What is transcranial direct current stimulation?
With tDCS a weak electrical current of 1 or 2 m Ampere is applied to the head with an electrode. The electrode is a non-metalic conductive rubber electrode, covered completely by saline soaked sponges.It is a noninvasive brain stimulation technique that utilizes low amplitude direct currents applied via scalp electrodes to inject currents in the brain and thus modulates the level of excitability. It doesn’t elicit seizures and it doesn’t require anesthesia. For comparison in ECT current of 9 m Ampere is used to elicit seizures during narcosis.
Advantages for tDCS above rTMS?
In comparison with rTMS, tDCS has some advantages
and disadvantages. The main advantages are that this is
- a simple, nonexpensive procedure, which is painless and allows inducing effects of opposite directions (facilitation or inhibition) on different parts of the brain
- It has a reliable sham condition, therefore providing more robust double blind clinical trials than TMS.
- tDCS is a good tool to be used simultaneously with cognitive training as it induces much less scalp sensation than rTMS and therefore is not prey to aspecific effects on attention
Limitations of tDCS compared to rTMS.
- tDCS is less focal than TMS, it is not focal enough to target localized areas and to map cognitive functions accurately
Efficacy in cognitive neurorehabilitation
Overall the use of tDCS in cognitive neurorehabilitation is limited. The data to date are insufficient to assess the therapeutic use of tDCS in cognitive rehabilitation.
tDCS has been used to modulate cognitive performance in healthy subjects. tDCS improved long-term memory consolidation for word pairs, enhanced working memory and language learning It also improved other language related tasks such as verbal fluency or picture naming. All these results were found in healthy subjects.
tDCS applied to the left frontotemporal cortex of nonfluent aphasic patients, resulted in a 34% improvement in the ability to name object pictures correctly. The effects of tDCS in patients with Alzheimer’s Disease. Results showed that after anodal tDCS in temporoparietal areas, accuracy on a word recognition memory task increased.
There is still a large field to explore with tDCS. The site of application, the excitability status of the underlying cortical tissue, and the timing, frequency, and duration of stimulation are still to be determined. Changes induced in cortical response induced by tDCS are dependent on a number of technical variables. Some are still subject of much debate because tDCS is marginally researched in the literature of neurostimulation. Depression trial discussed in a post on this blog suggest that
at least 4 weeks of treatment are necessary to achieve clinically meaningful benefits. Therefore, the duration of treatment remain a key point in further studies. Follow-up on efficacy is needed, the effects could be temporarily.
C MINIUSSI, S CAPPA, L COHEN, A FLOEL, F FREGNI, M NITSCHE, M OLIVERI, A PASCUALLEONE, W PAULUS, A PRIORI (2008). Efficacy of repetitive transcranial magnetic stimulation/transcranial direct current stimulation in cognitive neurorehabilitation Brain Stimulation DOI: 10.1016/j.brs.2008.07.002