This is an involuntary, rhythmic “to and fro” movement of the eyes. Most nystagmus are in the horizontal axis, such that the eyes are constantly moving to the left and right. There are many causes for this abnormal eye movement, though most nystagmus occur secondary to congenital motor deficits that begin in childhood. Sensory nystagmus can occur in a child who is born with poor vision and is associated with many other congenital visual defects such as albinism. Nystagmus that develop later in life are rare and can be caused by vestibular (inner ear) abnormalities, drug toxicities (including alcohol), or more serious neurologic problems like a tumor or stroke. Police officers sometimes look for nystagmus with their field sobriety tests, looking for horizontal nystagmus while asking people to look to their right and left.