Transcranial Direct Current Stimulation

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Transcranial Direct Current Stimulation

• Chris Rorden
• www.mricro.com
• www.cabiatl.com
• Method
• Designs
• Safety

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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.

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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.

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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.

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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)
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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)
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Effects persist

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

Duration
5min
7min
9min
Nitsche et al. (2003)
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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)
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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.

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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.

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Where to stimulate

• Sadleir et al. (2010) suggest effects will be
  diffuse.
• Datta (2009) suggest high density electrode
  placement could provide more specificity.

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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).

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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.

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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.

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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.

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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.