The Horizontal Gaze Nystagmus (HGN) test is one of the three standardized field sobriety tests (SFSTs) established by the National Highway Traffic Safety Administration (NHTSA) to assist officers in detecting drivers who may be under the influence of alcohol. While the HGN test is widely used in South Carolina DUI investigations, understanding its methodology, significance, and limitations is critical when defending against DUI charges. This test is the one where a police officer looks into someone's eyes as part of a DUI investigation.
What Is Horizontal Gaze Nystagmus?
Nystagmus is an involuntary jerking of the eyes that can occur when the eyes are moved to extreme angles. While nystagmus can have many causes—ranging from neurological conditions to fatigue—alcohol consumption can exacerbate this effect, making it more pronounced.
The HGN test is designed to observe specific jerking movements of the eyes that correlate with alcohol impairment. During the test, the officer asks the individual to follow a stimulus, such as a pen or finger, with their eyes while keeping their head still. The officer observes the eyes for the following six clues, three in each eye:
- Lack of smooth pursuit: The eyes do not follow the stimulus in a smooth motion and instead jerk noticeably.
- Distinct nystagmus at maximum deviation: The eyes exhibit pronounced jerking when looking as far to the side as possible.
- Onset of nystagmus prior to 45 degrees: The jerking begins before the eyes reach a 45-degree angle relative to the center.
The Science Behind HGN Testing
The presence of these clues correlates with the likelihood of a blood alcohol concentration (BAC) of 0.08 or higher. Specifically, if four or more clues are observed, studies suggest an approximate 77% accuracy rate for predicting a BAC of 0.08 or above. This relationship is based on Tharp’s Equation, derived from research conducted by Dr. Marcelline Burns and others under the NHTSA's guidance.
Tharp’s Equation provides a statistical basis for the test’s reliability by linking observed eye movements to alcohol impairment. However, it is important to understand that the HGN test does not definitively determine impairment. It only indicates a likelihood of a BAC over 0.08 when sufficient clues are present. Furthermore, numerous factors—including improper administration, individual physiology, and environmental conditions—can impact the accuracy of HGN results.
Vertical Gaze Nystagmus (VGN)
In addition to HGN, officers may also perform a test for Vertical Gaze Nystagmus (VGN), which examines whether nystagmus occurs as the eyes move vertically. The procedure is similar: the officer instructs the individual to follow a stimulus with their eyes as it is raised and lowered.
Unlike HGN, the presence of VGN is not commonly linked to lower levels of alcohol consumption. Instead, it is associated with higher BAC levels or the use of certain drugs. If VGN is observed, it may indicate a significantly elevated BAC or impairment due to substances other than alcohol. However, like HGN, VGN cannot confirm impairment on its own and must be considered in conjunction with other evidence.
Limitations and Defenses
While the HGN and VGN tests are based on established scientific principles, they are not infallible. Common challenges to these tests include:
- Improper administration: Officers must follow strict protocols, including positioning the stimulus at the correct distance (approximately 12–15 inches from the face) and maintaining proper timing. Any deviation can compromise the results.
- Medical conditions: Eye conditions, neurological disorders, and even fatigue can cause nystagmus unrelated to alcohol or drug use.
- Environmental factors: Poor lighting, distractions, or an unsteady testing environment can influence the accuracy of the observations.
- Individual variations: Some people exhibit natural nystagmus or have a lower threshold for nystagmus onset due to unique physiology.
Conclusion
The Horizontal Gaze Nystagmus and Vertical Gaze Nystagmus tests are frequently used tools in DUI investigations, but their results are not definitive proof of impairment. Instead, they offer officers a means of detecting the likelihood of a high BAC based on observable eye movements. Understanding the science behind these tests, as well as their limitations, is essential for building a strong DUI defense.
Call Us Today
If you are facing DUI charges in South Carolina, the consequences can be severe, but an experienced defense attorney can make all the difference. At the Law Office of James R. Snell, Jr., LLC, we have extensive experience defending DUI cases involving field sobriety tests, including HGN and VGN. James Snell is the author of South Carolina DUI Defense: The Law and Practice and a certified NHTSA DWI Detection and Field Sobriety Testing instructor. He has successfully represented hundreds of clients in Lexington, Columbia, and throughout South Carolina.
DUI cases are urgent, and prompt action is essential to protect your license, freedom, and reputation. Call us today at (803) 359-3301 for a free consultation, and let us put our knowledge and expertise to work for you.