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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).

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transcranial direct current stimulation
Transcranial Direct Current Stimulation
  • Chris Rorden
  • www.mricro.com
  • www.cabiatl.com
  • Method
  • Designs
  • Safety
tdcs vs tms
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.
tdcs vs tms1
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.
tdcs vs tens
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.
history of tdcs
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)

why a revival
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)

effects persist
Effects persist
  • Effects of tDCS persist after stimulation ends.
  • Longer stimulation, slower return to baseline.

Duration

5min

7min

9min

Nitsche et al. (2003)

typical design
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)

scientific concerns
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.
where to stimulate
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.
where to stimulate1
Where to stimulate
  • Sadleir et al. (2010) suggest effects will be diffuse.
  • Datta (2009) suggest high density electrode placement could provide more specificity.
where to stimulate2
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).

+

_

_

+

clever hans 1907
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)
our tdcs units
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.
theoretical safety concerns
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.
practical safety concerns
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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