What is the Horizontal Gaze Nystagmus Test?

The Horizontal Gaze Nystagmus (HGN) test is one of three standardized field sobriety tests (SFSTs) validated by the National Highway Traffic Safety Administration (NHTSA) for use by law enforcement to help determine whether a driver is impaired by alcohol or drugs. It is a test used in North Carolina that law enforcement typically administer during traffic stops when impaired driving is suspected. Though the NHTSA recognizes the HGN as a test that can indicate impairment, this test is not fool-proof. In fact, it has several disadvantages, which make admitting it as evidence problematic.

Attorney Steven N. Long uses his prior experience as a police officer and a prosecutor to review the types of and manner in which field sobriety tests like the HGN test were administered in your case to ensure you receive the best representation possible. 

How is the Test Performed?

The officer conducting the HGN test should provide clear verbal instructions to the driver. The police officer should tell the driver to stand still, place hands to the side, and keep head still. Then they must be properly instructed to look at a stimulus, like a pen or another object, and follow it with both eyes while the officer moves the stimulus from right to left.

The officer assesses the driver's eyes while moving the object from side to side. HGN tests, however, are very technical in their application. Proper administration involves specific requirements on distance between the stimulus and the driver's nose (12 - 15 inches), timing and length of holds (hold the stimulus for at least 4 seconds at the maximum deviation), and how many times and ways the stimulus is passed back and forth.

The HGN test is meant to measure the involuntary jerking of the eye – called "nystagmus." A driver with a high blood alcohol concentration may exhibit involuntary jerking of the eyes as the driver gazes toward the side while following the stimulus.

How is the Test Evaluated?

Law enforcement look for three clues while administering the HGN test. Each eye is assessed for these three clues, so there are actually a total of six possible indicators of impairment. If the officer determines four clues exist, that indicates the driver's blood alcohol content (BAC) level is above 0.08 percent.

  1. Clue 1 involves the onset of nystagmus in either eye before 45 degrees.
  2. Clue 2 involves nystagmus in either eye when the eye gazes as far as possible to the right or left – in this case, there is a sustained and distinct jerking.
  3. Clue 3 occurs when the driver is unable to follow the stimulus – known as lack of smooth pursuit. 

Each clue requires specific motions or manners in which the stimulus is held or passed. Each clue also requires different timing. For example, the movement of the stimulus to determine the lack of smooth pursuit should be two seconds out and back to each eye while it is four seconds to move the stimulus from eye to the driver's shoulder to determine the onset of nystagmus prior to 45 degrees.

Challenging the HGN Test

HGN tests can be challenged effectively by arguing against their reliability (these tests are highly subjective) or proving improper administration of the test (these tests require following strict and specific technical rules). Also, these tests can be challenged based on matters not associated with the test itself, but matters related to the driver or to the environment at the traffic stop.

Common Challenges to the HGN Test

  • Unreliable based on officer's subjective estimations and preconceived notions
  • Unreliable based on officer's failure to administer the test properly
  • Unreliable based on external factors

Common External Causes of Poor HGN Tests

  • Bad weather
  • Administered at night in darkness or during the day with a glaring sun
  • Bad roads or other environmental issues
  • Patrol car lights flashing or other lighting issues
  • Driver's pre-existing health issues or medications, like ear disorders, eye disorders, head injury or brain damage, excessive amounts of caffeine, antihistamines, barbiturates, illness like the flu or vertigo

Keep in mind that there are more than 38 non-alcohol-related causes for nystagmus, and each of these can lead to a poor HGN test. An experienced DWI defense attorney can highlight these weaknesses and argue reasonable doubt exists in the State's case against you. 

Officer Error

Law enforcement officers are human, and they make mistakes. Field sobriety tests should be administered properly. The instructions are strict and very technical. An error on the officer's part can lead to exclusion of certain evidence against you.

Can I beat a DWI charge in North Carolina?

Like any other traffic or criminal offense, it is possible to beat a DWI charge. However, it is not an easy task. It requires a thorough understanding of the law and a thorough understanding of detection techniques, field sobriety tests, breath tests, and blood tests. Understanding the latter tests is critical to identifying errors (technical or human-made errors) to challenge the reliability of test results.

Aside from errors or unreliable test results, an alleged DWI offender may have had their constitutional rights violated. This happens more often than you might imagine. A violation can lead to the inadmissibility of some or all evidence. Without sufficient evidence, the case may be dismissed, or a not guilty verdict may be achieved.

If you plan to fight your DWI charge, it is in your best interest to have an attorney represent you. The law is very complex. The evidence can be highly technical and scientific. Law enforcement and other expert testimony can be damaging. All these things can lead to a conviction, unless you have the necessary skills and knowledge to successfully challenge them in court.

Attorney Steven N. Long knows the law. His vast prior experience and training in the technical, scientific make-up of field sobriety tests, breath tests, and blood tests make him uniquely qualified to best represent you in a DWI charge.

Contact our firm today at (252) 514-0062 or submit the online form to schedule a free case review for your DWI charge.