DUI Lawyer | Joel Brand, Esq.

Field Sobriety Test: Horizontal Gaze Nystagmus

When a law enforcement officer pulls you over on suspicion of driving under the influence (DUI) in California, one of the tools they may use to determine impairment is the field sobriety test (FST). Among these tests, the Horizontal Gaze Nystagmus (HGN) test is one of the most common and scientifically validated methods used. Understanding how the HGN test works, its scientific basis, and its implications in a DUI case is crucial for anyone facing such charges. This article delves into the details of the HGN test, its administration, and its role in DUI cases.

What is Horizontal Gaze Nystagmus?

Horizontal Gaze Nystagmus refers to an involuntary jerking of the eye that occurs as the eyes gaze to the side. This physiological phenomenon can be exaggerated by alcohol and certain drugs. The test is based on the principle that nystagmus becomes more pronounced when a person is impaired by alcohol or specific drugs.

The Science Behind HGN

Nystagmus is a natural, involuntary eye movement that can occur in various forms, such as during spinning (rotational nystagmus) or with certain neurological conditions. Horizontal Gaze Nystagmus specifically refers to the jerking that happens when a person looks to the side.

When a person consumes alcohol, it affects the central nervous system, impairing the brain’s ability to control eye muscles and smooth pursuit movements. This impairment causes the eye to jerk more noticeably when it moves to a maximum deviation, which is the basis for the HGN test. The scientific validity of the HGN test has been recognized in numerous studies, making it a reliable indicator of impairment when administered correctly (Burns & Moskowitz, 1977).

How the HGN Test is Administered

The HGN test is typically administered at the roadside by a law enforcement officer trained in its procedures. Here is a step-by-step breakdown of how the test is conducted:

  1. Instruction Phase: The officer asks the suspect to remove their glasses if they are wearing any. They are then instructed to focus on a stimulus, usually a pen or small flashlight, held about 12-15 inches from their face and slightly above eye level.
  2. Initial Check for Equal Tracking: The officer moves the stimulus smoothly from the center of the suspect’s face to the left and right. This is to ensure that both eyes track the stimulus equally and to check for signs of resting nystagmus, which could indicate a medical condition unrelated to alcohol consumption.
  3. Smooth Pursuit: The officer then moves the stimulus slowly to the side and back to the center. They observe whether the eyes can follow the stimulus smoothly or if they exhibit jerking movements.
  4. Distinct and Sustained Nystagmus at Maximum Deviation: The stimulus is moved to the side as far as possible and held there for at least four seconds. The officer watches for distinct jerking when the eye is at maximum deviation.
  5. Onset of Nystagmus Prior to 45 Degrees: The stimulus is moved slowly from the center towards the side, and the officer looks for the jerking movements starting before the eye reaches a 45-degree angle. This angle is typically marked by aligning the pen with the suspect’s shoulder.

Each eye is tested separately, and the officer looks for three clues in each eye: lack of smooth pursuit, distinct nystagmus at maximum deviation, and onset of nystagmus prior to 45 degrees. If the officer observes four or more of these six clues, there is a high likelihood that the suspect’s BAC is 0.08% or higher.

Legal and Procedural Considerations

The Role of HGN in DUI Cases

The HGN test is a critical piece of evidence in DUI cases, but it is not infallible. Several factors can affect its reliability:

  • Proper Training and Administration: The officer administering the test must be properly trained and follow standardized procedures. Any deviation from these procedures can call into question the validity of the test results.
  • Medical Conditions: Certain medical conditions, such as inner ear disorders, neurological conditions, or even eye strain, can cause nystagmus unrelated to alcohol or drug impairment.
  • Environmental Factors: External factors such as flashing lights, passing traffic, and the suspect’s fatigue can affect the test results.

Challenging the HGN Test in Court

Given these potential issues, a skilled DUI attorney can challenge the HGN test in several ways:

  • Questioning Officer Training: Demonstrating that the officer lacked proper training or did not follow standardized procedures can undermine the test’s reliability.
  • Highlighting Medical Conditions: Presenting medical evidence that the suspect has a condition causing nystagmus can cast doubt on the test results.
  • Environmental and Situational Factors: Pointing out environmental factors or situational stressors that could have affected the test can also be a valid defense strategy.

Conclusion

The Horizontal Gaze Nystagmus test is a valuable tool for law enforcement in identifying impaired drivers. However, it is not without its limitations and potential for error. Understanding the science, proper administration, and legal implications of the HGN test is crucial for anyone facing DUI charges in California. A knowledgeable DUI attorney can effectively challenge the test’s validity, potentially influencing the outcome of the case.

Citations

  1. Burns, M., & Moskowitz, H. (1977). Psychophysical Tests for DWI Arrest. U.S. Department of Transportation, National Highway Traffic Safety Administration.
  2. California Vehicle Code § 23610.