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In order to measure the electrical responses of the eye and visual system an optical (flash) stimulator is required. The Ganzfeld stimulator is designed to provide a uniform light stimulus to the whole of the visual field for use in electroretinography (ERG) and visual evoked cortical response (VER or VEP). Although the most commonly used stimulus is a high-intensity, short-duration flash of white light, control of the flash brightness and colour is available, as is the level of steady background illumination.

The Ganzfeld stimulator comprises a hemisphere, typically 50 cm in diameter, into which the patient looks. The eyes are held at the approximate centre of the hemisphere by placing the chin on a rest. The inside of the hemisphere is made of white translucent material and the flash is provided by a strobe and set of colour filters mounted behind this. The strobe timing, intensity and selection of colour filters are often controlled electronically. A small central fixation light is provided in the rear wall, and additional red lights may be included at each side of this to allow electro-oculography (EOG) to be performed. In this test the two side fixation lights are illuminated alternately and the patient is asked to look at whichever light is on.

Such devices are found in the ophthalmology department or in the neurophysiology department.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details