Efficacy of Transcranial Direct Current Stimulation for Treating Working Memory Dysfunction and Depression For Patients With Temporal Lobe Epilepsy

Transcranial direct current stimulation (tDCS) is a powerful technique to modulate brain activity:

TDCS is based on the application of a weak direct current to the scalp that flows between two relatively large electrodes-anode and cathode. During tDCS, low amplitude (1-2 mA), constant currents are applied via the scalp electrodes and penetrate the skull to enter the brain. Although there is substantial shunting of current in the scalp, sufficient current penetrates the brain to modify the trans-membrane neuronal potential as shown by two recent modeling studies (Miranda et al. 2006; Wagner et al. 2007a), and thus influence the level of excitability and modulate the firing rate of individual neurons. When tDCS is applied for a sufficient duration, cortical function can be altered beyond the stimulation period (Nitsche and Paulus 2001) and the direction of the cortical excitability changes depends on current orientation.

Several well-conducted animal studies on the effects of tDCS dating back to the 1950s and 60s showed that tDCS is a powerful technique to modulate brain function. These studies demonstrated that polarizing currents applied to the surface of the brain result in a modulation of the cortical activity. Surface anodal polarization of the cortex increases spontaneous unit discharges (Burns 1954; Creutzfeld et al. 1962) and initiates paroxysmal activity (Goldring and O'Leary 1951), whereas cathodal polarization generally depresses these events. Furthermore, low-level surface polarization facilitates acquisition of learned motor responses and induces prolonged changes in patterns of evoked cortical unit discharges (Bindman et al. 1964). Finally, Purpura et al. (1964), studying pyramidal tract cells from cats, showed that prolonged periods of polarization may produce progressive membrane and post-synaptic potential changes as well as after-effects (Purpura and McMurtry 1965).

Based on this evidence, recent human studies have been performed and collectively have shown that motor cortex (M1) stimulation with tDCS changes motor cortex excitability depending on the stimulation polarity: while anodal stimulation increases cortical excitability, cathodal stimulation decreases it (Nitsche et al. 2003; Nitsche and Paulus 2000; Nitsche and Paulus 2001). Similar modulatory effects have also been described in the visual cortex (Antal et al. 2004; Antal et al. 2001). A recent tDCS study has shown that anodal tDCS of the primary motor cortex not only affects cortical activity, but induces significant changes on thalamic activity (Lang et al. 2005). It should be noted that application of tDCS in humans has advanced significantly in the last 10 years and it is therefore different from the human application used in the '60s and '70s (Wagner et al. 2007b)

Furthermore, tDCS offers several advantages as compared with other techniques of noninvasive brain stimulation (i.e., repetitive transcranial magnetic stimulation (rTMS)): (1) small size of the electrodes and stimulator, thus allowing portable use for instance to be used at home, (2) simple and non-expensive technique that can easily be translated for use in clinical practice, (3) long-lasting effects - the modulatory effects of tDCS last longer as compared to rTMS - for instance, 13 minutes of stimulation changes brain excitability for up to 2 hours (Nitsche and Paulus 2001), and (4) more easily blinded with sham tDCS in the setting of clinical trials (Gandiga et al 2006)