True about electroretinogram?
ELECTRORETINOGRAPHY( ERG ): Measures the changes in the resting potential of the eye, induced by the stimulation of the eye with a light stimulus. Components : *'a' wave - Negative wave, representing the activity of Rods and Cones *'b' wave - Positive wave, representing the activity of inner retinal layers. *'c' wave - Positive wave, related to retinal metabolism, paicularly associated with pigmented epithelium. types of electrogram Burian-Allen Electrode- (commonly used electrode for flash ERG) variable lens sizes consisting of an annular ring of stainless steel surrounding the central polymethylmethacrylate (PMMA) contact-lens core with a lid speculum. Dawson-Trick-Litzkow Electrode- low-mass conductive Mylar thread consisting of individual fibers impregnated with metallic silver ERG-Jet Electrode- a disposable plastic lens with a gold-plated peripheral circumference Mylar Electrode- aluminized or gold-coated Mylar Skin Electrode- may be used as a replacement for corneal electrodes by placing an electrode on the skin over the infraorbital ridge near lower eyelid; due to decreased amplitudes and variable responses, the skin electrode is primarily used for screening purposes only Cotton-Wick Electrode- Burian-Allen electrode shell fitted with a cotton wick which is useful for minimizing light-induced aifact Hawlina-Konec Electrode- Teflon-insulated thin metal wire (silver, gold, platinum) with three central windows, 3 mm in length, molded to fit into the lower conjunctival sac Recording electrodes: over cornea, bulbar conjunctiva or skin of lower eyelid Protect corneal surface with non-irritating ionic conductive solution (aificial tears or contact lens solutions containing sodium chloride and no more viscous than 0.5% methyl cellulose) Topical anesthesia for contact lens electrodes- Reference electrodes: This electrodes connects to the negative input of the system Bipolar electrode- incorporated within the contact lens-speculum assembly- may produce lower amplitude than the monopolar electrode at a separate area Monopolar electrodes may be attached to skin near each temporal obrbital rim, avoid placing over muscle masses Common electrodes: Connected to common input of the system Earlobe/mastoid/forehead Reference : Parsons' Diseases of the eye: 21st Edition ; Page 110 - 111
a wave is a positive wave
a wave arises from pigment epithelium
b wave arises from rods and cones
c wave is a positive wave
Optics and refraction