Transcranial brain stimulation: potential and limitations

Abstract

The brain adapts to new requirements in response to activity, learning or reactions to environmental stimuli by continuous reorganization. These reorganization processes can be facilitated and augmented, or also inhibited and prevented, by transcranial neurostimulation. The most common methods are electrical or magnetic stimulation. However, few studies have dealt with the newer methods using near infrared or ultrasound stimulation.Transcranial magnetic stimulation (TMS) allows the pain-free transfer of very short bursts of high intensity electrical energy through the skull and can induce action potentials. By varying the number and intensity of the stimuli, and the stimulus sequence, repetitive TMS (rTMS) can induce either inhibitory or facilitatory effects in the brain. A differentiation is made between short-lived interference with ongoing brain activity, and plastic changes that persist for a longer period beyond the end of the stimulation.Weaker electric fields in the 1 mA range can be applied painlessly through the skull. These probably exert their effects by modulating neuronal membranes and influencing the spontaneous firing rate of cortical neurons. They encompass the range from transcranial direct current stimulation (tDCS) to high frequency alternating current stimulation (tACS) in the kilohertz range. In view of the multitude of physically possible stimulation algorithms, hypothesis-driven protocols based on cellular or neuronal network characteristics are particularly popular, in the effort to narrow the choices in a meaningful manner. Examples are theta burst stimulation or tACS in the so-called “ripple” frequency range. It is, of course, not possible to selectively stimulate individual neurons using transcranial stimulation techniques; however selective after-effects can be achieved when used in combination with neuropharmacologically active drugs. The use of these methods for neuroenhancement is now a topic of intense discussion.