An event-related potential (ERP) is the measured brain response that is the direct result of a specific sensory, cognitive, or motor event. More formally, it is any stereotyped electrophysiological response to a stimulus. The study of the brain in this way provides a noninvasive means of evaluating brain functioning.
An event-related potential (ERP) is any stereotyped electrophysiological response to an internal or external stimulus. In simple terms it is any measured brain response that is the direct result of a thought process or perception.
ERPs can be reliably measured using electroencephalography (EEG). This method utilizes surface electrodes to measure the electrical activity of the brain (specifically the cortex) through the skull and scalp. As the EEG reflects many thousands of simultaneously ongoing neuronal processes, the brain’s response to a specific stimulus or event of interest is rarely visible in the ongoing EEG. In actual recording situations, even the most robust ERPs emerge only after many dozens of individual presentations of the stimulus of interest are averaged together. This technique cancels out noise and spontaneous EEG and enhances the voltage response to the stimulus making it stand out clearly from the averaged out background.
While evoked potentials may reflect the processing of the physical stimulus, they may also be modulated or even mediated by the „higher“ processes involving memory, expectation, attention, or changes in mental state.
Evoked Potentials (EPs)
Event-related potentials (ERP) are also referred to as evoked potentials (EP) and are a measurement of the brain’s direct response to a specific sensory, cognitive, or motor event. EPRs have the ability to measure (to the millisecond) the speed in which the brain is able to process this information. This fast-paced processing is what allow us as humans to receive, filter, and process billions of pieces of information in order to make split-second decision every second of every day. Due to the sensitivity of ERP testing, we are able to detect changes in this processing speed that is related to cognitive decline. If this testing is performed early enough, these changes can be seen before they become physically noticeable. The ERP can detect slowing in physical reaction times and decision-making skills, as well as stress disorders, memory loss, and other neurological disorders.
Memory functions and cognitive processes within the brain can be measured using event-related potentials (ERPs). These waveforms represent time-locked neuronal responses generated in response to specific events or stimuli. The latency, or time delay, between the onset of the stimulus and a patient’s physical response reflects brain processing speed, while waveform amplitude reflects neuronal recruitment and subsequent activation of the recruited neurons to process the information.
Fundamental elements of memory include the degree of attention to a stimulus and the subsequent encoding of information for storage and retrieval. P300a and P300b are two ERP components useful in measuring these aspects of memory. The P300b component has been exceptionally well-studied in regard to memory loss disorders, such as Mild Cognitive Impairment (MCI) and Alzheimer’s disease (AD). When comparing individuals with AD to age-matched controls, individuals with AD had longer P300b latency measures and low amplitudes. P300b latency and amplitude have been shown to predict the progression of mild cognitive impairment. Additionally, P300b metrics demonstrate superior sensitivity over conventional assessments, such as the MMSE, in detecting early preclinical memory loss.
Visual evoked potential (VEP)
A visual evoked potential is an evoked potential caused by a visual stimulus, such as an alternating checkerboard pattern on a computer screen. Responses are recorded from electrodes that are placed on the back of your head and are observed as a reading on an electroencephalogram (EEG). These responses usually originate from the occipital cortex, the area of the brain involved in receiving and interpreting visual signals.
When is the VEP used?
A doctor may recommend that you go for a VEP test when you are experiencing changes in your vision that can be due to problems along the pathways of certain nerves. Some of these symptoms may include:
- Loss of vision (this can be painful or non-painful);
- Double vision;
- Blurred vision;
- Flashing lights;
- Alterations in colour vision; or
- Weakness of the eyes, arms or legs.
These changes are often too subtle or not easily detected on clinical examination in the doctor’s surgery. In general terms, the test is useful for detecting optic nerve problems. This nerve helps transfer signals to allow us to see, so testing the nerve allows the doctor to see how your visual system responds to light. The test is also useful because it can be used to check vision in children and adults who are unable to read eye charts.
What does the VEP detect?
The VEP measures the time that it takes for a visual stimulus to travel from the eye to the occipital cortex. It can give the doctor an idea of whether the nerve pathways are abnormal in any way. For example, in multiple sclerosis, the insulating layer around nerve cells in the brain and spinal cord (known as the myelin sheath) can be affected. This means that it takes a longer time for electrical signals to be conducted from the eyes, resulting in an abnormal VEP. A normal VEP can be fairly sensitive in excluding a lesion of the optic nerve, along its pathways in the anterior part of the brain.
What the results may show
The VEP is particularly useful in detecting past optic neuritis. This refers to inflammation of the optic nerve, associated with swelling and progressive destruction of the sheath covering the nerve, and sometimes the nerve cable. As the nerve sheath is damaged, the time it takes for electrical signals to be conducted to the eyes is prolonged, resulting in an abnormal VEP. This may be seen in multiple sclerosis – one of the most common causes of optic neuritis (as above). Abnormal VEP’s are seen in multiple sclerosis patients due to the presence of optic neuritis.
The following are less easily differentiated but may cause abnormal VEPs:
- Optic neuropathy – this can be due to damage of the optic nerve from a number of causes, including: a blockage of the nerve’s blood supply, nutritional deficiencies, or toxins. As the nerve is damaged, electrical signals do not conduct properly. Examples include diabetes in the advanced stages which can be associated with damage to the blood vessels and nerves supplying the eyes, or toxic amblyopia which is a condition of the eyes associated with decreased vision, due to a toxic reaction in part of the optic nerve.
- Tumours or lesions compressing the optic nerve – if the optic nerve is compressed, the pathway for conduction is affected and an abnormal VEP is seen.
- Glaucoma – patients who suffer from glaucoma have increased intraocular pressure (ie pressure inside the eye). This can result in damage to the optic nerve, leading to prolonged VEPs.
- Ocular hypertension (high pressure) – this refers to any situation in which the pressure in the eye is higher than normal. There are no signs of glaucoma, but patients may be at increased risk of developing glaucoma later in life.
Clinical usefulness of the VEP
- The VEP is a standardised and reproducible test of optic nerve function
- It is more sensitive compared to magnetic resonance imaging (MRI) in detecting lesions affecting the visual pathway in front of the optic chiasm (area in the optic pathway where the optic nerve crosses sides)
- It is usually less costly compared to other investigations such as MRI
- If results of the VEP are negative, this can be useful in excluding certain disorders.