tRNS

Transcranial random-noise stimulation (tRNS) is a non-invasive brain stimulation technique that delivers a small electrical current with randomly varying amplitude and/or frequency to the scalp through surface electrodes. The current is typically applied for several minutes at intensities around 1 to 2 milliamperes and is used to modulate cortical excitability and information processing in targeted brain regions. The stimulation signal is defined as random within a specified frequency range, with high-frequency bands such as 100 to 640 Hz commonly used in many protocols.

The proposed mechanisms involve stochastic resonance, where background neural noise can enhance the detection and processing of weak signals, and plasticity-related processes that may involve NMDA receptor activity. Aftereffects can persist for minutes to hours after stimulation, suggesting transient modulation of cortical networks rather than long-term structural changes.

Common protocols vary by target region and task. Montages are chosen to reach specific cortical areas (for example, motor or visual cortex). Typical duration ranges from 10 to 30 minutes, with current intensities around 1–2 mA. The frequency content is randomized within a defined band, producing a sustained noisy input during the session.

Evidence for efficacy is mixed and often task- and site-dependent. tRNS has been associated with improvements in perceptual learning, visual and tactile discrimination, and some cognitive functions in healthy individuals, with effects sometimes enhanced by concurrent task practice. Compared with other forms of non-invasive stimulation, tRNS is generally well tolerated and can produce fewer cutaneous sensations, though responses are variable across studies.

Safety considerations include common minor sensations such as tingling or itching under the electrodes. Phosphenes can occur with occipital stimulation. Contraindications include implanted electronic devices in the head, a history of seizures, pregnancy, or other medical conditions requiring caution. As with any stimulation method, protocols should follow device guidelines and clinical judgment.