Transcranial random noise stimulation

Transcranial random noise stimulation
Transcranial random noise stimulation
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Transcranial random noise stimulation (tRNS) is a non-invasive brain stimulation technique and a form of transcranial electrical stimulation (tES).

Technique

tRNS involves the application of a random electrical oscillation over the cortex. tRNS can be applied in three frequency ranges: the entire spectrum from 0.1 to 640 Hz, in the low band (0.1-100 Hz), or in the high band (101-640 Hz). The effect of tRNS at frequencies of 100-640 Hz with decreasing duration of the stimulation (4,5, and 6 minutes) on motor cortical excitability was studied. Significantly increased facilitation at 5 and 6 minutes and none at 4 minutes was found, suggesting that a minimal duration of 5 minutes is necessary to observe an effect. Although tRNS has shown positive effects in various studies, the optimal parameters, as well as the potential clinical effects of this technique, remain unclear.

Effects

Terney et al (2008) was the first group to apply tRNS in humans. They showed that by using an alternate current along with random amplitude and frequency (between 0.1 and 640 Hz) in healthy subjects, the motor cortex excitability increased significantly (i.e. increased amplitude of motor evoked potentials) for up to 60 minutes after 10 minutes of stimulation. The study included all the frequencies up to half of the sampling rate (1280 samples/s) i.e. 640 Hz, however the positive effect was limited only to higher frequencies.

Fertonani et al. (2011) tested the role of high- and low-frequency tRNS on perceptual learning compared to anodal and cathodal tDCS. tRNS was applied for a duration of 4 minutes in five experimental blocks (22 minutes total), with a current of 1.5 mA and frequencies in the low (0.1-100 Hz) and high range (100-640 Hz). The study concluded that only the high-frequency tRNS subjects showed better accuracy on the perceptual task compared with the other groups.

Snowball et al (2013) reported an improvement of calculation- and memory-recall based learning that was associated with hemodynamic responses, suggesting an efficient neurovascular coupling on the dorsolateral prefrontal cortex. The effects persisted for up to 6 months after stimulation. This study showed that tRNS could potentially have long-term effects.

Malquiney et al (2011) have applied tRNS on the dorsolateral prefrontal cortex to assess its effects on working memory by using a randomly alternating level of current between —500 and +500 µA with a sampling rate of 1280 samples/s and high range frequencies (101-640 Hz), providing a current of 1 mA. This study did not find any significant changes in working memory.

In fMRI studies, a reduction of the Blood-oxygen-level dependent (BOLD) response in the motor cortex could be observed during tRNS and finger tapping.

Mechanism of action

The physiological mechanisms underlying the effects of tRNS are not well known, however many hypotheses have been suggested. The robust changes in cortical excitability observed after tRNS could be attributed to the repeated opening of sodium channels or to the increased sensitivity of neuronal networks to modulation. tRNS may influence cortical oscillations, leading to changes in excitability. These proposed mechanisms are consistent with the observation that reversing electrode polarities in tRNS does not interfere with the augmentation in cortical excitability, suggesting that tRNS-induced cortical excitability is independent of current flow direction.

Since tRNS is a repetitive, random, and subthreshold stimulation, it is speculated that tRNS induces direct temporal summation of neural activity because the time constant of a neuron is sufficiently long to permit the summation of two stimuli presented in close succession.

The effects of tRNS may also be explained in the context of the stochastic resonance phenomenon. tRNS is, by definition, a stimulation that induces non-finalized random activity in the system (i.e., noise). The presence of neuronal noise might enhance the sensitivity of the neurons to a given range of weak inputs.

Comparison with other tES techniques

This technique is relatively newer than other transcranial electrical stimulation techniques; therefore, speculation about its possible mechanisms of action in cognition is rare to date. Researchers using tRNS and tACS to enhance cognition is still fewer than those supporting tDCS.

tRNS stimulation differs from tDCS in that instead of constant direct current delivery, current levels are randomly generated, with a normal distribution around a specific mean intensity. Other parameters related to the stimulation electrodes, like position and size, are similar to tDCS. Compared to tDCS, tRNS has the advantage of being more comfortable, which makes it potentially advantageous for setting and blinding studies. tRNS is easier to blind than tDCS with the 50% perception threshold for tDCS at 400 µA while this threshold was at 1200 µA in the case of tRNS.

tACS (transcranial alternating current stimulation) is a frequency-specific stimulation method that is also thought to influence oscillatory neuronal activity. This method differs from tRNS in that a sinusoidal current is applied at a fixed frequency rather than a randomly presented range of frequencies. Often, tACS is applied at frequencies that mirror the predominant frequency bands observed in EEG in different regions of the brain.

References

Technique

Effects

Mechanism of action

Comparison with other tES techniques

See also

References