What is Electrooculography?
Electrooculography (EOG) is a diagnostic test used to evaluate the movement of the eyes. The test involves the placement of electrodes around the eyes to measure the electrical activity generated by eye movements. The results of the EOG test are interpreted by a specialist, who evaluates the electrical activity generated by eye movements, looking for abnormalities such as excessive or insufficient eye movements or unusual patterns of movement.
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Table of Contents
Physiological Basis of Electrooculography (EOG)
Electrooculography (EOG) is an electrophysiological technique used to measure eye movements and assess the function of the retinal pigment epithelium (RPE), which is a layer of cells at the back of the eye. It works by detecting the corneo-retinal standing potential, a stable electrical potential difference that exists between the front (cornea) and back (retina) of the human eye. The eye essentially acts as a tiny electrical dipole, with the cornea being positive and the retina being negative.
The Eye as an Electrical Dipole
At the heart of EOG is the concept that the eye behaves like a natural electrical dipole. This is because the front of the eye (specifically the cornea) is electrically positive relative to the back of the eye (the retina, especially the retinal pigment epithelium or RPE), which is electrically negative.
This standing electrical potential, often called the corneo-retinal potential, is typically around 6–10 millivolts (mV) in humans under normal conditions.
This potential is mainly generated by:
✔ The active transport processes of the retinal pigment epithelium (RPE).
✔ Ionic gradients and metabolic activities that maintain the polarity between the cornea and the retina.
How Eye Movements Generate Measurable Signals
When a person moves their eyes horizontally to the left or right, the orientation of the corneo-retinal potential relative to external electrodes changes:
✔ Turning the eyes to the right brings the positively charged cornea closer to the right electrode.
✔ Turning the eyes to the left does the opposite.
This change in the position of the electrical dipole results in a change in the voltage recorded between the electrodes placed near the outer corners of the eyes (canthi).
These voltage changes are proportional to the amplitude of the eye movements and are captured as the EOG signal.
Light and Dark Adaptation Phases
One of the unique features of EOG is how the corneo-retinal potential changes between dark and light conditions, which helps assess the functional integrity of the RPE:
🌑 Dark Phase
✔ When the eyes are kept in darkness for about 15 minutes, the potential gradually declines, reaching its lowest point known as the dark trough.
✔ This decline is thought to result from reduced metabolic activity and ion transport in the RPE cells under dark conditions.
💡 Light Phase
✔ When the eyes are then exposed to bright, diffuse light, the potential starts to increase, reaching its highest point called the light peak.
✔ This rise occurs because light stimulates the RPE to enhance ion transport, especially the movement of chloride ions, which increases the transepithelial potential difference.
Arden Ratio: A Functional Indicator
The ratio of the light peak to the dark trough—known as the Arden ratio—is a key clinical parameter:
Arden Ratio = Dark Trough Amplitude／Light Peak Amplitude
✔ A normal Arden ratio is typically greater than 1.8–2.0.
✔ Lower values suggest dysfunction in the RPE, which can occur in retinal dystrophies such as Best disease.
Why is the RPE Central?
The RPE plays a critical role because it:
✔ Maintains ion gradients essential for retinal function.
✔ Contributes significantly to the standing potential recorded in EOG.
✔ Responds to light by modulating ion transport, which is what generates the measurable change between dark and light phases.
Disorders affecting the RPE impair these processes, leading to abnormal EOG findings, even if the photoreceptors themselves may still function reasonably well.
Purpose of Electrooculography
Electrooculography (EOG) is a diagnostic test that measures the resting electrical potential between the cornea and the retina. Although technically straightforward, EOG serves multiple important purposes in clinical practice and research. Its primary applications revolve around assessing the health of the retinal pigment epithelium (RPE), monitoring eye movements, and supporting the diagnosis and management of specific retinal and neurological disorders.
A. Assessing Retinal Pigment Epithelium (RPE) Function
One of the main purposes of EOG is to evaluate the functional integrity of the RPE, which plays a crucial role in maintaining photoreceptor health and the visual cycle. The test tracks the natural fluctuation in the corneo-retinal potential during periods of dark and light adaptation:
✔ In darkness, the potential gradually declines (dark trough).
✔ With light exposure, the potential increases, reaching a peak (light peak).
The Arden ratio (light peak / dark trough) is calculated to quantify this change. A normal ratio indicates healthy RPE activity, while a low ratio suggests RPE dysfunction. This is especially valuable in diagnosing and monitoring hereditary retinal dystrophies like Best vitelliform macular dystrophy and certain types of retinitis pigmentosa.
EOG plays a targeted role in the evaluation of several retinal conditions:
✔ Best vitelliform macular dystrophy: Characteristically shows a markedly reduced Arden ratio even when the electroretinogram (ERG) is relatively normal.
✔ Adult-onset foveomacular vitelliform dystrophy: Also shows subnormal EOG responses.
✔ Helps differentiate between diseases primarily affecting the photoreceptors (which show abnormal ERG) and those involving the RPE (which show abnormal EOG).
By isolating RPE dysfunction, EOG complements other retinal tests such as ERG and visual evoked potentials (VEP).
C. Evaluation of Ocular Motility Disorders
Beyond retinal assessment, EOG is also used to record eye movements by detecting changes in the corneo-retinal potential as the eye rotates:
✔ It provides an objective method to measure horizontal and vertical eye movements.
✔ Clinicians use this to evaluate conditions such as nystagmus, internuclear ophthalmoplegia, and other ocular motility disorders.
✔ In sleep studies (polysomnography), EOG electrodes help track rapid eye movement (REM) phases.
Although modern techniques like video-oculography and infrared eye trackers have become more common, EOG remains useful in settings where those methods are not available.
D. Research Applications
In research, EOG is used to:
✔ Study the physiology of eye movements and the dynamics of the corneo-retinal potential.
✔ Investigate RPE function in experimental models of retinal disease.
✔ Evaluate the impact of systemic diseases (e.g., diabetes) or medications on retinal health.
It is a simple, non-invasive tool that can offer insights into both ocular and neurological systems.
Complementary Diagnostic Role
While EOG alone rarely makes a definitive diagnosis, its value lies in combining results with other tests:
✔ ERG (measures electrical activity of the photoreceptors and inner retina)
✔ Fundus photography or optical coherence tomography (OCT) for structural details
✔ Visual acuity tests and perimetry for functional vision assessment
Together, these tools provide a comprehensive picture of retinal and RPE health.
Procedure of Electrooculography (EOG)
Electrooculography is a relatively simple, non-invasive test used to measure the electrical potential between the front (cornea) and back (retina) of the eye. The procedure requires patient cooperation and typically lasts between 30–45 minutes. Here’s a step-by-step breakdown:
1. Preparation and Setup
Patient preparation
🔹 The patient is comfortably seated in a dimly lit room.
🔹 The skin around the eyes is cleaned with an alcohol wipe to reduce resistance and ensure good electrode contact.
🔹 The patient is instructed to remain relaxed, minimize blinking during measurement intervals, and follow light cues carefully.
Electrode placement
🔹 Five electrodes are usually used:
♦ Two active electrodes are placed on the skin at the outer canthi (one on each side of the eye) to record horizontal eye movements.
♦ One electrode is often placed above the eye, and another below to monitor vertical eye movements if needed.
♦ A ground electrode is attached to the forehead (usually at the center).
🔹 The electrodes detect the voltage change that occurs as the eyes move horizontally or vertically, leveraging the natural corneo-retinal potential.
2. Dark Adaptation Phase (Dark Trough Measurement)
🔹 The patient is asked to sit in complete darkness for about 10–15 minutes.
🔹 During this phase, the baseline corneo-retinal potential typically drops to its lowest point, known as the “dark trough.”
🔹 While in darkness, the patient tracks a small, slow-moving target (usually a red or dim light) moving horizontally between two fixed positions (typically ±15° or ±30° from the midline).
🔹 Each time the patient moves their eyes from one side to the other, a voltage change is recorded.
The repeated measurements help establish an average value of the corneo-retinal potential during darkness.
3. Light Adaptation Phase (Light Peak Measurement)
🔹 After dark adaptation, the room lights are turned on or the patient is exposed to bright diffuse light.
🔹 This light adaptation phase lasts about 10–15 minutes.
🔹 The light causes the corneo-retinal potential to gradually increase, reaching its highest point known as the “light peak.”
🔹 During this time, the patient continues following the moving light target from side to side as before.
🔹 The voltage changes are recorded in the same way as during the dark phase.
4. Calculation and Interpretation
The main result derived from EOG is the Arden ratio:
Arden ratio = Dark trough／Light peak
✔ A normal Arden ratio is typically greater than 1.8–2.0.
✔ A ratio below this range suggests dysfunction of the retinal pigment epithelium (RPE).
These data are then reviewed alongside the patient's clinical context and other test results (e.g., ERG, fundus imaging).
5. Duration of the Test
The entire EOG procedure generally takes 30–45 minutes, with most of the time spent on dark and light adaptation.
6. Special Considerations
🔹 The test requires good patient cooperation: patients must accurately follow the target light without moving their head.
🔹 Movements of the head or excessive blinking can cause artifacts in the recording.
🔹 EOG is safe, non-invasive, and does not require pupil dilation, which makes it comfortable for most patients.
🔹 It can be performed on both children (if cooperative) and adults.
Advantages of Electrooculography (EOG)
✔ Non-invasive: It only requires electrodes placed on the skin around the eyes.
✔ Relatively simple setup: Compared to some other eye-tracking methods.
✔ Can detect eye movements even with closed eyes: Useful in sleep studies.
✔ Wide field of view: Can measure large eye movements.
Limitations of Electrooculography (EOG)
✖ Susceptible to artifacts: Muscle activity from facial movements or blinks can interfere with the signal.
✖ Signal drift: The baseline potential can shift over time, making precise eye position measurement challenging without calibration.
✖ Less precise for small eye movements: Video-based eye trackers often offer higher resolution for subtle eye movements and gaze tracking.
✖ Does not directly measure gaze point: Unlike some other eye-tracking technologies, EOG provides information about eye movement direction and amplitude, but not the exact point of gaze on a screen.
Electrooculography remains an important electrophysiological tool in the field of ophthalmology and visual sciences. By measuring the corneo-retinal standing potential and calculating the Arden ratio, EOG helps assess the function of the retinal pigment epithelium and diagnose retinal dystrophies. D
