Banner for The Criminal Justice Systems: A Guide for Law Enforcement Officers and Expert Witnesses in Impaired Driving Cases

Toxicologists, Crash Reconstructionists,
Optometrists, and Other Medical Personnel


Horizontal Gaze Nystagmus (HGN) and its application to impaired driving has been known for years. However, it is likely brand new to the juror hearing this testimony in an impaired-driving case. While we’ve all heard “the eyes are the windows to the soul,” how many of us knew our eyes can provide strong evidence of impairment due to alcohol and some other drugs? That is the challenge faced by prosecutors and optometrists who need to convey this “novel” phenomenon to a jury—and sometimes a judge.

As with each of the other expert categories, the initial line of questioning will address your professional training, background and experience. The prosecutor will also want to know if you are familiar with any of the scientific studies done on HGN (and there are several). It is certainly a good idea to familiarize yourself with as many of these studies as possible.7 Questions will then become specific to your knowledge of the HGN and, specifically, the protocol used by police:

  • Where did you go to optometry school?
  • Please briefly describe the curriculum.
  • Do you have to be licensed?
  • By whom?
  • What’s involved in getting a license? Are you now licensed?
  • How long have you been licensed?
  • Did any of your coursework or additional training include the effects of alcohol and other drugs on the central nervous system?
  • Please describe that training.
  • Do certain drugs have the potential to affect eye movements?
  • Does alcohol have the potential to affect eye movements?
  • Please briefly describe how alcohol affects eye movements.

You will be asked to explain in layman’s terms the meaning and significance of lack of smooth pursuit, maximum deviation, angle of onset, and nystagmus. Simple analogies can be critically important: “Lack of smooth pursuit is similar to a car’s windshield wiper dragging across a dry windshield—it catches a little along the way instead of moving in one smooth arc as it does on a wet windshield.” Additional questions may include whether or not HGN is affected by contact lenses, poor eyesight, or lighting conditions (“Could the officer’s flashlight shining in the defendant’s eyes cause HGN? Or the flashing lights on top of the patrol car?”).

You may also be asked to discuss the differences between HGN and other types of nystagmus—what causes them and how they differ in appearance. You will be asked about the use of HGN testing in your practice, and its use as a test employed by police:

  • Are you familiar with the test used by police?
  • Is it similar to anything you use in your practice? How is it similar? How is it different? Do these differences affect the results of the police test?
  • In your opinion, is this a test which can be taught to and correctly used by police?

Finally, one very important question will be “Do you have an opinion as to whether the presence of HGN is a reliable indicator of the use of a central nervous system depressant, such as alcohol?” Having answered that question and a couple of follow up questions addressing the specific case, you will then be “turned over” to opposing counsel to answer more questions. The area of attacks might include a potential for bias on your part, your familiarity with published studies attacking HGN, your unfamiliarity with the defendant in this case, and the claim that a certain percentage of the population has naturally occurring HGN. Always answer truthfully. REMEMBER: This case is not about HGN to the exclusion of all other indications of impairment. HGN is one factor to consider in the totality of all the evidence.


7 For additional information on HGN scientific studies contact the National District Attorneys Association’s National Traffic Law Center at or phone: 703.549.4253