Interocular Differences

Causes of Interocular Differences in ERG Recordings

Interocular differences are differences in amplitude or timing between the right and left eyes. Differences such as these may yield exciting findings in experiments where treatments are administered to just one eye. However, in the case of investigating interocular differences where they are not expected, here are a few common causes. 

1 - Unequal angle of electrodes

Electrodes should be equally angled on both eyes. When applying electrodes, ensure that the angles at which the eyes meet the stimulator are equal. Oftentimes, the stimulators may be placed unequally if the researcher is applying both left and right side stimulators while standing on just one side of the mouse. This can be easily tested as the source of interocular differences by removing the electrodes and reapplying them with more care. If the differences remain the same after re-application, then the electrode position is not to blame.

2 – Uneven dark adaptation 

A quality dark-room will only have dim, red, long-wavelength light sources which do not light adapt the animals. In a scenario where this is not the case, animals may become unevenly light-adapted while waiting to be tested. To test this, turn off all room lights for approximately 5-minutes before testing the mice again. If the light-adaptation was a factor, the interocular differences should diminish. In any case, be sure to use appropriate red-lights.  

3 – Uneven dilation 

Ensure that the pupils are maximally dilated. ERG amplitudes are directly influenced by the amount of light that is entering into the eye. If one eye is letting in more light than the other, large differences in ERG amplitudes will arise. 

4 – Hot swap stimulator-electrodes

To rule out a technical problem with the stimulator-electrodes, swap the stimulator from one eye to the other. Unplug the stimulator from the right eye and plug it into the left, then vice versa. Be sure to unplug and re-plug ALL of the connections when moving the entire stimulator. Logically, if the stimulator were the problem, then the interocular difference would follow the stimulator. If there does seem to be an issue with the stimulator itself, then please contact Diagnosys Support. 

If you have checked all of these possibilities, consider that the cause of interocular difference may be a genuine biological response from the animal.