tDCS course Chapter 2 Biophysics of tDCS - #19 - Apr 26, 2025

Mike: 00:00:15

Welcome to the Neurostimulation Podcast.

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I'm Michael Passmore, clinical

associate professor in the Department

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of Psychiatry at the University of

British Columbia in Vancouver, Canada.

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The Neurostimulation podcast is

about exploring the fascinating

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world of neuroscience and

clinical neurostimulation.

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We look at the technology, how it

works, what are the latest research

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breakthroughs, and importantly, how

those research breakthroughs are

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being translated into treatments that

can improve health and wellbeing.

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I want to also emphasize that this

podcast is separate from my clinical

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and academic roles, and is part of my

personal effort to bring neuroscience

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education to the general public.

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Accordingly, it's important for me

also to emphasize that the information

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shared here is intended for educational

purposes only and not as medical advice.

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If you have specific questions that

pertain to your own health, I would really

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encourage you to consult a professional,

whether that's a doctor or other

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professional in the healthcare area, to

get tailored and specific answers to your

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questions and to ensure that you have a

professional and comprehensive evaluation.

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Today's episode is presented

by ZipStim Neurostimulation.

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ZipStim is the clinic that I operate.

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You can check us out at zipstim.com.

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That's Z-I-P-S-T-I M.com.

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In this episode, we're going to be

discussing the next chapter in the

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important textbook on transcranial

direct current stimulation.

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So please stay tuned.

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I think you're really going to

get a lot out of this episode.

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So today we're taking a comprehensive

journey into the biophysics behind one

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of the most accessible and exciting

non-invasive neuromodulation techniques

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in clinical neuroscience today.

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Transcranial direct current

stimulation or tDCS.

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Today's episode is going to be

based on chapter two of the textbook

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Practical Guide to Transcranial

Direct Current Stimulation.

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This is a foundational reference for

clinicians and researchers alike.

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So whether you are a seasoned

neuromodulation specialist, a student,

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a curious clinician, or just someone

who's interested in how this technology

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works and how it might be helpful for

you, a friend or a family member, we're

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going to unpack together how tDCS works.

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We're going to start with a look

at the physics, diving into the

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physiological effects and ending where

this fascinating field is headed.

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Let's start with segment

one, the essence of tDCS.

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This is simplicity with subtle power.

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We're going to begin with a bold claim.

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tDCS is powerful because

it is simple, while other

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neuromodulation techniques like.

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Transcranial magnetic stimulation or

TMS or deep brain stimulation, DBS

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require complex equipment and intense

electrical or electromagnetic pulses.

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tDCS relies on a low amplitude constant

electrical current, typically between one

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and two milliamperes delivered through

scalp electrodes, so non-invasively.

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It doesn't make neurons fire directly.

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But it makes them more

or less likely to fire.

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This essentially makes

tDCS a priming tool.

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It enhances or dampens the brain's

existing patterns of activation if

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you apply it before or during a task,

for example, in motor rehabilitation,

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or training of working memory, it

may improve the effectiveness of that

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task by modulating the underlying

cortical excitability and plasticity.

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And here's the key word, sub-threshold.

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Well, there are two keywords, really

non-invasive and sub-threshold.

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So tDCS does not involve any invasive

procedure into the body or brain.

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tDCS also doesn't force

electrical action potentials.

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Instead, it shifts the

likelihood of neuronal firing.

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Think of it kind of like tuning a radio.

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We're not amplifying or increasing

the volume of the sound.

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But we're adjusting the dial until

you find the clearest signal.

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Let's move on to segment two,

where we're going to talk about the

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physics of electrical current flow

from the skin to the brain cortex.

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To understand how transcranial

direct current stimulation affects

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the brain, we need to talk a

little bit about electrical fields.

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When electrical current flows from the

anode electrode to the cathode electrode,

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a portion of that current is absorbed or

redirected by the skin, scalp, the skull,

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and the cerebrospinal fluid, which is the

fluid that bathes the outside of the brain

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and which is found inside the spinal cord

and the so-called ventricles of the brain.

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However, a measurable fraction of that

electrical current actually reaches

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the cortex of the brain or the actual

brain tissue, enough to change the

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polarization tendencies of the neurons

in that affected brain tissue according

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to computational modeling studies.

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One milliampere of

current produces about 0.2

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to 0.5

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volts per meter in the cortex, depending

on head anatomy and the positioning of the

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electrodes, which is technically referred

to as the montage, how the electrodes

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are situated on the scalp, to give you a

sense of the scale, that's about 200 times

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weaker than the electrical field induced

by transcranial magnetic stimulation.

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But with tDCS, it's a continuous

application of energy, and

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this is the sustained exposure.

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That is what gives tDCS its unique

capacity to influence neuronal

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membrane potentials over time.

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Here's a practical analogy, TMS or

transcranial magnetic stimulation

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is more like ringing a doorbell.

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But tDCS is like gently pushing

the door continuously until

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it eventually swings open.

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In segment three, we're going

to talk about neurons and how

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they behave under the influence,

polarization and plasticity.

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Let's look a little bit more

carefully at neuronal polarization.

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So when transcranial direct current

stimulation is applied, anodal

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stimulation tends to cause somatic

depolarization of cortical parametal

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neurons making them more likely to

fire, whereas cathodal stimulation

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tends to cause hyperpolarization and

reduces neuronal firing probability.

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But it's not just about whether

a neuron is excited or inhibited.

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The direction and strength of

that current affects different

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compartments of the neuron.

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Here's some examples.

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Anodal electrical current may

depolarize the soma and hyperpolarize

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distal dendrites, whereas cathodal

current might do the reverse.

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These complex patterns influence

not only the firing rate of neurons,

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but also synaptic plasticity.

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This is the way that neurons strengthen or

weaken their connections to one another.

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In fact, animal studies have shown that

applying tDCS during synaptic activity

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can amplify long-term potentiation,

which is an important underlying

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mechanism that's involved with memory

function, and this is felt to occur,

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especially via NMDA receptors and

L type calcium channel modulation.

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For segment four, we're going to

talk about what happens over time,

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acutely versus after effects.

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Now in a single tDCS session, there

will be immediate changes in membrane

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potential, but there's more tDCS, at

least in the studies, and the current

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clinical guidelines for therapeutic

applications involve repeated sessions.

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So each session, depending on the disorder

that's being targeted, each session is

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going to last around a half an hour,

and we're going to be likely prescribing

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half an hour sessions for say two weeks.

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And so with repeated tDCS sessions,

and especially when paired with

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active tasks, tDCS can lead to

lasting neuroplastic changes.

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Examples of these changes include

strengthening of motor engrams during

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post-stroke rehabilitation, reduced

hyperactivity in cortical pain networks,

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and improved emotional regulation

by way of prefrontal-limbic circuit

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modulation in treatment of depression.

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These kinds of after effects may last

minutes to hours, and with repetitive

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treatments potentially longer.

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This is why many clinical protocols

involve daily sessions over several weeks.

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So, here's what seemed to be an

emerging clinical pearl based on

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the research that's coming out.

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In this area, tDCS seems to work best

when it's paired with meaningful activity.

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So for example, if you are looking at

post-stroke motor rehabilitation, if

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you're stimulating the motor cortex

with tDCS, it seems to have an improved

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effect when the patient is attempting

or actually moving themselves.

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If you're stimulating the prefrontal

cortex in order to try to improve

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cognitive function or mood symptoms,

then it might be helpful to have

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the patient engage in cognitive or

emotional awareness types of tasks.

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The research is pointing in the

direction of the brain responding

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best when it has a reason to change,

when it's being encouraged to change

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by volitional kinds of activities

that the patient is undertaking

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during the sessions and accumulate.

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The brain seems to respond best when

it has a reason to change when there

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is a volitional kind of activity,

whether that's a cognitive activity

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and or a motor activity and/or a mood

related activity that the patient

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is attempting to do simultaneously.

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And the modulation due to the tDCS

seems to be facilitating that or

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encouraging these changes by way of

the kind of priming and potentiation

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that we've been talking about.

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In segment five, we're going to

talk a bit about dosing tDCS.

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Certainly the research is pointing

in the direction of our understanding

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that it's not one size fits all

when it comes to dosing duration of

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individual sessions and or duration of.

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Courses.

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So let's break down the key

parameters of  a tDCS session.

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There really are five parameters

that we would focus in on.

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The first is the electrical

current intensity.

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This is typically between one and

two milliamperes or milliamps.

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The second is the stimulation duration

in a single session, and that's

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ranging between 10 to 30 minutes,

depending on the target symptoms.

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The third thing is the

electrode placement or montage.

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And so where the tDCS electrodes

are placed on the scalp matters

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enormously because this is going

to influence the underlying brain

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areas that are being targeted.

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The fourth thing is

electrode size and material.

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So larger electrodes are going

to spread the current and smaller

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electrodes are going to focus it.

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The fifth parameter is ramp time.

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So gentle increases and gentle decreases

seems to improve tolerability, basically

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reducing uncomfortable side effects.

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Other things to include in other things

to consider include the electrical current

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density, which is the current per square

centimeter, as well as the total charge,

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which is the intensity times duration.

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These parameters influence

not only effectiveness, but

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safety and patient comfort.

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Researchers know that it's important

to consider and report all of these

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in the method section of studies.

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This is essential for things like

credibility and study reproducibility for

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clinicians, consistency matters, as well

as adherence to emerging and evolving

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clinical treatment guidelines, and helping

to ensure that we're providing the cutting

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edge treatment that patients deserve.

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Changing even one variable, like

electrode size or placement may

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dramatically alter outcomes.

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So we want to be quite careful with that.

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In Segment six, we're going to talk

about modeling, personalization,

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and the rise of so-called HD tDCS.

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Thanks to computational modeling,

we can now visualize how electrical

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current flows through individual brains.

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MRI imaging based head models show that

standard electrode montages often produce

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diffuse current flow affecting more than

just the targeted underlying brain region.

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This has led to high definition

or HD tDCS, which uses small

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gel based electrodes in

multi-channel arrays on the scalp.

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The most well-known configuration is the

4x1 ring, where a central electrode is

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surrounded by four return electrodes,

allowing much more focused stimulation.

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Other emerging technologies include EEG

triggered tDCS, individualized modeling

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based on head scans, closed loop systems

that adapt stimulation in real time.

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This really is the future

personalized, dynamic, and precise

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non-invasive brain stimulation.

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In segment seven, we're going to talk

about historical roots, all the way

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from electric fish to modern clinics.

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Let's take a step back for a moment

and consider the amazing history that

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has brought us to where we are today.

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The use of electrical stimulation applied

to the body dates back to the Roman

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Empire, and maybe even before that, but

at least we know that in the Roman Empire,

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physicians applied electric torpedo fish

to relieve headaches in the 19th century.

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Italian physicist, Giovanni Aldini

(actually Galvan's nephew) experimented

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with Galvanism on the heads of

patients with psychiatric illnesses.

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In the mid 20th century crude tDCS devices

reemerged under the names electro sleep

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and cranial electrotherapy stimulation,

often without rigorous control trials.

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But fast forward to the year 2000 when

Nitsche and Paulus published their

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landmark study showing that weak direct

current applied to the human scalp could

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reliably alter cortical excitability.

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That experiment launched the modern

era of tDCS research and the field

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really hasn't looked back ever since.

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In segment eight, we're going to talk

about mechanistic nuances and ask some

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open questions for future consideration.

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Despite decades of study, tDCS

mechanisms are still being refined.

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Some open questions include, what's the

role of glial cells, the surrounding

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structure that provides support and

nourishment for the neurons, and

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what about the neurovascular unit?

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To what extent is the vasculature of

the brain implicated in tDCS's mechanism

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and/or the underlying pathophysiology

of the disorders that tDCS is targeting?

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How does individual anatomy

change electrical current flow?

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Why do some individuals seem

to respond better than others?

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Can we optimize the timing of stimulation

in order to harness, uh, brain

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state dependence kind of situation?

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These questions are the focus of

current clinical trials, many of which

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are combining tDCS with things like

behavioral therapy, digital health

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tools, or pharmacologic augmentation.

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In segment nine, we're going to

talk about real world potential

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and clinical applications.

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We're going to close by revisiting

the clinical promise here.

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tDCS is being actively investigated for

treatment of things like major depressive

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disorder, chronic pain conditions

including fibromyalgia and migraine.

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We've reviewed that already in previous

episodes and we're going to look into that

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in much more detail in future episodes.

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Also, stroke rehabilitation is a

major active area of ongoing research

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and emerging clinical applications.

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Alzheimer's disease and other

neurocognitive disorders, including and

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very excitingly mild cognitive impairment

as a potential target in order to help

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stall or perhaps even prevent people from

converting to things like Alzheimer's

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disease and other forms of dementia

or major neurocognitive disorder, at

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least the neurodegenerative types,

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Parkinson's Disease or other motor

based neurodegenerative disorders, A

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DHD, attention deficit hyperactivity

disorder, learning disorders,

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addiction, craving reduction in

substance use related disorders.

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The list is actually expanding

because of all of the potential and

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promise of this particular technology.

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It's also interestingly, perhaps

not surprisingly, being used

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off-label for things like improving

athletic performance, perhaps

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cognitive performance in students

and so-called cognitive hackers.

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This obviously raises ethical

questions, but it also underscores

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the potential overall versatility

and promise of this technology.

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Let's recap.

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What have we learned today?

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tDCS is elegant in its simplicity,

powerful in its subtlety,

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and promising in its breadth.

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It doesn't override the brain function.

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It seems to be guiding it, and that

perhaps is its most profound quality.

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We're still early overall in this

story, particularly in terms of clinical

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applications, but by understanding the

principles through rigorous research.

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By understanding the physics, the

neurophysiology, and the overall

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clinical potential, we move closer to

making this kind of brain modulation

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an everyday tool in mental health

applications, rehabilitation applications,

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and general human flourishing.

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Thanks for joining me today on

the Neurostimulation Podcast.

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If you found today's episode helpful,

I really would encourage you to

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like, subscribe, leave comments or

a review, share this with family,

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friends, and colleagues, anyone you

think that might benefit from this.

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Tune in next time where we explore

another fascinating conversation in

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the world of health, wellness, and

neuroscience, or another interesting

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topic in clinical neurostimulation,

whether that's an emerging research

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finding or more information based on

the textbook that we're exploring today.

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I really appreciate your time,

your interest, and your attention,

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and we will see you next time.

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Take care, stay curious and be well.