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Answer transcribed from Brightway's Interview with Dr. Bryce Appelbaum:
Nystagmus is basically the oscillation of the eyes. It's usually horizontal but it can be vertical, pendular, and look different depending on the reason for it. It's essentially the brain sending the signal to the eyes to look at a particular place but just getting these impulses where the eyes are fatigued and constantly searching for a better quality image to fixate on. If there's trouble with holding fixation or discrimination ability, you can't keep your eye on the target and the brain's still telling the eyes to focus there, then a lot of times they're shaking. When that happens, you're not able to use the sweet spot of the vision or the line of sight, which is what allows you to see 20-20 or in clarity. For many people, working on fixation or how to keep the eye steady to inhibit some of these extra movements is very challenging. For some people, nystagmus is better with converging, for some, it's worse than with converging. There's often a null point where a position where the magnitude of the eye movements or direction of the eye movements is different. Taking where somebody is operating better and then expanding that is a lot easier than nystagmus at all positions, but finding the right lens combination with glasses or prism can often help pretty dramatically.