Nystagmus

Nystagmus is simply a specific type of eye movement. This is a major area of study in neuro-ophthalmology and it happens to be the best way we have of studying the vestibular system. By monitoring eye movements during and after specific provocative stimuli (as is performed during VNG and rotary chair testing), we gain a great deal of knowledge regarding disease states involving our inner ears and brain.
Nystagmus is often described by the direction and/or type of eye movement. In vestibular medicine, the most commonly described type of nystagmus is "jerk" nystagmus, whereby the eye moves slowly in once direction and quickly in the opposite direction. For example, if the eye moves slowly to the rightt, then quickly back to the left, we call this "left-beating" nystagmus. Here is an example:

Upbeat Nystagmus:

Below are some examples of nystagmus that would be due to a central (brain) disease state.

Downbeat Nystagmus (note is more pronounced in gaze right or left):

Downbeat Nystagmus only noted in gaze right or left and with fixation (staring at a light):