Electroencephalography

Electroencephalography. It is a method to record electrical activity generated by the cerebral cortex of the brain, typically using electrodes placed on the scalp. The resulting recording, known as an electroencephalogram, provides a macroscopic view of neural dynamics and is a fundamental tool in both clinical neurology and neuroscience research. Its development was pioneered by Hans Berger in the 1920s, building upon earlier discoveries in animal electricity by Luigi Galvani and the identification of the evoked potential.

History

The foundational discovery of bioelectrical phenomena is credited to Luigi Galvani in the late 18th century. The first recordings of electrical activity from the exposed brain of animals were performed by Richard Caton in 1875. The pivotal breakthrough for human application came from Hans Berger, a Jena psychiatrist, who in 1924 recorded the first human electroencephalogram, coining the term and identifying the alpha wave. His work was later validated and expanded by prominent figures like Edgar Douglas Adrian. The field accelerated with the development of the 10-20 system by Herbert H. Jasper and others, standardizing electrode placement. Key clinical correlations were established at institutions like the Massachusetts General Hospital and the Montreal Neurological Institute, linking specific patterns to disorders like epilepsy.

Method

The standard procedure involves attaching multiple electrodes to the scalp according to the international 10-20 system. These electrodes, often made of silver chloride, are connected to differential amplifiers within an EEG machine which greatly magnifies the tiny voltage fluctuations. The signals are then digitized for computer analysis. Activation procedures such as photic stimulation using a stroboscope or hyperventilation are frequently employed to provoke latent abnormalities. For more precise localization, as in pre-surgical evaluation for epilepsy surgery, clinicians may use subdural grid or depth electrode implants, developed at centers like the Cleveland Clinic.

Clinical use

The primary clinical application is in the diagnosis and management of epilepsy, where it can identify interictal spikes and characterize seizure types. It is crucial in diagnosing encephalitis such as that caused by herpes simplex virus, and in assessing encephalopathy related to hepatic failure or Creutzfeldt-Jakob disease. It plays a vital role in evaluating brain death and monitoring cerebral function during carotid endarterectomy or in the intensive care unit. The technique is also used to investigate sleep disorders like narcolepsy at institutions like the Stanford Sleep Center.

Research use

In cognitive neuroscience, it is employed to study the neural correlates of perception, attention, and memory, often by analyzing event-related potentials like the P300. It is a core tool in psychophysiology research at universities like UCLA and the Max Planck Institute. The technique is integral to brain-computer interface research, enabling control of external devices, with pioneering work conducted at the Wadsworth Center. It also provides insights into the mechanisms of anesthesia and is used in neurofeedback protocols.

Interpretation

Interpretation involves analyzing the waveform's frequency, amplitude, morphology, and spatial distribution. Normal adult wakefulness is dominated by posterior alpha waves. Beta waves are prominent frontally, while theta wave and delta wave activity is normal during slow-wave sleep but abnormal in wakefulness. Specific patterns have diagnostic significance; for example, spike-and-wave discharges are indicative of absence seizures, and periodic sharp wave complexes are associated with Creutzfeldt-Jakob disease. The interpretation of epileptiform activity was systematized by researchers like J. Kiffin Penry.

Safety and limitations

The procedure is non-invasive and generally very safe, with no known risks from the electrical recording itself. Limitations include its poor spatial resolution compared to functional magnetic resonance imaging or magnetoencephalography, as the skull and scalp blur the electrical signals. It is largely insensitive to activity originating in deep brain structures like the hippocampus or brainstem. The recording can be easily contaminated by artifacts from eye movement, muscle activity, or heart signals, requiring skilled technologists for acquisition.