To conduct the horizontal nystagmus test, the arresting officer asks a driver to look at a stylus while moving it right to left and back and forth across a driver’s field of vision.  The officer is looking for involuntary jerking of the eyes, known as nystagmus. The presence of nystagmus during the test could mean the individual has consumed alcohol earlier in the night, but it is not a definite indicator of impairment.  There are dozens of other neurological conditions that can cause nystagmus.