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Electrooculography (EOG/E.O.G.)
is a technique for measuring the resting potential of the retina.
The resulting signal is called the electrooculogram. The main
applications are in ophthalmological diagnosis and in recording eye
movements. Unlike the electroretinogram, the EOG does not represent
the response to individual visual stimuli.
Eye movement measurements: Usually, pairs of electrodes are placed
either above and below the eye or to the left and right of the eye.
If the eye is moved from the center position towards one electrode,
this electrode "sees" the positive side of the retina and the
opposite electrode "sees" the negative side of the retina.
Consequently, a potential difference occurs between the electrodes.
Assuming that the resting potential is constant, the recorded
potential is a measure for the eye position.
The eye acts as a dipole
in which the anterior pole is positive and the posterior pole is
negative. 1. Left gaze: the cornea approaches the electrode near the
outer canthus of the left eye, resulting in a positive-going change
in the potential difference recorded from it. 2. Right gaze: the
cornea approaches the electrode near the inner canthus of the left
eye, resulting in a positive-going change in the potential
difference recorded from it (A, an AC/DC amplifier).
The EOG is used to assess the function of the pigment epithelium.
During dark adaptation, the resting potential decreases slightly and
reaches a minimum ("dark trough") after several minutes. When the
light is switched on, a substantial increase of the resting
potential occurs ("light peak"), which drops off after a few minutes
when the retina adapts to the light. The ratio of the voltages (i.e.
light peak divided by dark trough) is known as the Arden ratio.
In practice, the
measurement is similar to the eye movement recordings (see above).
The patient is asked to switch the eye position repeatedly between
two points (usually to the left and right of the center). Since
these positions are constant, a change in the recorded potential
originates from a change in the resting potential.
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