February 5, 2009 |
by Warren Grill, senior technical editor
February 5, 2009
There is renewed interest in transcranial stimulation as a means to manipulate brain activity and treat neurological disorders. Low-level DC current applied to the skull using surface electrodes, called transcranial direct current stimulation, can modulate cortical function in humans.
Last year, we highlighted applications of tDCS to improve verbal fluency, perhaps as a treatment for cognitive impairment, and to treat depression [NBR Aug07 p1]. Now this technique has been extended to transcranial alternating current stimulation, which uses low-amplitude sinusoidal currents at frequencies matched to the intrinsic rhythms of the brain. These intrinsic activities are observed in electroencephalographic and field potential recordings and are divided in distinct frequency bands: theta (4-8 Hz), alpha (8-14 Hz), beta (14-22 Hz), and gamma (>30 Hz). Publishing in Current Biology, the team of Ryota Kanai and colleagues from University College London and Georg-August University, Göttingen, Germany, report that the ability to evoke visual percepts (phosphenes) by transcranial activation of the visual cortex was strongly dependent on the stimulation frequency.
Eight healthy subjects reported on visual sensations produced by low-amplitude transcranial currents applied over the visual cortex at varying amplitudes and frequencies, in both the light and the dark. In the light, stimulation applied in the beta range enhanced the perception of phosphenes (flickering light), while stimulation delivered in other frequency bands (theta, alpha, or gamma) had no impact. Similarly, the threshold currents for phosphene perception were lowest (about 500 µA) for sinusoidal currents in the beta frequency range. However, in the dark, stimulation applied in the alpha range enhanced the perception of phosphenes, while stimulation delivered at other frequencies had no impact, and threshold current were lowest for frequencies in the alpha range.
A previous report by the same group, published in the journal Brain Stimulation earlier this year, indicated negligible effects of tACS on the excitability of the motor cortex. However, that study used current intensities (about 400 µA) below the thresholds found in the present study to evoke phosphenes by stimulation of the visual cortex, suggesting that higher amplitude currents may have been effective.
The strong dependence of the effect on frequency suggests that tACS was interacting with ongoing brain activity. This frequency dependence is reminiscent of the effects of so-called slow wave stimulation that we reported previously [NBR, Nov06 p1]. Alternating electric currents, subthreshold for direct stimulation, when applied at 0.75 Hz during slow wave sleep enhanced word recall, while stimulation at 5 Hz did not.
Pharmacological treatments for neurological disorders often have their effects through modulation of abnormal patterns of neuronal activity and network interactions. The same activity patterns may also be susceptible to modulation with applied electric fields, and this will open up new opportunities to treat neurological diseases with neurotechnology. These new results highlight the importance of both amplitude and frequency in effective modulation of cortical excitability with transcranial stimulation.
