Recording of electrical signals from eye is electroretinography. Since retina is extension of the cerebral cortex, it has special relationship with the brain. Due to the exposed position of eye, it is easy to record the potentials. The voltage value of cornea is 20mV positive when compared to eye fundus. Fundus is located behind the interior of eyeball. When retina is illuminated with external light, the electric potential changes slowly. The changes are recorded as Electroretinogram. Two electrodes are used to record the signals. Ag/AgCl electrode is placed on the contact lens and a distant electrode placed on the cheek. This technique is mostly used for research rather than clinical purpose. From the diagram below, it is seen that two electrodes are placed above and below the nose. They measure the vertical movement of eye. The obtained signals are given to the amplifier. Another pair of electrodes is placed on either side of the eye.
They record the horizontal movement of eye and send the signals to another amplifier. A bipolar electrode recording setup is used. Specially made contact lenses are used, such that it should not fall out of eye when illuminated with light.
In our eyes, the outer layer of retina has photopigments, which acts as photoreceptors. When light falls on the retina, photo pigments absorbs the photons. This results in breakdown of the photopigments and release ions, which changes the membrane potential. Due to this, an action potential is generated that travels down through optic nerve. The electrodes pick up the generated action potential and give to the amplifier. Then it goes to the recorder.
The below graph shows an electroretinogram signal. The constant DC horizontal line in the recorder appears, when there is no incidence of flash of light. When there is a light for 2 seconds, an electroretinogram appears. The point A responds to flash of light produced due to ERP (Early Receptor Potential). At this point light flash induces change in photopigments molecules. Point B is generated due to LRP (Later Receptor Potential). This occurs with a delay of 1 to 5 milliseconds. The point C arises due to offset of light stimulus. The point D is due to the off response of ERP and LRP.
Hence, amplitude is proportional to light intensity and duration of light applied on the eye.
Applications of Electroretinography
- To study the effect of drugs on eye movement systems.
- To analyze the semicircular canals.
- To diagnose the neurological disorders.
- Level of anesthesia during surgery can be indicated with study of eye movement.