Optokinetic nystagmus is for stabilising the pictures we see when we are observing moving objects for example if we are rotating on an office chair. If we wear a mask while rotating however, the optokinetic nystagmus will not occur since we don't see. But I read that vestibular nystagmus will occur even if we are wearing a mask. What is vestibular nystagmus exatly and how does it differ from optokinetic nystagmus?
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The vestibular reflexes are the main cause for stabilizing eye movements. The labyrinth canals in the inner ear detect changes in head direction, movement and acceleration. This is important for balancing body and visual field. These are fast involuntary reflexes thanks to almost direct connections between the sensory hair cells in the ear and the eye muscle innervating neurons.
The optokinetic reflexes are complementing the vestibular reflexes for slow movements or visual field motions independent of head movement (source and further reading). It needs the visual input.
In my studies we actually did this experiment ourselves, measuring the eye muscle activity while rotating with opened or closed eyes. The results were basically the same in both conditions. With opened eyes you try to fixate something in the moving image for longer and have therefore larger jumps in the eye movements, but with closed eyes the saccades are still there.
Edit: The two senses can also be in conflict with each other. For example reading a book in a car (your vestibular system detects motion, while your visual field is steady) or watching a movie in the cinema (your visual system detects motion while your vestibular system doesn't). This causes motion sickness.
