Many neurological symptoms cannot be fully understood by looking at anatomy alone. An MRI, eye examination or hearing test may provide important information about physical structures, yet these investigations do not always show how efficiently sensory information is travelling through the nervous system.
At Sydney Neuro & Integrative Health, we use visual evoked potentials and auditory evoked potentials as part of selected neurological assessments. These non-invasive neurodiagnostic tests allow us to measure the brain’s electrical response to carefully controlled visual or auditory stimulation.
Rather than asking only whether a person can see an image or hear a sound, evoked potential testing examines a more specific question:
How accurately and efficiently does the nervous system transmit that information from the sensory organ toward the brain?
This can provide valuable objective information when assessing patients with complex visual, auditory, vestibular or neurological symptoms.
What are evoked potentials?
Whenever the nervous system receives a sensory stimulus, it produces a small and highly organised electrical response. An evoked potential test records this response using electrodes placed on the scalp.
The electrodes do not deliver electricity into the brain. They simply detect and record the nervous system’s naturally occurring electrical activity while a patient looks at a visual pattern or listens to a series of sounds.
Specialised software then averages many responses together. This helps separate the relevant neurological signal from background activity such as muscle tension, eye movements and unrelated brain activity.
The clinician primarily evaluates:
- Latency: how long the response takes to appear after the stimulus
- Amplitude: the strength or size of the recorded response
- Waveform morphology: the shape and organisation of the response
- Side-to-side differences: whether the two eyes, ears or pathways respond differently
- Reproducibility: whether the response remains consistent when the test is repeated
These findings are interpreted alongside the patient’s history, neurological examination and any other relevant investigations.
A visual evoked potential, or VEP, assesses the functional integrity of the visual pathway extending from the eye and optic nerve toward the visual areas of the brain.
What is a visual evoked potential test?
During testing, electrodes are positioned over the scalp, particularly near the occipital region at the back of the head where the primary visual cortex is located. The patient usually watches a screen displaying a reversing black-and-white checkerboard pattern. Each eye may be tested separately so that the responses can be compared.
The checkerboard does not simply test whether the patient can recognise the image. Its precisely timed reversals create a repeatable visual stimulus, allowing the equipment to measure how long the signal takes to reach the visual cortex.
Pattern-reversal VEPs are widely used because their timing and waveform are generally more consistent than responses produced by less structured visual stimuli. Current international standards describe VEPs as an established method of assessing visual pathway function from the retina toward the primary visual cortex.
What can a VEP tell us?
One of the most clinically important measurements is the timing of the principal positive response, commonly known as the P100. This response usually occurs at approximately 100 milliseconds after the checkerboard reverses, although interpretation must always use appropriate reference data and account for individual and technical factors.
A delayed response may indicate that visual information is travelling more slowly than expected somewhere along the pathway. Differences between the two eyes can also be clinically meaningful, particularly when one pathway appears to conduct more slowly or produce a less organised response than the other.
Depending on the clinical presentation, VEP testing may contribute useful information when investigating:
- Unexplained changes in vision
- Possible optic nerve dysfunction
- Visual symptoms associated with neurological conditions
- Differences in visual pathway function between the eyes
- Symptoms following concussion or other neurological injury
- Blurred or altered vision that is not fully explained by a routine eye examination
- Visual disturbance occurring alongside dizziness, headache or balance dysfunction
- The functional effects of known or suspected demyelinating disease
VEP testing has particular value when there is concern about the optic nerve or the visual pathway behind the retina. It can sometimes demonstrate a functional conduction delay even when structural abnormalities are subtle or when visual acuity alone does not reflect the entire problem.
However, a VEP result is not specific to one diagnosis. Abnormalities may also be influenced by reduced visual acuity, incorrect optical correction, poor fixation, cataracts, retinal or macular dysfunction, nystagmus, fatigue and difficulty maintaining attention. This is why the result must be interpreted in the context of ophthalmological findings and the broader neurological assessment.
What are auditory evoked potentials?
Auditory evoked potentials measure the electrical responses generated as sound travels through the auditory nerve and brainstem pathways.
The form most relevant to neurological assessment is commonly called a brainstem auditory evoked potential, or BAEP. It may also be described as an auditory brainstem response.
During the test, the patient wears earphones and listens to a series of controlled clicking sounds. Each ear is generally stimulated separately. Electrodes positioned on the scalp and around the ears record the electrical responses produced during the first few milliseconds after each sound. The test records the nervous system’s response objectively.
Clinical standards describe these short-latency responses as a method of assessing and approximately localising dysfunction affecting the auditory nerve and auditory pathways within the brainstem.
What does an auditory evoked potential measure?
A typical brainstem auditory response contains a series of small waves, traditionally labelled with Roman numerals. Particular attention is usually given to waves I, III and V and to the time intervals between them.
These waves broadly reflect the sequential transmission of sound-related activity through the auditory nerve and different levels of the brainstem. Because several structures can contribute to each response, the waves should not be treated as a simple one-to-one map of individual anatomical structures.
The clinician examines whether the waves are present, whether their timing is within the expected range and whether the intervals between different components are prolonged. Responses from the left and right ears are also compared.
This may assist in evaluating:
- Possible auditory nerve or brainstem pathway dysfunction
- Asymmetrical neurological findings involving the ears
- Auditory symptoms occurring with dizziness or imbalance
- Tinnitus accompanied by other neurological signs
- Unexplained changes in sound perception
- Suspected disruption of sensory conduction through the brainstem
- Neurological symptoms in which an objective assessment of auditory pathway function may add useful information
Auditory evoked potentials provide information about neural transmission, but they do not replace a comprehensive hearing assessment. Conductive hearing loss, cochlear dysfunction and other peripheral hearing conditions can alter the recorded response. Formal audiology may therefore be necessary to determine whether an abnormality originates in the ear itself or farther along the auditory pathway. Clinical guidelines specifically emphasise the importance of interpreting these results in conjunction with audiometric findings.
How these tests support our clinical assessments
Patients attending our clinic often present with symptoms that involve more than one neurological system. Visual disturbance may occur alongside dizziness, headaches, balance impairment, neck dysfunction, cognitive fatigue or autonomic symptoms. Tinnitus or altered sound sensitivity may coexist with vestibular problems, post-concussion symptoms or signs suggesting altered brainstem function.
In these cases, evoked potentials can add another objective layer to the assessment.
For example, a standard neurological examination may identify asymmetry in eye movements, balance, reflexes, coordination or sensory function. Vestibular testing may demonstrate difficulty stabilising vision during head movement. Posturography may show abnormal reliance on visual information for balance. A qEEG may identify patterns of cortical activity that require further clinical correlation.
Visual and auditory evoked potentials examine a different aspect of nervous-system function: the timing and integrity of sensory signal transmission.
By combining these findings, we can develop a more integrated understanding of how the patient’s symptoms may relate to the visual system, auditory pathways, vestibular system, brainstem and broader neurological networks.
Visual and auditory symptoms can be difficult for patients to describe. Terms such as blurred vision, visual motion sensitivity, muffled hearing, sound intolerance, head pressure or “feeling disconnected” may mean different things to different people.
Objective information rather than symptom reporting alone
Evoked potential testing does not depend entirely on the patient finding the perfect words to describe their experience. It provides a measurable physiological response that can be compared between sides and against appropriate reference values.
This can be particularly valuable when symptoms fluctuate, when several systems appear to be involved or when conventional testing has not fully explained the patient’s experience.
Functional testing complements structural imaging
An MRI provides detailed structural images. Evoked potentials provide functional information.
The two forms of testing answer different questions. A scan may identify a lesion, inflammation, compression or another anatomical change, whereas an evoked potential assesses whether the relevant sensory pathway is transmitting information with expected timing and organisation.
A normal evoked potential does not exclude every neurological disorder, and an abnormal result does not establish a diagnosis by itself. Nevertheless, the findings may help determine whether further investigation or referral to a neurologist, neuro-ophthalmologist, ophthalmologist, audiologist or ear, nose and throat specialist is appropriate.
What should patients expect?
Both tests are non-invasive and do not involve radiation.
Small recording electrodes are applied to the scalp using conductive paste or gel. For visual testing, the patient is seated in front of a patterned screen and asked to focus on a central point. Each eye may be assessed independently. Patients who ordinarily wear glasses should generally bring them, because clear focus is important during pattern-based testing.
For auditory testing, sounds are delivered through earphones or inserts while the patient remains relaxed and still. The clicks are repetitive but carefully controlled. Movement, jaw clenching and excessive muscle tension can interfere with the very small electrical responses being measured, so patients are encouraged to remain comfortable and relaxed.
The test duration varies according to the clinical question, the number of pathways being assessed and whether recordings need to be repeated to confirm their reliability.
Patients should attend with clean, dry hair and avoid applying oils, gels, sprays or other styling products to the scalp. They should also inform the clinician about significant visual impairment, hearing loss, hearing aids, ear problems or difficulty tolerating flashing patterns or repetitive sounds.
The benefit for our patients
The principal benefit of evoked potential testing is that it provides objective information about sensory pathway function that cannot be obtained from symptom questionnaires or a routine physical examination alone.
When clinically indicated, the results may help us:
- Clarify whether visual or auditory pathways warrant further investigation
- Identify meaningful differences between the two sides
- Correlate sensory pathway findings with balance, vestibular and neurological results
- Establish an objective baseline
- Inform referrals and communication with other healthcare practitioners
- Avoid relying on a single test or one isolated explanation for complex symptoms
- Develop a more individualised management and rehabilitation plan
Where repeat testing is appropriate, evoked potentials may also provide an objective method of comparing pathway function over time, provided the testing conditions and methodology remain consistent.
A considered part of a broader neurological evaluation
Visual and auditory evoked potentials are not screening tests for every patient, nor are they standalone diagnostic tools. We recommend them when the patient’s history and examination indicate that objective assessment of the visual or auditory pathways may contribute meaningful information.
At Sydney Neuro & Integrative Health, our aim is to understand how different parts of the nervous system are working together rather than viewing each symptom in isolation. Evoked potential testing can help connect the patient’s reported experience with measurable neurological function and guide decisions regarding further investigation, referral and care.
Would evoked potential testing be relevant to your symptoms?
Patients experiencing unexplained visual disturbance, auditory symptoms, dizziness, balance problems, tinnitus or other neurological concerns can contact Sydney Neuro & Integrative Health to discuss whether a comprehensive neurological assessment, including visual or auditory evoked potentials where clinically appropriate, may be suitable.
Could VEP or AEP testing be relevant to your symptoms?
If you are experiencing unexplained visual disturbance, altered sound perception, tinnitus, dizziness, imbalance or other complex neurological symptoms, you are invited to book a complimentary 10-minute consultation with Dr Daniel Yazbek.
This brief conversation provides an opportunity to discuss your concerns, understand our diagnostic approach and determine whether a comprehensive clinical evaluation and visual or auditory evoked potential testing where clinically appropriate, may be relevant to your situation.