limits prevents electrode damage and tissue injury. . Glossary entry was adopted because it worked, visibly, in the operating room, and the mechanism has been argued about for thirty years since. The current synthesis is that it disrupts pathological synchrony by driving axons at a regular high frequency, changes local firing, and induces plasticity over time. Medicine is full of treatments whose effectiveness preceded their explanation; a student who can say “it works, and here is what we think and do not know about why” is more credible than one who claims certainty either way.
Reading the evidence
For each modality, ask: is there a sham-controlled trial, and what did it show? What is the effect size, and in whom? What is the mechanism claimed, and what is the evidence for it specifically (not just that the treatment works)? Cochlear implantsCochlear implant The most successful neural prosthesis: an electrode array in the inner ear that stimulates the auditory nerve in up to 22 bands. Related: Vocoder (noise vocoder)Vocoder (noise vocoder) A simulation of what a cochlear implant delivers: the loudness envelope of each frequency band, applied to noise in that band. . Glossary entry and DBSDeep brain stimulation (DBS) Electrodes implanted deep in the brain and pulsed continuously, the standard treatment for advanced Parkinson's disease and essential tremor. Related: Charge densityCharge density The charge injected per pulse divided by electrode area; keeping it below safe limits prevents electrode damage and tissue injury. . Glossary entry for tremor pass every question. Spinal cord stimulation for pain passes with a modest effect and large placebo. tDCSTranscranial direct current stimulation (tDCS) Passing a milliamp or two through the head between sponge electrodes; trivially easy to build, weakly supported by evidence, and not for self-experiment. Related: Transcranial magnetic stimulation (TMS)Transcranial magnetic stimulation (TMS) A coil on the head that induces currents in the cortex with a brief magnetic pulse; approved for depression, and only for lab use with a trained operator. . Glossary entry fails the first question in most applications. Learning to run this checklist is the clinical spine’s core skill.
Deep dive Why high-frequency stimulation can silence and drive at once 3 min
A neuron’s cell body and its axon respond differently to the same pulses. At 130 Hz, cell bodies near the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry often stop firing in their own pattern (depolarization block, or inhibitory afferent activation), while axons passing by are driven to fire at the stimulation frequency. Downstream, the target receives a regular 130 Hz input instead of the pathological bursty beta pattern, and regular input is, in effect, informationally silent. So DBSDeep brain stimulation (DBS) Electrodes implanted deep in the brain and pulsed continuously, the standard treatment for advanced Parkinson's disease and essential tremor. Related: Charge densityCharge density The charge injected per pulse divided by electrode area; keeping it below safe limits prevents electrode damage and tissue injury. . Glossary entry both silences the nucleus’s own output pattern and replaces it with a regular one. Computational models (including work in the Dorval lab on this campus) showed this “informational lesion” account, and it reconciles the older silencing and driving hypotheses.
Explain what this page was about to your roommate in three sentences. No jargon they would not know.
We see deep brain stimulation for tremor use pulses near 130 Hz with widths of 60 to 90 microseconds and currents of 2 to 4 milliamps from a chest generator, and it helps over a hundred thousand people while cochlear implants help about a million. We compare that stronger record to scalp direct current at 1 to 2 milliamps for 20 minutes, which has large and inconsistent results and no approved use in the United States. We judge each method by asking for a sham controlled trial with a fake treatment arm, the size of the benefit and in whom, and what proof supports the claimed mechanism.
Yours does not have to match. Check that the three ideas are there.