tDCS & tACS

Transcranial Direct Current Stimulation (tDCS) and Transcranial Alternating Current Stimulation (tACS) are non-invasive neuromodulation techniques that aim to influence brain activity and connectivity. tDCS involves the application of a low, direct electrical current to specific areas of the scalp using electrodes. This modulates the resting membrane potential of neurons, making them more or less likely to fire, thereby enhancing or inhibiting neural activity. The effects of tDCS can be categorized into "online" and "offline" phases.

During the "online" phase of tDCS, which occurs while the stimulation is being applied, the electrical current induces immediate changes in neuronal excitability by altering the membrane potential. When the anode (positive electrode) is placed over a brain region of interest, it promotes neuronal depolarization, making neurons more likely to fire, while the cathode (negative electrode) has the opposite effect, leading to hyperpolarization and reduced firing.

In the "offline" phase, which occurs after tDCS has been applied, there are longer-lasting stimulation effects. Besides its direct effects on membrane potentials, tDCS can induce changes in signaling related to gamma-aminobutyric acid (GABA) and glutamate. GABA, the brain's primary inhibitory neurotransmitter, plays a crucial role in broadly reducing neuronal excitability. On the other hand, glutamate, the brain's primary excitatory neurotransmitter, broadly increases neural excitability. These effects can outlast the actual stimulation period, depending on the type of stimulation, leading to long-term depression or long-term potentiation of the stimulated brain areas.

tACS delivers oscillating electrical currents at specific frequencies to entrain neural oscillations. This rhythmic stimulation can synchronize or desynchronize brain regions, potentially enhancing communication between them.

Both tDCS and tACS have gained popularity in neuroscience research and clinical applications. tDCS is used to investigate cognitive processes, like learning, executive functioning, and motor functions, while tACS is particularly valuable for studying brain oscillations, sensory perception, and memory consolidation. These non-invasive neuromodulation techniques offer promising opportunities for exploring the neural basis of human experiencing and behavior and can within limits even be used to enhance cognitive functioning.