Transcranial direct current stimulation (tDCS) involves applying weak electrical currents to the head, to generate an electromagnetic field which modulates the activity of brain neurons. tDCS is known to selectively modulate neuronal excitability and can be used in conjunction with fMRI, transcranial magnetic stimulation (TMS) or centrally acting drugs. It is being investigated as a treatment for a variety of conditions such as stroke recovery, depression and migraine.
Extensive neurophysiological experiments have shown evidence that Direct Current (DC) penetrates the brain to modify neuronal transmembrane potentials, thereby influencing the level of excitability and modulating firing rates.
Animal studies suggest, that if cathodal stimulation - negative pole or electrode (cathode) is located near the cell body or dendrites - it decreases the resting membrane potential and therefore hyperpolarises neurones, whereas if anodal stimulation - positive pole or electrode (anode) is located near the cell body or dendrites - it causes depolarisation by increasing resting membrane potentials and spontaneous neuronal discharge rates.
Current research suggests that electrodes placed on the forehead could produce noticeable psychological changes that were dependent on the direction of the field.
Lang et al claim that anodal polarisation of the motor cortex increased the motor response to TMS of the same area; reduction of this response was observed with cathodal polarisation, the effects of which seem to last for an appreciable amount of time after exposure. Investigators are currently testing the validity of these claims and the effects of tDCS on other brain areas and functions.
- In studies of the visual system Antal et al working with Professor Walter Paulus in Göttingen, have used tDCS to modulate visual cortex excitability measured using TMS, and to enhance visuomotor learning.
- In cognitive studies tDCS has been used to change probabilistic learning in adults and to study working memory. These studies have stimulated V1, V5/MT, parietal cortex and prefrontal cortex.
- Studies applying tDCS to the motor cortex have revealed that tDCS can modulate motor cortex excitability (measured using TMS) for up to 90 minutes post stimulation. Exploiting this long term change in brain sensitivity has allowed exploration of motor learning and plasticity.
- The long term effects of tDCS make it an effective tool to use in conjunction with centrally acting drugs and one study has shown that dopaminergic modulation combined with tDCS can enhance the effects of tDCS for up to 24 hours.
- The rehabilitation potential of tDCS remains to be explored but already, in rehabilitation following stroke, tDCS has been combined with measures of plasticity in double blind trials to give promising indications of a positive role for non invasive DC stimulation in rehabilitation.