Nystagmus is a jerking movement of your eyes visible to your examiner. The physician will describe these eye jerks in detail and can ordinarily localize the problem precisely to one of the ear canals, most frequently the posterior canal on the right or left side. Sometimes the exam is problematical. Some persons are very afraid of being tilted and manipulated because this is the very thing that brings on their troublesome symptoms and they studiously avoid such movements. Sometimes the sufferer just happens to come in when they are not symptomatic or they have a problem that is difficult to localize.
Some persons have the problem in both ears.
The ear contains tiny sand grains of Calcium Carbonate call otoliths which literally means ear rocks. These fellows are meant to be heavy and like to fall on receptors in the ear called hair cells when you tilt your head. In this way your inner ear relays information to your brain about the position of your head. Lots of times these little rocks come off of their membrane and break free going from a part of the inner ear called the utricle falling into one of the semicircular canals, most commonly the posterior canal. I ve borrowed a figure from Tim Hain, MD an expert on dizziness, but such pictures are readily available from a lot of sources.