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Transcranial Direct Current Stimulation






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Transcranial Direct Current Stimulation. Chris Rorden www.mricro.com www.cabiatl.com Method Designs Safety. tDCS vs TMS. Transcranial magnetic stimulation Relatively expensive (~$50,000). Moderate sized effects (e.g. mild speech arrest).
Transcranial Direct Current Stimulation

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Slide 1

Transcranial Direct Current Stimulation

  • Chris Rorden

  • www.mricro.com

  • www.cabiatl.com

  • Method

  • Designs

  • Safety

Slide 2

tDCS vs TMS

  • Transcranial magnetic stimulation

    • Relatively expensive (~$50,000).

    • Moderate sized effects (e.g. mild speech arrest).

    • Safe, but there are reports of inducing seizures when high amplitude and frequency are combined.

    • Causes resting neurons to fire.

      • Very brief pulse stops interrupts processing for ~30ms, can be used repetitively.

      • Depending on frequency, sustained TMS can induce excitability reduction (long-term depression) or enhancements (long-term potentiation) that can persist for hours or days.

Slide 3

tDCS vs TMS

  • Transcranial direct current stimulation

    • Very inexpensive (~$250 for iontophoresis unit).

    • Believed to be exceptionally safe.

    • Does not cause resting neurons to fire (Purpura and McMurtry, 1965; Terzuolo and Bullock,1956).

    • Believed to modulate the firing rate of active neurons.

      • Depending on polarity, tDCS can induce cortical excitability reduction or enhancement can persists for hours.

Slide 4

tDCS vs TENS

  • Transcutaneous Electrical Nerve Stimulation systems are used to treat pain.

  • TENS pulsed 2-160Hz, 5-80 mA.

  • At slow frequency and high amplitude TENS induces muscle contraction.

  • In contrast, tDCS uses constant 1-2mA.

Slide 5

History of tDCS

+Anodal

  • ’50-60s exposed cortex of animals: diminish (cathodal) or enhance (anodal) cortical excitability and activity.

  • Lippold & Redfearn (1964) report scalp tDCS relieves depression in humans.

-Cathodal

Bindman et al. (1964)

Slide 6

Why a revival?

Ardolino (2005)

  • New methods provide converging support

    • Confirmed using consistent behavioral measures : corticospinal excitability, measured with TMS; TENS (Nitsche 2000; Ardolino 2005).

    • Confirmed using imaging: e.g. one sees less task related activation following cathodal stimulation (Baudewig et al., 2001)

    • Mechanism: change in membrane potential, NMDA receptor efficacy for longer duration effects (Nitsche, 2004).

Baseline

0min

60min

Baseline

After -tDCS

Baudewig et al. (2003)

Slide 7

Effects persist

  • Effects of tDCS persist after stimulation ends.

  • Longer stimulation, slower return to baseline.

Duration

5min

7min

9min

Nitsche et al. (2003)

Slide 8

Typical design

  • Convention is to conduct behavioral task during and/or immediately after stimulation.

  • E.G. Dockery reports that prefrontal tDCS polarity influences learning of Tower of London task – with effects seen 6-12 months later.

Dockery et al. (2009)

Slide 9

Scientific concerns

  • Current is very small (1-2mA)

    • So tiny, many doubt neural effects are real.

  • Behavioral effects typically very small

    • ‘File drawer problem’ most null results not counted.

    • Electrode placement crucial.

    • Controlling for experimenter demand crucial.

Slide 10

Where to stimulate

  • Null result if stimulated region not involved with task.

  • Our Visor neuronavigation system allows you to identify regions based on fMRI or MRI data.

Slide 11

Where to stimulate

  • Sadleir et al. (2010) suggest effects will be diffuse.

  • Datta (2009) suggest high density electrode placement could provide more specificity.

Slide 12

Where to stimulate

  • Stimulation region not well focused.

  • Must create electrical circuit: both anode and cathode.

    • If both on scalp, are effects due to facilitation or inhibition?

    • If one electrode on shoulder/limbs (Baker, 2010), perhaps spinal influence.

    • One option is large, diffuse electrode over mastoid (Elmer, 2009).

+

_

_

+

Slide 13

Can a horse perform arithmetic?

Actually, animal was responding to body language of human observers.

tDCS effects are small.

Small effects vulnerableto experimenter demand.

Double-blind rare but crucial.

I personally remain scepticalof many findings: we need scientific rigor.

Clever Hans (1907)

Slide 14

Our tDCS units

  • Our tDCS units designed for iontophoresis.

  • Can deliver up to 4mA: contemporary studies do not exceed 2mA.

  • Disposable sponge electrodes.

  • Optional USB system can ensure double blind research.

Slide 15

Theoretical safety concerns

  • Potential side effects with tDCS

    • electrode-tissue interface could lead to skin irritation and damage.

    • Stimulations could lead to excitotoxic firing rates.

    • Tissue damage due to heating.

  • Rat studies suggest injury only when current density is several orders of magnitude beyond those used in humans (Liebetanz et al. 2009).

  • Standard doses in humans does not appear to alter serum neuron specific enolase (NSE), a sensitive marker of neuronal damage (Nitsche et al, 2003).

  • Datta (2009) heating in humans is negligible.

  • Slide 16

    Practical safety concerns

    • Subtle but common side effects

      • Nitsche et at. (2003) reports that in more than 500 participants the only side effects are initial scalp tingling or sensation of a light flash.

      • Some studies suggest that higher current densities can lead to skin irritation.

      • If cognitive effects are prolonged, perhaps we should warn participants about driving or other hazardous tasks after a treatment session.

        • Koenigs (2009) note one neurologically healthy participant reported a couple hours dysphoria following cathodal tDCS.


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