Electrical stimulation to treat medical conditions is not a new therapy; it has been used to treat diseases for centuries. The first electricity sources used for electrical stimulation were produced by animal electricity. Antique Egyptians knew about the electrical proprieties of Nile catfish, but it is unclear if (and how) they experimented with them for clinical purposes. The first reported evidence of electrical stimulation arrives some centuries later in antique Greece times, when Plato and Aristotle described the ability of the torpedo fish to generate curative effects by its electric discharges (Althaus, Reference Althaus1873; Rockwell, Reference Rockwell1896; Harris, Reference Harris1908).
The first evidence of transcranial stimulation in history comes in Roman Empire times, when Scribonius Largus (the physician of the Roman Emperor Tiberius Claudius Nero Caesar) described how placing a live torpedo fish over the scalp could relieve headache in a patient (Scribonius Largus, Reference Ruello1529). Perhaps the first person known to have been cured by torpedo fish electricity was Anthero, a freed slave of Tiberius Caesar, who suffered gutta (probably gout) (Cambridge, Reference Cambridge1977). In the late 11th century, the Muslim physician in Persia, Ibn-Sidah also suggested the use of torpedo fishes to treat epilepsy (Priori, Reference Priori2003), placing the live catfish on the brows of the patients (Delbourgo, Reference Delbourgo2006). The use of electric fish stimulation also spread to Africa, where Jesuit missionaries in early modern Abyssinia (Ethiopia) reported that locals used catfishes as a method of expelling ‘Devils out of the human body’ (Delbourgo, Reference Delbourgo2006). Fish electricity was maybe the most popular type of electrical stimulation for more than 10 centuries though it is not clear how the effects were measured.
In 1660 the German scientist Otto von Guericke invented the first electrostatic generator (Comroe & Dripps, Reference Comroe and Dripps1976), a frictional crank-controlled machine. This device could be considered the first stimulator device and its variations were used later by scientists like the Italian anatomist Leopoldo Marco Antonio Caldani in 1756 to stimulate muscles in sheep and frogs (Caldani, Reference Caldani and Haller1760). The Middlesex Hospital (England) was probably the first hospital to purchase an electrostatic machine in 1767 (Cambridge, Reference Cambridge1977).
In 1745 Ewald Georg von Kleist invented the Leyden jar, the first capacitor in history (Keithley, Reference Keithley1999). This device could store electric charge produced from an electrostatic generator. Experimenters, like Anton de Haen in 1755 (Priestley, Reference Priestley, Dodsley, Johnson, Davenport and Cadell1767) and Benjamin Franklin in 1757 (Franklin, Reference Franklin1757), were able to combine electrostatic generators and the Leyden jar for therapeutic electrification (McWhirter et al. Reference McWhirter, Carson and Stone2015).
In 1773 the anatomist and physiologist John Hunter studied the torpedo fish thoroughly. These investigations were undertaken at the request of John Walsh, who showed that the ‘shocks’ produced by the torpedo fish were caused by the generation of electricity (Walsh, Reference Walsh1773). These kind of animals or fishes have an electric organ that, on brain command, generate a three-dimensional dipole field around their bodies, discharging single-cycle pulses from below 1 Hz to about 65 Hz at rest (Hopkins, Reference Hopkins2009). Electric fish electricity is not direct current (DC); nevertheless it is the first reported kind of stimulation in history.
Unlike fish electricity and electrostatic electricity, DC is the flow of electrical charge that does not vary with time, generating a constant signal (Belove & Drossman, Reference Belove and Drossman1976). The birth of the DC generator was in the 1st century BC with the so-called Baghdad battery, attributed to the ancient Persian civilization (Scrosati, Reference Scrosati2011), but other references attribute the invention to the Parthians, calling it the Parthian galvanic cell (Keyser, Reference Keyser1993). This invention remained forgotten until the 20th century, when the archeologist Wilhelm Köning discovered it in Iraq and it was possibly used for medical purposes. In the 18th century Luigi Galvani invented a DC battery (galvanic battery) and his nephew, Giovanni Aldini, was one of the first persons to utilize DC for clinical applications. Aldini's most detailed account of DC clinical treatment concerns Luigi Lanzarini, a 27-year-old farmer suffering from melancholy madness (major depression), who had been committed to Santo Orsola Hospital, in Bologna, Italy on 17 May 1801, but first assessing the effects of galvanic currents on his own head (Fitzgerald, Reference Fitzgerald2014). The patient's mood progressively improved so that Lazarini was apparently completely cured several weeks after the beginning of the treatment (Parent, Reference Parent2004).
Aldini's work was the milestone which began the era of DC stimulation for neurological and psychiatric conditions. Later in 1802, Hellwag and Jacobi reported the use of transcranial DC, also reporting the first evidence of phosphenes using DC (Hellwag & Jacobi, Reference Hellwag and Jacobi1802; Paulus, Reference Paulus2010). Around 1880 the application of brain stimulation treatments on patients was particularly popular among German psychiatrists, pioneers in electrotherapy, an early tDCS method. Wilhelm Tigges, Rudolph Gottfried Arndt (Steinberg, Reference Steinberg2013a ) and Erb (Reference Erb1883) tried to establish clear rules on the most beneficial application methods and doses in order to investigate which results it may produce and under what circumstances (Steinberg, Reference Steinberg2014). The experimental designs with larger groups in electric therapy research protocols were a common factor in this age. For example, Arndt used 12 psychotic patients in his 1870 experiment. Despite being very detailed, Arndt's reports do not provide exact data about the strength of the applied current. Due to controversial reports (some with positive results and others with negative) and the lack of understanding of operating principles, electrotherapy was repeatedly suspected of attaining result through suggestion only (Steinberg, Reference Steinberg2013a ). Many other researchers used DC for the treatment of mental disorders during the 19th century and the early part of the 20th century, but the variation of procedures, unclear descriptions, few qualitative details and the misunderstood effect of polarization led to variable and/or inconclusive results. The use of DC stimulation was abandoned from the 1930s (Steinberg, Reference Steinberg2013b ).
In 1957 DC reappeared in electrosleep therapy and around 1960–1963 electro-anesthesia research incorporated DC bias. In 1964, motivated by animal studies that reported lasting changes in excitability using prolonged scalp DC stimulation, Lippold and Redfearn used 50–500 µA DC currents in 32 healthy subjects, and reported that anodal current induced an increase in alertness, mood and motor activity, whereas cathodal polarization induced quietness and apathy (Lippold & Redfearn, Reference Lippold and Redfearn1964; Guleyupoglu et al. Reference Guleyupoglu, Schestatsky, Edwards, Fregni and Bikson2013). Despite several follow-up research works, from the 1970s DC stimulation was once again abandoned, probably due to the introduction of new psychiatric drugs (Dubljević et al. Reference Dubljević, Saigle and Racine2014).
It was not until 1998 when the usage of DC was advocated and the modern tDCS was born, when Priori and his colleagues investigated the influence of DC in the brain by testing its effects on cerebral cortex excitability using transcranial magnetic stimulation (Brunoni et al. Reference Brunoni, Nitsche, Bolognini, Bikson, Wagner, Merabet, Edwards, Valero-Cabre, Rotenberg, Pascual-Leone, Ferrucci, Priori, Boggio and Fregni2012). Characteristics of tDCS, such as the fact that it is non-invasive, mostly well tolerated and its mild adverse effects, have sparked great interest and increase in clinical studies recently (Brunoni et al. Reference Brunoni, Nitsche, Bolognini, Bikson, Wagner, Merabet, Edwards, Valero-Cabre, Rotenberg, Pascual-Leone, Ferrucci, Priori, Boggio and Fregni2012).
The transcranial DC stimulators have evolved from a simple galvanic battery in the 18th and 19th centuries, passing from vacuum tubes and transistors to microprocessors and microcontroller technologies in the 20th century. Progress in microcontroller technology has enabled electronic and biomedical engineers to build precise tDCS devices with a better control of stimulation parameters (Paulus & Opitz, Reference Paulus and Opitz2013) at reduced costs. The future tDCS designs will focus on obtaining simple systems by way of reducing size, power consumption, weight and enhanced portability (Kouzani et al. Reference Kouzani, Jaberzadeh, Zoghi, Usma and Parastarfeizabadi2016).
The tDCS is a promising tool for basic neuroscientists, clinical neurologists and psychiatrists in their quest to causally probe cortical representations of sensorimotor and cognitive functions, to facilitate the treatment of various neuropsychiatric disorders (Schlaug & Renga, Reference Schlaug and Renga2008) and enhance neurological functions in healthy humans (Dubljević et al. Reference Dubljević, Saigle and Racine2014). Although tDCS has demonstrated benefits, the scientific community must communicate carefully about their findings, by providing neutral data to the media and public, since the popular media sometimes consider tDCS as a ‘miracle device’ (Riggall et al. Reference Riggall, Forlini, Carter, Hall, Weier, Partridge and Meinzer2015). Such sensationalistic news about the benefits of tDCS leads people to self-administer stimulation, as we can see in some Internet do-it-yourself tDCS forums (Wexler, Reference Wexler2015).
The authors wish to thank Jorge Barraza-Díaz and Gerardo Reyes-Acevedo for their technical support.
Declaration of Interest