Introduction to EEG

Electroencephalography (EEG) is a technique used to measure electrical activity in the brain. This electrical activity is measured by electrodes on the brain and presented to the computer through an EEG amplifier. For about a century, EEG has been used in research to understand brain functioning or investigate diseases. More recently, these recordings have even been used to control computers, neuromarketing, and social interaction.

What is the origin of EEG signals?

The brain consists of millions of neurons. When recording EEG, not all brain activity from all structures can be measured or distinguished. EEG signals represent the summed electrical activity of populations of cortical neurons, meaning that activity from other structures in the brain may not be distinguishable in the EEG [1]. The electrical signals, from action potentials, are the method of communication between neurons. When these action potentials travel along a neuron, a current dipole is generated (see Figure 1). When many cortical neurons ‘fire’ action potentials synchronously (and are oriented in the same direction), electrodes on the scalp can measure this activity.

The amplitudes of the brain waves measured from the scalp are usually in the range of 1 µV and 100 µV. [2] Often, brain waves are irregular and no pattern can be recognized. However, sometimes distinct patterns can be recognized in the EEG, such as the four types of brain waves in Figure 2 below.

Figure 2: Types of Brain Waves (Picture source for alpha, beta, theta, and delta)

EEG Electrodes

EEG is measured non-invasively with electrodes on the scalp. There are different types of EEG electrodes on the market, such as gel, wet and dry electrodes. Each type has advantages and disadvantages regarding data quality and ease of use. The highest signal quality can be obtained using gel electrodes, which are the most commonly used. These electrodes are often integrated into a headcap that comes in different sizes. The placement of these electrodes is standardized and follows the 10-20 system (Figure 3) for an even distribution of electrodes over the head [3].

EEG Amplifier

The function of an EEG amplifier is to take the weak electrical signal at the scalp and to magnify its amplitude so that it can be further processed, recorded, or displayed on a computer. The EEG is always represented as a potential difference between recording sites, which is amplified. As a result, signals common to all electrodes are suppressed. For EEG there are different ways of referencing and amplifying a signal; an electrode can either be compared to one electrode (a common reference) or it can be compared to an averaged set of (all) electrodes.

Figure 1: Origins of the EEG signal in the brain. Adapted from Bear et al1 Origins of the EEG signal in the brain. Adapted from Bear et al [1]

Types of Brain Waves

1. Alpha Waves (8-12 Hz): Associated with relaxation, eyes closed, and resting states; prominent in the occipital region, and involved in blocking visual input and memory processing.

2. Beta Waves (14-30 Hz): Linked to intellectual activity, focus, and alertness; found in the parietal and frontal regions; involved in motor control and attention.

3. Theta Waves (4-7 Hz): Seen during daydreaming or light sleep; prominent in the parietal and temporal regions; associated with memory storage and response inhibition.

4. Delta Waves (0.5-3 Hz): Present during deep sleep; found in the cortex; linked to long-term memory storage and restoration during sleep.

More information regarding the types of brain waves can be read in our blog here.

Equipment for Measuring EEG

Figure 3: The electrode layout of the 10-20 system (left) and corresponding brain regions (right) (pictures adapted from photo and photo).

Key Factors to Consider When Recording EEG

When done right, EEG can provide valuable insights into brain functioning. However, it is essential to address various artifacts to ensure accurate measurement of EEG signals.

External EEG Artifacts

As EEG is a biological potential with very low amplitude, it is highly sensitive to contamination from unwanted sources of interference [4]. The most common external artifact (not originating from the body) that affects all electrodes is mains interference. This artifact is caused by environmental sources that surround the body and can be seen as 50 or 60 Hz noise (Figure 4).

Another common problem is cable movement artifacts. The cables between the electrode and the amplifier can be capacitively coupled to mains. A current can flow between the cables from one electrode to the other if this capacitive coupling is not the same for all cables. Using actively shielded cables, the capacitive coupling to mains is prevented and so 50/60 Hz noise is not introduced into the signal. Read more about external artifacts here.

Figure 5: Blinking artifact on a few EEG traces. The blinking artifact with an amplitude of roughly 100-200 microvolts is mostly visible in the frontal electrodes.

Figure 4: Mains interference seen toward the right of the EEG.

Physiological EEG Artifacts

Artifacts originating from electrical activity produced by body parts other than the brain can sometimes be recorded at the scalp. For example, blinking causes a major artifact on the EEG as seen in Figure 5. Moreover, electrical activity caused by facial muscles like the jaw muscles are commonly present in the EEG.

You can read more about how to recognize these physiological EEG artifacts here.