Methods of therapeutic cortical stimulation.

In the nineties, epidural cortical stimulation (ECS) of precentral region has been performed to treat drug-resistant neuropathic pain and repetitive transcranial magnetic stimulation (rTMS) of prefrontal region has shown antidepressant effects in episodes of major depression. These were among the first attempts to treat neurological or psychiatric disorders with cortical stimulation. Actually, a variety of invasive and noninvasive techniques of cortical stimulation could serve therapeutic purpose, including ECS, rTMS, but also transcranial electrical stimulation using pulsed currents (TCES) or direct currents (tDCS). This review presents the methods of therapeutic cortical stimulation that are currently applicable and some of their principles. In particular, it must be emphasized that the site(s) of action can be distant from the site of stimulation because axons with remote projections are more prone to be activated than local cell bodies. Hence, cortical stimulation may activate, inhibit or otherwise interfere with the activity of various cortico-subcortical networks, depending on stimulus frequency and intensity, current polarity, and the configuration of the induced electric field. Functional and clinical effects occur during or beyond the time of stimulation. The existence of after-effects relates to processes of synaptic plasticity induced by the stimulation. Cortical stimulation may also have neuroprotective effects against disease-related excitotoxic phenomena. Considering the multiple techniques and the various potential clinical indications, it is a challenge to determine the place of cortical stimulation in the treatment of neurological and psychiatric diseases, in particular by the side of deep brain stimulation.