Electroencephalogram (EEGs) and Event-Related Potentials (ERPs)

Do you know the difference between electroencephalogram (EEGs) and event-Related Potentials (ERPs)?

EEGs (electroencephalograms) are a method of measuring brain activity using electrodes (from 25 to 34, and more for deeper insights into brain activity) placed on the scalp with a conductive gel. These electrodes are able to detect the tiny electrical activity that occurs when action potentials ‘fire’ in a neurone, also known as nerve impulses. This neuronal activity is detectable.

EEG performed on a person's head, Flaticon

These electrical charges are then graphed over time to give an indication of the level of activity occurring in that particular area of the brain, usually in response to a stimulus or test. These waves indicate some form of functional activity in a particular brain area, likely in response to the stimulus, event, or test the person is experiencing.This activity is then plotted as a result on a graph that provides the data in the form of 4 different types of waves known as:

• Alpha.

• Beta.

• Theta.

• Delta.

These waves differ in two points: amplitude and frequency.

Ten seconds of simulated EEG data, Wikimedia Commons

Sometimes the waves differ depending on whether a person is asleep or awake. The example above shows some recognisable patterns of waves. They are consistent and can be easily categorised.

However, these types of waves can be synchronised or desynchronized.

1. Synchronised waveforms usually occur when the person is asleep or focused on a task and can be categorised as alpha, beta, delta, and/or theta waves.

2. Desynchronized waveforms are more the norm when people are awake, as their brain rapidly switches attention and function. This usually results in a mixture of the above wave types, which is why amplitude and frequency vary so much.

EEGs are also good for detecting diseases such as epilepsy in a person because epilepsy interrupts the signals normally sent through neurons in the brain. This results in abnormal behaviours or responses such as muscle spasms and seizures.Identifying recognisable waveforms during sleep allows clinicians to determine if the person is suffering from a sleep disorder. EEGs are also used to detect brain activity in other diseases such as Alzheimer's disease.

What are ERPs (event-related potentials)?

Event-related potentials (ERPs) are very similar to EEG because they use electrodes placed on the scalp to measure electrical activity in the brain.ERPs differ, however, in the way they measure responses to a stimulus, namely by exposing the participant to the stimuli many times.

The graph above shows the neuron's activation potential and time after a stimulus.Latency is the time elapsing between showing the stimulus to the participant and their response to it. ERPs and EEGs usually have a short latency in the first 100ms, referred to as sensory ERPs because the senses respond reflexively to the stimulus.After 100ms comes the actual response to the stimulus, where the information has been processed cognitively.

Strengths and weaknesses: Electroencephalogram (EEGs) vs Event-Related Potentials (ERPs)

Here we discuss the strengths and weaknesses of event-related potentials and electroencephalograms, as both are similar techniques with similar advantages and disadvantages.However, one of the major things we can suggest by using EEGs and ERPs is that these areas can be associated with specific responses and behaviours when measured.

Strengths

• Low cost: Compared to other brain scanning techniques, EEGs and ERPs are a low-cost alternative. They do not require extensive equipment and are more accessible than machines such as fMRIs (functional magnetic resonance imaging).
• Useful for clinical diagnosis: They are helpful in diagnosing diseases such as epilepsy, a condition that results from disturbed electrical activity in the brain.
• High temporal resolution: both techniques have a high temporal resolution because they record activity within milliseconds of its occurrence, unlike fMRI, which has a delay. It is highly accurate in timing the electrical activity in the brain.
• ERPs are robust: Because ERPs use an averaging method, their results are more reliable and can be attributed to activity because random activations are filtered out that have nothing to do with the brain activity occurring as a result of the stimulus.
• Non-invasive: Electrodes are placed on the scalp to detect activity. It is a non-invasive method of detecting brain activity.
• Mobility: The subject can move during the EEG or ERP examination, unlike fMRI, which requires the subject to standstill. This procedure allows for a wider range of tests to be performed on participants than is the case with other forms of brain scans.
• Functional localisation: This method provides valuable evidence and information to support the idea of localising functions within the brain and will help map these areas for future efforts.
• Tracking cognitive development: Both of Bells studies show how helpful EEGs were in testing children in different situations; in memory tasks at 8 months of age, and how robust these tests were.¹

Weaknesses

• Low spatial resolution: Because the electrodes are on the scalp, they can only detect electrical activity deep in the brain. For example, it cannot tell what the amygdala is doing, only what the surface activity is doing. The spatial resolution is lower compared to other techniques, such as fMRI.
• Not accurate: EEG electrodes can provide a good estimate of where the electrical activity is occurring, but it is not accurate. It tells you where the electrical activity is the strongest, but not exactly where it comes from.
• Uncomfortable: The many electrodes combined with the gel make the device uncomfortable for participants. Also, the gel stays in the hair until it is washed off, so it can be very inconvenient for participants.
• EEGs lack the robustness of ERPs: EEGs capture activity that occurs throughout the brain, and do not use the averaging technique, making ERPs robust in comparison.
  • As a result, more EEG samples are needed to provide accurate results, which is both time-consuming and inconvenient, and sometimes increases costs.

¹Martha Ann Bell and Kimberly Cuevas, Using EEG to Study Cognitive Development: Issues and Practices, Journal of cognition and development: official journal of the Cognitive Development Society, 2012.

¹Martha Ann Bell, A psychobiological perspective on working memory performance at 8 months of age, Child Development, 2012.