Ensure that the electrode is clean before use! If there is dried wetting/hypromellose solution on the tip it will affect the response. Ultrasonic cleaners with Diagnosys-approved inserts are best for cleaning the electrodes. If no ultrasonic cleaner is available, gently place an alcohol-soaked kim wipe (or alcohol pad) on the tip of the electrode and twist. Do not rub in a circular motion!
* Despite relying on visual acuity, this is a dilated test! The stimulator imposes its own aperture on the eye, so make sure that the animal is maximally dilated, and that the electrode is positioned so that it’s pointing down the optical axis.
* When the stimulator is placed correctly, you should be able to see a bit of the cornea above the stimulator. If you are unable to visualize the cornea, you may be putting too much pressure on the eye, and may need to pull the stimulator back slightly. (This has been the most helpful tip for most of the users we’ve passed it along to)
* Make sure that the eye is clear and without cataracts before starting the test! This test is dependent on visual acuity and vision-compromising cataracts will adversely affect the response. Xylazine can induce cataracts, probably due to the loss of the blink reflex and subsequent exposure of the cornea. Make sure that the eye stays hydrated before and during the test!
* Make sure that there are no whiskers stuck to the eye. This can interfere with the optics and affect the response.
* If you are not getting a response – look in the surveillance port directly above the stimulator aperture to determine that the stimulus is running.
* Be careful with the amount of hypromellose/wetting solution used. Too much can make it difficult to visualize the eye in the dark.
* While this is not technically a dark adapted protocol, we find we get the best results in the dark. Dark adapted animals tested in the dark is best, but the dark adaptation need not be total.
Most problems with the PERG wind up having to do with either the condition of the electrode (it has to be clean) or the placement of the electrode. Let us know if these tips help at all!
The image below is a close up of the Celeris PERG. The aperture is the clear black dot in the middle of the non transparent layer. This is where the light from the OLED screen is coming through.