PATTERN GENERATOR

Typical Number in Hospital: 2

Cost Bands: 4

References:

In the diagnosis of disorders of the visual system and in the investigation of some neural diseases which may also affect vision, it is common to measure the visual evoked potential (VEP), sometimes called the visual evoked response (VER). In this test a flash or pattern change stimulus is provided and the response is detected from electrodes over the visual cortex. The result is an electrical potential with characteristic waves demonstrating the passage of the signal through various stages of neural transmission and processing. The response is affected by background illumination, the intensity change occurring during the stimulus, and the change in the geometric pattern of the stimulating image. The effect of the intensity change can be eliminated by the use of a pattern reversal stimulator which changes from one image to the next without change in intensity.

The commonest form of this pattern stimulus is a checkerboard of black and white squares which alternates the black and white areas so that there is no change in average light level.

There are two main types of pattern generator. The first employs one or two slide projectors illuminating a screen with the alternate patterns by back projection. The second type uses a television type of cathode ray tube with a computer generator providing the patterns. A timing pulse is provided from the stimulator to the recording apparatus to start the measurement system at the time of reversal.

Such apparatus is used in the ophthalmology department or in the neurophysiology department.