The Electrooculogram EOG

EOGElectrophysiological test that records Potential difference between electrodes placed around the eyes.To assess eye movements and position. Corneal Retinal PotentialEye tiny battery carries potential field/ dipole Cornea of the eye is electrically positive relative to the back of the eye.

EOG This positive potential behaves as if it were a single dipole oriented from the retina to the cornea. Such corneoretinal potentials are well established and are in the range of 0.4 - 1.0 mV . Eye movements thus produce a moving (rotating) dipole source and, accordingly, signals that are a measure of the movement may be obtained . The chief application of the EOG is in the measurement of eye movement.

Measuremant of the clinical EOG The calibration of the signal may be achieved by having the patient look consecutively at two different fixation points located a known angle apart and recording the concomitant EOGs . By attaching skin electrodes on both sides of an eye the potential can be measured by having the subject move his or her eyes horizontally a set distance . Typical signal magnitudes range from 5-20 V/.

EOG

Measuremant of the clinical EOG

Measuremant of the clinical EOG

Measuremant of the clinical EOG A ground electrode is attached usually to either the forehead or earlobe. Either inside a Ganzfeld, or on a screen in front of the patient, small red fixation lights are place 30 degrees apart .

Measuremant of the clinical EOG

Saccadic Response Saccadic movements describe quick jumps of the eye from one fixation point to another. The speed may be 20 - 700/s.

Saccadic Response

The standard mehtod

The standard mehtod Typically the voltage becomes a little smaller in the dark reaching its lowest potential after about 8-12 minutes, the so-called "dark trough. When the lights are turned on the potential rises, the light rise, reaching its peak in about 10 minutes. When the size of the "light peak" is compared to the "dark trough" the relative size should be about 2:1 or greater . A light/dark ratio of less than about 1.7 is considered abnormal.

Practical notes, instruments and definitionsElectrodes: recording the EOG is relatively undemanding as regards the electrodes. These shoud be relatively non-polarisable such as standard medical EEG or ECG electrodes, of a size appropriate for attachment to the side of the nose.

Practical notes, instruments and definitionsAmplifiers: for the lowpass filter, 30 Hz is sufficient.Amplifier saturation: EOG potentials measured during saccadic eye movements can vary by about 5:1 in amplitude between subjects, which, with the light rise, may mean a total amplitude range of up to 15:1. thus, the operator must be able to see the recordings of the saccades to ensure saturation dose not occur, and to adjust the amplifier gain setting accordingly.

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