What is the definition of Nystagmus in the context of field sobriety tests?

Nystagmus, in the context of field sobriety tests, refers to the involuntary jerking or movement of the eyes. This physiological response is crucial in assessing potential impairment due to substances such as alcohol or drugs. During field sobriety tests, officers observe this eye movement as an indicator of intoxication; for instance, the Horizontal Gaze Nystagmus (HGN) test specifically looks for this eye reaction in response to a moving stimulus, which can be influenced by the level of intoxication.

The other options, while related to potential signs of impairment, do not specifically define nystagmus. Brightness sensitivity relates to visual acuity under varying lighting conditions, balance issues while walking pertain to a person's coordination and stability, and difficulty in understanding instructions can indicate cognitive impairment, but none of these directly describe the eye movement characteristic of nystagmus. Thus, the correct definition of nystagmus is specifically tied to the involuntary eye movement observed during sobriety testing.