Transcranial brain stimulation: Past and future

This article provides a brief summary of the history of transcranial methods for stimulating the human brain in conscious volunteers and reviews the methodology and physiology of transcranial magnetic stimulation and transcranial direct current stimulation. The former stimulates neural axons and generates action potentials and synaptic activity, whereas the latter polarises the membrane potential of neurones and changes their sensitivity to ongoing synaptic inputs. When coupled with brain imaging methods such as functional magnetic resonance imaging or electroencephalography, transcranial magnetic stimulation can be used to chart connectivity within the brain. In addition, because it induces artificial patterns of activity that interfere with ongoing information processing within a cortical area, it is frequently used in cognitive psychology to produce a short-lasting ‘virtual lesion’. Both transcranial magnetic stimulation and transcranial direct current stimulation can produce short-lasting changes in synaptic excitability and associated changes in behaviour that are presently the source of much research for their therapeutic potential.