WEEK ONE also

Bismillahirrahmanirrahim.

In this post, I will explain more about the EOG and how electrooculography works. Just for reminding, electrooculography (EOG) is a type of biomedical technique for measuring the resting potential of the retina while the electrooculogram is the resulting signals that comes from the potential difference caused by the eye movements.

The electrooculogram (EOG) is the electrical signal produced by the potential difference between the retina and the cornea of the eye. This difference is due to the large presence of electrically active nerves in the retina compared to the front of the eye. Many experiments show that the corneal part is a positive pole and the retina part is a negative pole in the eyeball. The movements of the eye create dipole which can be measured using bipolar electrodes to provide information about eye movements, e.g., angular amplitude and velocity.

The diagram above represents the schematic of the eye and the polarity of the potential difference (top). Angular deviation of eye position changes the vector of the dipole, resulting in amplitude changes in the EOG recording.

For EOG, there are two basic channel configurations. The first one is two channel configuration and the second is four channel basic configuration. 

Two Channel Configuration

In this configuration, the electrode is placed at the area between the eye and the hairline of both eyes. The electrodes must be centered to match the pupil of the subjects’ both eyes. Reference electrodes are placed at the in the middle of the subject's forehead. Because eye movements are largely binocularly synchronous in both sleeping and awake subjects, this electrode configuration will provide out-of-phase deflections on each electrode for most types of eye movements.

two channel configuration EOG electrode

Four Channel Configuration

For this configuration, the electrodes are placed at same position as two channel configuration but there are another two electrodes placed one above the eyelid and another below the eye. The advantage of this technique is that simultaneously recording vertical and horizontal eye movements allows for eye position and vector to be calculated.

four channel configuration EOG electrode

Positive or negative pulses will be generated when the eyes rolling upward or downward. The amplitude of pulse will be increased with the increment of rolling angle, and the width of the positive (negative) pulse is proportional to the duration of the eyeball rolling process.

When the eyes are stationary or when the eyes are looking straight ahead, there is no considerable change in potential and the amplitude of signal obtained is approximately zero.

When the eyes are made to move upwards, then there results an action potential, which when measured will give a value of -0.06v to +0.06v. Similarly a downward movement of the eyes will give a similar voltage but with opposite polarities to that obtained due to the left movement. The potential difference is differ depend on the subject. 

the graph represent the potential difference when the eyes
rolling upward, downward, left or right.

That's the basic of EOG. In the next post, I will show the lab test on the EOG. But using the EOG equipped in UniKL BMI's Medical Electronics Laboratory :)

Barakallah.

*updated*
For slide presentation given to students as guideline click here.