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). • 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 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 • 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 +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? 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 of tDCS persist after stimulation ends. • Longer stimulation, slower return to baseline. Duration 5min 7min 9min Nitsche et al. (2003)
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 • 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 • 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 stimulate • Sadleir et al. (2010) suggest effects will be diffuse. • Datta (2009) suggest high density electrode placement could provide more specificity.
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). + _ _ +
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 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 • 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 • 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.