How Reliable is the Horizontal Gaze Nystagmus Test?

The horizontal gaze nystagmus test (HGN) is called one of the most reliable field sobriety tests. Police officers will rely on HGN to determine whether a driver is intoxicated or impaired. Like other field sobriety tests, however, this one can produce a false positive.

What is the Horizontal Gaze Nystagmus Test?

HGN is one of the tests approved by the National Highway Traffic Safety Administration for the purpose of detecting impairment in drivers.

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Source: Wikipedia

The test is used to detect the involuntary eye movements or jerking when one is asked to look to the side. Alcohol consumption usually exaggerates this jerking and makes it easier to detect. Some prescription drugs could also produce a positive result on this field sobriety test.

There’s a simple reason why the HGN test works to detect DUIs – alcohol has a suppressive effect on the functioning of the nervous system. Thus, one’s ability to control the voluntary sideways movement of the eye is reduced. A person isn’t aware of the exaggerated jerking because there’s no control over it.

HGN is one component of the three-part field sobriety test. The other two elements are the one-legged stand test and the walk and turn test. Out of the three, HGN is seen as most effective by the National Highway Traffic Safety Administration.

Can the HGN Test Produce a False Positive?

While the science is there and the test is a pretty reliable one, it’s still possible to get a false positive with it.

The consumption of high amounts of caffeinated beverages can increase the involuntary eye jerking when looking sideways. The same applies to taking an aspirin or smoking cigarettes – nicotine has a similar effect.

Individuals who have a condition known as lazy eye and those who have sustained a brain injury in the past could also get a false positive. A few other causes of false positives do exist and the most common ones include:

  • Old age
  • Nearsightedness and astigmatism
  • A vitamin B12 deficiency increases the risk of experiencing neurological problems
  • Diabetic neuropathy
  • Stroke
  • Influenza and streptococcus infections
  • Muscular dystrophy
  • Multiple sclerosis
  • High blood pressure
  • Glaucoma
  • Vestibular apparatus disorders
  • Exposure to toxins and chemicals like those found in solvents, dry-cleaning products and carbon monoxide
  • The use of antihistamines for allergies

Admissibility of the HGN Test Results and How to Challenge Them

Drivers who are stopped on suspicions of being impaired are not required to do a standard field sobriety test like HGN. If you ever are pulled over, you can refuse to have the HGN administered. Even if you do agree to testing and you get a positive result, there are still things an experienced DUI attorney can do about it.

Declining to field sobriety testing cannot be held against you in court. Thus, unless you panic and you do something rash, there’s absolutely no reason to submit to the request of a police officer. Make your refusal clear but be polite throughout the encounter.

In Arizona, the results of field sobriety tests can be used as evidence against you in court.

horizontal gaze nystagmusYour lawyer can challenge the test results on several fronts. For a start, the administration of the HGN test could be incorrect. There are several conditions that will have to be met for the results to be considered accurate and reliable. Any failure in those standard procedures will negate the validity of the test.

In addition, you and your attorney will attempt to establish that the positive was caused by a medical condition or another issue rather than the consumption of alcohol. You already have a good idea about the things that could lead to a positive. If any of the conditions above apply to you, make sure that your lawyer is informed.