The McGill Physiology Virtual Lab

Biological Signals Acquisition

EOG tests > intro

Recording eye movements of a stationary subject: a reference electrode is placed on the forehead, electrodes are placed on the right and left temples for lateral eye movement detection (for vertical eye movement detection, one can also place electrodes above and below an eye). This technique is also an indirect way to evaluate the tracking and scanning of visual targets.

We move our eyes constantly during our daily activities to keep our line of sight pointed at a target of interest. In order to generate an eye movement along any axis, there are three antagonistic pairs of muscles which are attached to the globe of the eye.
These sets of muscles function to move the eye horizontally (left versus right), vertically (up versus down) and torsionally (clockwise versus counter clockwise).

There are four different types of conjugate eye movements. These eye movements fall into two specific categories:

  1. Eye movements that function to stabilize the position of the eye in space during head movements (Reflex eye movements).
  2. Eye movements that function to redirect the line of sight to follow a moving target or to attend to a new target of interest (Voluntary eye movements).

The Technique: Electro-oculography (EOG)
In the 1920's, it was discovered that by placing electrodes on the skin in the region of the eyes, one could record electrical activity which changed in synchrony with movements of the eye in the head. It was initially believed that these potentials reflected the action potentials in the muscles that are responsible for moving the eyes in the orbit.
However, it is now generally agreed that these electrical potentials are generated by the permanent potential difference which exists between the cornea and the ocular fundus (cornea-retinal potential, 10-30mV: the cornea being positive).
This potential difference sets up an electrical field in the tissues surrounding the eye. As the eye rotates, the field vector rotates correspondingly. Therefore, eye movements can be detected by placing electrodes on the skin in the area of the head around the eyes. Vertical movements of the eyes are best measured by placing the electrodes on the lids, while horizontal eye movements can be best measured by placing the electrodes on the external canthi (the bone on the side of the eye).
Limitations of the Technique

The underlying assumption of this method of recording eye movements is that the movement of the electric field in the conducting tissues surrounding the eye is related, in a simple (usually assumed to be linear) way to the movements of the eye itself. Due to the non-uniformity of these tissues and the shapes of the tissues surrounding them, this can only be an approximation to the biological reality. However, for horizontal eye movements within the range of 30 degrees, the potential measured is assumed to be linear to the actual movement of the eye in the orbit. The resolution of EOG is considered to be about 1 degree. Because it is a relatively simple technique, EOG is still commonly used clinically for testing eye movements in patients.

For a fixed eye position, the EOG is far from being constant in magnitude, but can be influenced by a number of external factors. These factors include
  1. the noise generated between the electrodes' contacts and the skin
  2. the metabolic state of the tissues (pO2, pCO2, and temperature)
  3. visual stimulation
  4. contraction of facial muscles

In addition, recorded EOG, particularly for vertical eye movements, is quite sensitive to movements of the eye lids. In summary there are a number of external factors which can complicate the interpretation of the EOG, and for that reason EOG is considered highly sensitive to artifacts. The considerable artifacts which can be introduced through the contact between the electrode contacts and the skin can be minimized by reducing the resistance between the electrodes and the skin.

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