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  • Writer's pictureBrian Foley

Conroe Criminal Defense Attorney - Cross Examining a Police Officer on HGN

Conroe Criminal Defense Attorney - Brian Foley - Board Certified in Criminal Law.

Challenging the Horizontal Gaze Nystagmus Test in a DWI Case: A Guide to Cross-Examination


Driving while impaired (DWI) cases can be complex legal battles, often hinging on the validity of field sobriety tests conducted by law enforcement officers. One such test is the Horizontal Gaze Nystagmus (HGN) test, which measures involuntary eye movements as a potential indicator of intoxication. Cross-examining a police officer about their administration of the HGN test is a crucial element in building a strong defense in a DWI case. In this blog post, we will explore how to effectively cross-examine a police officer regarding the HGN test to challenge its reliability and potentially secure a favorable outcome for your client.

Understanding the Horizontal Gaze Nystagmus Test

Before delving into cross-examination strategies, it's essential to understand what the HGN test entails. During the HGN test, an officer observes a suspect's eyes as they follow a slowly moving object, typically a pen or a small flashlight. The officer looks for specific signs of nystagmus, an involuntary jerking of the eye, which may become more pronounced when a person is under the influence of alcohol or drugs. If the officer observes certain clues, they may use this as evidence of impairment.

Cross-Examination Strategies

  1. Officer's Qualifications: Begin by establishing the officer's qualifications to conduct the HGN test. Ask about their training, certification, and experience in administering the test. Determine whether they followed the proper protocols and guidelines during the test.

  2. Environmental Factors: Question the officer about the testing conditions. Were there any distractions, poor lighting, or adverse weather conditions that could have affected the test's reliability? These factors may have a significant impact on the results.

  3. Suspect's Physical Condition: Explore the suspect's physical condition at the time of the test. Did they have any medical conditions or injuries that could have influenced their performance? Additionally, ask if the suspect was on any prescription medications that could cause nystagmus.

  4. Clues Observed: Have the officer describe the specific clues they observed during the HGN test. Was there a lack of smooth pursuit, distinct nystagmus at maximum deviation, or onset of nystagmus prior to 45 degrees? Challenge the officer to provide a clear and detailed account of their observations.

  5. Alternative Explanations: Present alternative explanations for the observed nystagmus. For instance, stress, fatigue, or even the suspect's naturally occurring nystagmus can mimic the signs of intoxication. Challenge the officer to differentiate between alcohol-induced nystagmus and other potential causes.

  6. Training Records: Request access to the officer's training records and materials related to the HGN test. This can help you uncover any inconsistencies or inadequacies in their training, which can be used to question their expertise in administering the test.

  7. Legal Guidelines: Familiarize yourself with the relevant legal guidelines and standards for conducting the HGN test in your jurisdiction. If the officer deviated from these standards, it can weaken the credibility of their testimony.


Successfully challenging the Horizontal Gaze Nystagmus test in a DWI case requires a thorough understanding of the test, as well as strategic cross-examination techniques. By questioning the officer's qualifications, the testing environment, the suspect's physical condition, and alternative explanations for nystagmus, you can build a compelling argument to challenge the test's reliability. Ultimately, a strong cross-examination can help cast doubt on the prosecution's case and improve your client's chances of a favorable outcome in their DWI case.

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