The introduction of the galvanometer with astatic needles has been mainly associated with Leopoldo Nobili, a physicist in Florence, and was further refined in 1858 by William Thompson (Lord Kelvin) in England. Galvanometers were able to faithfully demonstrate continuous electrical currents and their variations in intensity, but could not detect extremely brief electrical phenomena (Fig. 2). In 1875, Richard Caton, a scientist from Liverpool, England, placed two electrodes of a galvanometer over the scalp of a human and became the first to record brain activity in the form of electrical signals. Caton used a beam of light projected on the mirror of the galvanometer and reflected onto a large scale placed on the wall. With this type of visualization, Caton found that “feeble currents of varying direction pass through the multiplier when the electrodes are placed at two points of the external scalp surface.” This initial experiment led to the concept of the graphic recording of registered electrical brain signals, the technique that underlies present-day EEG. Caton noted that the surface of the gray matter was positively charged with respect to deeper structures in the cerebrum. He also noted that the electric currents of the cerebrum changed in relation to the underlying function with neurofunctional active regions exhibiting negative variations of electric current. Hence, Caton has also been credited with pioneering the work on evoked potentials.

Concurrent with Caton’s work, physiologists in Eastern Europe began to report their observations on cerebral electrical activity with another discovery of greater impact on the neuroscientific world – the capability of the cerebral cortex to be electrically stimulated as described by Gustav Fritsch and Julius Eduard Hitzig in 1870. These discoveries were followed by several observations of spontaneous electrical activity in the brains of animals and the studies of electrical responses of the human brain after electrical stimulation. These included the first EEG evidence of epileptic activity during a seizure in a dog following electrical stimulation reported by Napoleon Cybulski, a Polish physiologist. Seven years after the study of Fritsch and Hitzig, Vasili Y. Danilevsky wrote his thesis on electrical stimulation and the spontaneous electrical activity of animal brains while working at the University of Kharkov. However, Danilevsky was disappointed as he had expected better correlation of the spontaneous regional electrical brain activity with psychic and emotional processes. Adolf Beck, a Polish physician and physiologist at the University of Lwów, Poland, further investigated the spontaneous electrical brain activity of rabbits and dogs using nonpolarizable electrodes and observed the disappearance of rhythmical oscillations during illumination of the eyes (i.e., “alpha blocking”).

In 1903, Willem Einthoven, a Dutch doctor and physiologist, invented a string galvanometer, an instrument with greater sensitivity of detection but which required photographic recording. The string galvanometer became the standard instrument for EEG at the turn of the century with Pravdich-Neminsky, a Ukrainian and then Soviet physiologist, reporting electrical activity recordings using this technique in animal brains in 1912.

In the 1920s, Hans Berger, a German neuropsychiatrist and the discoverer of the human EEG, was the first to describe the existence of human EEG signals [1]. Berger first used a string galvanometer in 1910, to later migrate to an Edelmann model followed by a more powerful Siemens double-coil galvanometer. Unfortunately, it took more than 10 years for the scientific community to accept these scalp potentials as genuine brain signals.

In 1926, Berger started to use the more powerful Siemens double-coil galvanometer and published his first report of a human 3-min EEG recording in 1929. He described the alpha rhythm as the dominant component of human EEG signals and the alpha blocking response, a milestone in the history of clinical EEG [1]. For his one-channel EEG tracings, Berger used a bipolar recording technique with fronto-occipital leads along with a time marking line generated with a sine wave of 10 cycles/sec (Fig. 3). During the 1930s, Berger recorded the first EEG of human sleep, detecting sleep spindles. He followed this with the examination of human EEG patterns in hypoxic brain injury, in epilepsy, in the investigation of several diffuse and localized brain dysfunctions, and with the examination of changes in EEG signals with mental activities.

While EEG provides very large-scale, robust measures of neocortical dynamic function, one single scalp electrode provided estimates of synaptic sources averaged over tissue masses containing between 100 million and 1 billion neurons.

In 1932, a group of inventors in Berlin, Germany, lead by Jan Friedrich Tönnies, a German inventor and engineer, and investigators at the Rockefeller foundation in New York, USA, simultaneously built the first amplifiers designed to record cerebral potentials thus opening up the field of multichannel recordings that covered large brain regions [2]. By the 1940s, EEG technology was viewed as a genuine window on the mind, with important applications in neurosurgery, neurology, and cognitive science.

The first report of a prominent, transient, electrographic element termed an “epileptic spike” came from Fisher and Lowenbach in 1934 [3], inspiring further electrophysiological work in epileptology by Gibbs and Lennox, two neurologists and epileptologists from Harvard Medical School, USA. A few years later, Hallowell and Pauline Davis began the first investigations of EEG patterns during human sleep. This was followed by the first systematic studies of sleep EEG patterns and different stages of sleep in humans by Alfred L. Loomis and colleagues initiating EEG-based analyses of sleep disorders through the work of Kleitman in the 1940s at the University of Chicago, USA.

In the 1940s, the EEG started to become invasive, and the use of special implanted or “depth” electrodes and the exploration of deep intracerebral regions began. The space averaging of brain potentials resulting from extracranial recording is a random data reduction process forced by current spreading in the head volume conductor. In contrast, EEG electrodes implanted in brains provide more focal details but with very circumscribed spatial coverage that fails to provide a more global assessment of brain function. However, technical and ethical limitations of intracranial recording forced neurophysiologists to emphasize scalp recordings, which provide synaptic action estimates of larger scale sources that can be correlated to cognition and behavior.

Throughout the 1950s, clinical and experimental neurophysiological studies using EEG expanded rapidly worldwide with the discovery of cerebral recruiting responses, the effects of descending and mostly inhibitory influences of the brainstem reticular formation, and the use of EEG to locate brain regions that generated epileptic activity prior to surgical interventions. These studies were followed by EEG studies in newborns in the 1960s and investigations of evoked cortical potentials that became commonly used for prognosis and the monitoring of psychiatric and neurocritically ill patients in the late 1970s [4, 5]. Despite the large body and scientific impact of early investigations using EEG at that time, further discussion on this topic is beyond the focus of this chapter.

Since the first human EEG recordings in the early 1920s and their widespread acceptance 20 years later, it has been known that the amplitude and frequency content of EEG patterns reveals substantial information about the neurofunctional state of the brain. For example, the voltage record during deep sleep has dominant frequencies in the delta range near 1 Hz, whereas the eyes-closed waking state is associated with sinusoidal oscillations of an alpha frequency range near 10 Hz. Early in the history of EEG, it became clear that more standardized and automatic quantitative analyses would allow for reliable identification and correlation of EEG information to different neurofunctional states, such as distinguishing different sleep stages, determining the depth of anesthesia, identifying waxing and waning epileptic activity during seizures, and the analysis of encephalopathic states. Hence, in the 1950s, EEG became widely available, and almost every tertiary academic medical care center had at least one EEG machine. At the end of the decade, EEG was also in use in a large number of nonacademic hospitals and private practices in the 1960s.