Transcranial Direct Current Stimulation in the Treatment of Autism: Literature Review

Transcranial Electrical Stimulation (tES) is a non-invasive brain stimulation technique used to modulate neuronal activity through the application of weak electrical currents (1mA to 2 mA) to the scalp. Unlike Transcranial Magnetic Stimulation (TMS), tES is administered during task execution, as it more specifically influences the neurons involved in the task, thereby altering responses. The method includes various stimulation protocols such as Transcranial Direct Current Stimulation (tDCS), Transcranial Alternating Current Stimulation (tACS), and Transcranial Random Noise Stimulation (tRNS), each differing in the type of current delivered. tDCS, the most commonly used protocol, applies a continuous current, inducing depolarization (excitatory effects) with anodic stimulation or hyperpolarization (inhibitory effects) with cathodic stimulation. This technique is widely used for cognitive enhancement and produces both short-term and long-term effects on synaptic plasticity. tACS, on the other hand, utilizes alternating currents to synchronize neuronal oscillations, acting specifically on brain regions at defined frequencies. Lastly, tRNS delivers randomized currents within a frequency range, leveraging stochastic resonance to enhance cortical excitability and improve performance in concurrent motor or cognitive tasks. tRNS is also associated with prolonged aftereffects and is less discomforting for subjects. Safety guidelines for tES include limiting current density, electrode type, and duration (maximum 2 mA for 20 minutes) to prevent tissue damage. Exclusion criteria apply to individuals with metallic implants, neurological conditions (e.g., epilepsy), or certain medications. This study highlights the mechanisms, protocols, applications, and safety parameters of tES, offering insights into its potential for modulating brain activity and enhancing cognitive and motor performance.