What Is The “Horizontal Gaze Nystagmus Test”
The Horizontal Gaze Nystagmus (HGN) test, sanctioned by the National Highway Traffic Safety Administration (NHTSA), is a common assessment used by Bakersfield police during traffic stops to detect potential drunk driving. While recognized by the NHTSA as indicative of impairment, the HGN test is not flawless and presents challenges when presented as evidence due to its limitations.
Nystagmus refers to involuntary eye movements, particularly noticeable in horizontal gaze, which intensify under the influence of alcohol and certain drugs. Alcohol’s depressive effects on the nervous system affect reflexes and coordination, manifesting in jerky eye movements observed during the HGN test. These movements, uncontrollable and unaffected by visual clarity, serve as solid indicators of intoxication when sustained at maximum deviation.
Alongside the one-leg stand (OLS) and walk-and-turn (WAT) tests, the HGN test contributes to a comprehensive field sobriety assessment. Studies supported by the NHTSA since 1977 have consistently affirmed the HGN test’s validity, with increasing accuracy observed with officer experience and training, reaching up to 92%.
How The Horizontal Gaze Nystagmus (HGN) Test is Done
Initially, the officer instructs you to stand still with feet together and, if applicable, remove any eyewear. Then, you’re directed to fix your gaze without turning your head while the officer moves a small object, like a light, finger, or pen, horizontally across your field of vision. The officer looks for three specific indicators:
- Lack of Smooth Pursuit: If your eyes jerk or exhibit a bouncing motion while tracking the moving object. Impairment disrupts smooth eye tracking, resembling the movement of windshield wipers on a dry surface.
- Distinct and Sustained Nystagmus At Maximum Deviation: If your eyes continue jerking within a four-second period when held at their furthest sideways position.
- Onset of Nystagmus before 45 Degrees: If jerking, eye movements begin before reaching a 45-degree angle, suggesting a blood alcohol concentration (BAC) above .08. The earlier the onset, the greater the impairment level.
Each eye is examined for these clues, resulting in six clues observed (three per eye). The officer conducts two passes with the stimulus in front of your eyes, checking for each clue individually, and tallies the total clues. The HGN test purportedly demonstrates an 88% accuracy rate in detecting individuals with a BAC at or above .08 if four or more clues are present.
Additionally, the officer might assess for Vertical Gaze Nystagmus, where the stimulus moves vertically instead of horizontally. If your eyes involuntarily jerk or fixate at maximum deviation during vertical movement, it’s another impairment sign.
Notably, your eyes respond similarly to vertical and horizontal nystagmus tests. No substance, whether alcohol or drugs, selectively induces jerking in one direction only. Therefore, if Vertical Gaze Nystagmus appears while Horizontal Gaze Nystagmus does not (or vice versa), it raises concerns for officers, suggesting alternative reasons for perceived impairment may exist.
False Positives: Other Causes of Nystagmus
Nystagmus, a jerking of the eyes, can be triggered by factors beyond alcohol consumption, including seizures, certain medications, and medical conditions like brain tumors or inner ear problems.
Officers inquire about your health before conducting the Horizontal Gaze Nystagmus test, particularly eye-related issues. If you disclose relevant medical conditions such as partial blindness or inner ear diseases, the officer might skip the test to avoid false positives.
Additionally, officers assess for equal pupil size, resting nystagmus, and equal tracking ability. Discrepancies in these parameters may indicate an underlying medical problem. Furthermore, significant differences in test performance between horizontal and vertical movements or between eyes may signal a medical issue.
Failing A Horizontal Gaze Nystagmus Test and Legal Representation
Failing the Horizontal Gaze Nystagmus (HGN) test can result in DUI charges, which can be intimidating. However, nystagmus, the jerking of the eyes observed in the test, can be caused by various factors unrelated to alcohol, such as inner ear disorders, neurological issues, and even genetic predispositions.
It’s essential to secure legal representation if charged with DUI based on HGN or other field sobriety tests. A capable DUI defense attorney can challenge the administration and interpretation of the HGN test, consider alternative medical or environmental influences on the results, and navigate the complexities of DUI charges to potentially lessen the repercussions.
Having a lawyer familiar with the intricacies of such tests is crucial, given the potential for external influences on HGN results. They can contest the reliability of the findings and construct a defense informed by legal expertise and scientific understanding of field sobriety tests.
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Yuliya Kelmansky is an Expert Attorney who has over 15 years of practice defending a variety of cases.
