Visual Electrophysiology Unit

An electroretinogram (ERG) measures the electrical response from the retina to different types of stimulation. To perform the test, we need to use eye-electrodes to measure the electrical signals from the retina. There are different types of ERGs.  

Full-field ERG (ff-ERG)

• The full-field ERG is the most common test of overall retinal function. An electrode placed on the eye measures responses from the retina to flashes of light, under dark- and light-adapted conditions to assess the function of rods and cones, respectively. 
• Because the measurements are taken from the eye itself, this test can be performed awake (typically six years and older) or under sedation/general anesthesia. 
• The test takes about 40 minutes. For accuracy in measurement, the pupils are dilated, and patients are encouraged not to blink or squint during the flashes. Contact lenses must be removed due to the use of eye-electrodes.

Multi-focal ERG (mfERG)

• The mfERG helps to assess cone function in the central retina under normal lighting conditions, but requires pupil dilation. An electrode placed on the eye measures the response from your cones to a large honeycomb pattern displayed on a screen. The pattern honeycombs quickly change between black/white during the test. This allows us to calculate how the cones from the central retina are functioning.
• The test takes about 15 minutes. For accuracy in measurement, the pupils are dilated, and patients are encouraged not to blink or squint during the recording. 

Pattern ERG (pERG)

• The PERG measures the overall electrical response from the central retina, with your best vision. An electrode placed on the eye measures the response from the central retinal cones and ganglion cells, while the eyes are stimulated by a checkerboard pattern from a screen. 
• The test takes about 20 minutes. For accuracy in measurement, glasses must be worn if used, and there should be minimal blinking and good focus. Contact lenses cannot be worn for this test.   

Hand-held ERG

• The hand-held ERG is a quick screening tool to measure overall retinal function and is useful in very young children or patients with co-morbidities who cannot undergo sedation or anesthesia. For this test, an electrode placed on the lower eye-lid measures responses from the retina to flashes of light under dark- and light-adapted conditions. The hand-held ERG can provide general information on rod and cone system function.  Please note that the hand-held ERG is not as in-depth or sensitive as the full-field ERG, and each eye is tested separately.
• The test takes about one hour. For any measurement to take place, the eyes must be open enough for the device to track the pupil.  

The EOG assesses the function of a specific layer of the retina, the retinal pigment epithelium (RPE). The EOG measures the RPE function using electrodes placed at the inner and outer corner of each eye. For this test, patients are required to make quick eye-movements (left to right) between two lights, over time in the dark (15 minutes) and then in the light (15 minutes). The measurements are taken during the eye-movements. 

The duration of the test itself is approximately 40 minutes. For accuracy in measurement, the pupils are dilated, the patient’s head must be very still and straight, and the eye movements must be quick between the two lights. Patents are encouraged not to squint or make other eye movements during the recordings.

A VEP measures how a specific stimulus (patterns or flashes) is processed by the visual cortex of the brain, with your best vision. The VEP response relies all parts of the visual pathway, from the front of the eye to the brain but is primarily used to measures optic nerve/pathway function. The signal is measured at the visual cortex using electrodes placed on the back of the scalp.  

There are multiple types of VEP protocols offered, which are tailored to assess specific parts of the visual pathway based on patient ability and cooperation.   

• Flash stimulus – uses a bright flash to measure overall visual potential (the ability to see light)  
• Pattern reversal stimulus – uses checkerboard pattern of various sizes to measure optic nerve or pathway function  
• Pattern appearance stimulus – uses checkerboard pattern of various sizes and/or contrast to objectively measure visual function and ability   
• Multi-channel testing– uses checkerboard and flash stimuli with multiple electrodes across the back of the scalp to assess for albinism.  

The duration of the test itself ranges from 10 to 20 minutes. For accuracy in measurement, the patient must be awake, looking towards the screen with good focus, and have the best possible visual acuity. Glasses or contact lenses must be worn if used for vision. Parental support and encouragement are required for young children.

The FST estimates the dimmest possible blue, red, and white flash of light that can be seen by patients with underlying retinal conditions. The FST relies on patient feedback to make this calculation. For this, patients are dilated and dark-adapted for 40 minutes before testing. During the test, patients must report if they see or do not see flashes of light using buttons until a reliable minim level of detection can be measured for each colour. No electrodes are used for this test. 

The duration of the test itself is approximately 1.5 hours. For accuracy in measurement, the patient is tested in complete darkness, must be alert at the time of testing, and respond to every flash. Each eye is tested separately and each colour is tested at least two times.

Color vision is the ability to see different wavelengths of light and helps us assess function of central retina and optic nerve/pathway. Different colour vision testing measures the type and extent of colour blindness in a patient.  

There are 3 common types of colour vision tests used in the VEU.

Hardy-Rand and Ritter (HRR) 

• A standard colour vision test that measures the degree of red-green and blue-yellow colour-vision. 
• Patients are required to name the shapes that stands out on a page of dots and the colour of these shapes. 

D-15 (Farnsworth and Lanthony) 

• D15 tests require patients to arrange 15 coloured discs into a box starting from a specific colour. The discs have to be arranged so the colours blend from one disc to the next as smoothly as possible, like a rainbow.  
• How the discs are arranged measures the degree of red-green, and blue-yellow colour vision.  

Mollon-Reffin “Miminal” Vision Test  

• Uses patient’s judgement of colours to measure the degree of red-green and blue-yellow colour-vision.  
• Patients are required to pick out a stand-out colour disc from a group of discs. The lightest coloured disc that can be picked-out reveals your level of red-green and blue-yellow colour-vision. This is a fun and easy test to do in younger children.

This is a one-year program offered through the University of Toronto and Visual Electrophysiology program. Prospective trainees are required to understand the principles of visual electrophysiology and develop skills in clinical interpretation. Weekly training sessions are included year-round to establish a thorough understanding of electrophysiology.

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Two-week observerships are available for practicing ophthalmologists with an interest in electrophysiology and electrophysiologists. These observerships are tailored to individual needs. We will work with you to enhance your knowledge and build experience in visual electrophysiology testing and interpretation.  

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If you’re interested in events related to visual electrophysiology, visit the International Society for Clinical Electrophysiology of Vision site for dates in 2021. 

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