Transcranial alternating current stimulation
Transcranial alternating current stimulation (tACS) is a noninvasive means by which alternating currents applied through the skull over the occipital cortex of the brain entrains in a frequency-specific fashion the neural oscillations of the underlying brain.
Two electrodes are used: a stimulating one over the target cortex, and a reference one elsewhere, such as on the top of the head or on the neck. The size of the stimulation electrode is around 3 x 4 cm and the reference electrode has three times the surface area so as to reduce current density and limit stimulating the skin. They are held in place by elastic bands, and the hair and skin are saturated with saline solution for about 5–10 minutes. There is an initial sensation on the scalp but, after the initial few minutes, this fades.
Transcranial alternating current stimulation applied over the visual cortex produces its effects depending upon frequency and illumination levels. When applied in an illuminated room, it induces most effectively the perception of continuously flickering light (phosphenes) with frequencies in the beta wave. But when in the dark such perception of flickering lights is most effective when the stimulation frequency when done in alpha wave range. The phosphenes are reported to be seen in the far peripheral areas of vision. This suggests that tACS stimulates the anterior part of the visual cortex on its medial wall where the peripheral areas of the visual field are processed. The reason for this could be that the current flows mostly in the cerebrospinal fluid between the two cerebral hemispheres and enters the cerebral cortex from the forward part of the visual cortex.
No effects are produced by current alternations at the theta wave or gamma wave frequencies. These experiments make tACS a potentially interesting tool to investigate the causal role of oscillatory synchrony in cortical processing. But the claim that tACS phosphenes originate in the visual cortex has been disputed; the alternative claim is that current spread from the occipital electrode evokes phosphenes in the retina.The frequency dependence of the phosphene strength can also be explained by retinal ganglion cell dynamics. Hence experiments involving tACS applied at the visual cortex needs appropriate control studies to rule out the retinal interference.
Transcranial alternating current stimulation applied over the motor cortex on one side brain at beta wave frequencies entrains activity in this range and increases the coherence between scalp-recorded electroencephalography (EEG) and electromyographic (EMG) activity in the first dorsal interosseous muscle of the opposite hand when it is held steady on a joystick. The brain waves led muscle EMG by 41.6 ms suggesting that the synchronization with muscle activity is caused by the brain. There is also a small but significant slowing of the reaction time when a person moves the joystick to a target.
Transcranial alternating current stimulation can modulate the ongoing rhythmic brain activity. Stimulation in the individual alpha frequency range elevates the endogenous EEG alpha band power.
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