Horizontal gaze nystagmus (HGN) is a condition where the eyes involuntarily jerk or twitch as they move laterally, typically observed when an individual’s gaze is directed to the side. During a DUI stop, an officer may conduct the HGN test by moving an object (such as a pen or finger) horizontally across the person’s field of vision while observing the eyes for any signs of nystagmus.

While HGN can indicate impairment, its reliability as a sole indicator is questioned for several reasons:

1. **Subjectivity**: Interpretation of nystagmus can vary between officers, leading to inconsistent results. Factors such as lighting conditions, the officer’s experience, and the subject’s eye characteristics can influence the observations.
2. **False Positives**: Nystagmus can be influenced by various factors other than alcohol or drug consumption, such as fatigue, certain medical conditions, prescription medications, or even environmental stimuli like flashing lights.
3. **Lack of Specificity**: HGN is not specific to alcohol impairment and cannot distinguish between alcohol and drug impairment. Different substances affect the body differently, and HGN alone cannot pinpoint the specific substance causing impairment.

Therefore, while HGN can be a useful tool in conjunction with other field sobriety tests and evidence, its limitations make it unreliable as a standalone indicator of impairment in DUI cases.