Electroencephalography is the neurophysiologic (neurophysiology is a part of physiologic science, concerned with the study of low-level functioning of the nervous system. Other related sciences are neurobiology, psychology, neurology, clinical neurophysiology, electrophysiology, ethology, higher nervous activity, neuroanatomy, cognitive science) or medical procedure of measuring the electrical activity of the brain. This is done by attaching electrodes on the scalp or subdurally as in special cases (beneath the dura mater or toughest and outermost layer surrounding the brain) or in the cerebral cortex (this is the brain structure in vertebrates and is essential in major brain functions like memory, attention, perceptual awareness, “thinking”, language and consciousness.)

What is recorded during the procedure is known as an electroencephalogram (EEG) which shows traces of electrical signals (postsynaptic potentials) or brainwaves to the guy on the street, from a large number of neurons. The EEG is used to test brain function; in clinical use, however, it records the voltage differences among parts of the brain, it does not measure electrical currents.

EEGs are popularly used in experimentation as they are primarily non-invasive; no need for an operation on research subject. Moreover, he does not need to speak, move, or even show any emotion for the data to recorded; the apparatus can even detect electrical signals resulting from covert responses to stimuli, like reading whatever his eyes comes across in the examination room. The EEG can detect the minutest changes in the brain’s electrical activity (down to a millisecond-level). It is among the few procedures that registers real time results or in medical parlance, a very high temporal resolution. The other common technique is MEG or Magnetoencephalography. ( It is an imaging technique that measures the magnetic fields the brain’s electrical activity produces.)

There are various forms of EEG, all very effective in monitoring and diagnosing certain abnormalities like epilepsy and syncope (fainting), sleep disorders, and more crucial of all, coma and brain death.

The procedure is even used to evaluate cases like dementia when other methods are not practical to employ like personal communication. There have been cases where it was used as a legal support in the assessment of brain death. Research is ongoing to find out if EEG can also assist in monitoring the medical treatment of depression, but nothing conclusive, it is still in the clinical stage.

In line with research, EEG is used by neuroscientists and biological psychiatrists in lab experiments to record cerebral activity so as to study the function of the brain in controlled behavior of human volunteers and animals. EEG patterns recorded during sleep sessions are always reliable when it comes to theories to explain sleep.

Conventional EEG involves attachment of electrodes on the scalp to obtain the recording. Prior to this, the area is prepared, usually by light abrasion and the application of a conductive gel to prevent any problems like dislodged equipment.

There is a system to follow for the full reliability and maximization of electrode placement. It is called the 10-20 system and is determined by measuring and marking the scalp. EEG was extremely useful before computer imaging was invented; it was practically the only means to pinpoint the location of a tumor. Today, with the advent of the MRI and CT scans, the procedure is not as frequently used for localizing many lesions.

How does an EEG work?

Each pair of electrodes is connected to a differential amplifier (one electrode per input). Voltage amplification between electrodes is placed at (typically) 1,000 “100,000 times, or 60 “100 dB of voltage gain. Each resulting voltage signal goes thru a high-pass filter and a low-pass filter, respectively set at 0.5 Hz and 35-70 Hz. The high-pass filter takes care of slow electrogalvanic signals, while the low-pass filter screens out the faster electromyographic signals.

There are three ways to arrange the electrode-amplifier relationships:

Common reference derivation

This uses one electrode as a reference somewhere along the scalp midline. Each amplifier is connected to this same electrode by one terminal (each amplifier has two). All the other electrodes are positioned relative to the reference point. The reference electrode links both earlobe electrodes.

Average reference derivation

The averaged signal of all the amplifiers is used as the common reference for each amplifier.

Bipolar derivation

Equal number of electrodes are connected in series to corresponding amplifiers (one amplifier for 2 electrodes). In this arrangement, amplifier 1 measures the difference between electrodes A and B; amplifier 2, on the other hand, measures the difference between B and C. The same goes for succeeding amplifiers and electrodes.

Whatever signal captured is then saved, printed out on paper, or displayed on a computer screen. EEG amplitude is pegged at about 100 µV when done from the scalp, and about 1-2 mV when taken from the surface of the brain.

EEG is not bereft of limitations; its got several: the sensitivity of scalp electrodes is not enough to capture the individual electric unit of signaling in the brain or technically termed action potentials. Moreover, it cannot be determined a hundred percent if the electrical activity is releasing inhibitory, excitatory or modulatory neurotransmitters. The EEG only detects activity of large groups of neurons, which by proportion, produces a greater voltage than that emitted by an individual neuron. EEG cannot also be positioned as strategically as other functional brain imaging techniques like functional magnetic resonance imaging (fMRI). Comprehensive detection can be gained with the use of EEG topography, which involves a large number of electrodes to triangulate or cover all angles of electrical activity.

It may not be all-encompassing, but EEG has several advantages in detecting brain activity. It is real-time, as earlier mentioned. Compared to other methods in exploring brain activity the EEG has a difference down to only sub-millisecond, while other methods have differences ranging from seconds to even minutes. The EEG is the sole direct measure of brain activity at present as the brain is believed to work through its electric activity. The other methods only rely on blood flow or metabolism which could be isolated from brain electric activity. More sophisticated methods employ EEG or MEG with MRI or PET for higher temporal and spatial resolution.