By Erin Gerstenzang

Before the early 1970s, officers used several different field sobriety tests to determine possible intoxication. But none of these tests were standardized. Officers used their own set of instructions and clues, making it very difficult to determine how reliable these tests were.

In 1975, the Southern California Research Institute, with funding from the National Highway Traffic Safety Administration (NHTSA), decided to come up with tests that all officers throughout the United States would administer and interpret the same way. They came up with the three standardized tests that officers use today. Then, they conducted several other studies to determine how accurate each one is.

The first standardized field sobriety test that officers rely on during a DUI investigation is the HGN, or the Horizontal Gaze Nystagmus test. Nystagmus refers to the involuntary jerking of the eyeballs. The jerking becomes more pronounced when someone is intoxicated by alcohol or certain drugs. This is why it is used to determine if probable cause exists for a DUI arrest.

Common Causes

Three drug categories can cause nystagmus to occur: Depressants, inhalants, and dissociative anesthetics. It’s important to know what can cause this condition so you can match up what the officer thinks is present with what the toxicology shows.

For example, if you have a case with marijuana, it doesn’t fall into one of those three drug categories known to cause nystagmus. So if the officer sees the standard six out of six clues, it won’t match up to the facts of your case. Either it’s a false positive or the officer is claiming to see something that was not there.

How the Officer Conducts the Test

The Horizontal Nystagmus Test (HGN) is a standardized field sobriety test where the officer instructs the suspect to follow (with his eyes) a stimulus to the left and to the right.

The first step the officer should take before conducting the test is to medically qualify your client and verbally instruct them on how to take the test.

Then, they should position the stimulus between 12-15 inches from the driver’s face and slightly above eye level.

The position of the stimulus is the first thing you want to make note of. Because if it is held too closely, too high, too far, or too low, it increases the false positive rate. Making it very important to distinguish the position correctly when reviewing the video.

Next, the officer will conduct a series of passes with the stimulus. During this phase of the test, it’s important for you to count the seconds for each pass.

Equal Tracking

The first series of passes is called the equal tracking section – – where the officer is checking to make sure your client’s eyes move together at the same speed and with a full range of motion.

The officer will always start with the defendant’s left eye. It should take approximately two seconds out and approximately two seconds back to the center, then repeat approximately two seconds out, and approximately two seconds back to the center.

So if you add all those times up it should take approximately eight seconds total.

There should be a distinct pause between the equal tracking section and the next section, which is a smooth pursuit of the stimulus.

Smooth Pursuit

For the smooth pursuit portion of the test, the stimulus is moved from the center position to the far left and back to the center position twice for each eye, moving at a speed that takes at least two seconds from the center position to the side position.

If a lack of smooth pursuit is detected, a “clue” is scored for the eye in which the officer observed a lack of smooth pursuit. The acceptable tolerance for this test is anywhere between 12-20 seconds.

Maximum Deviation

The next series of passes is designed to determine whether the person has distinct nystagmus at maximum deviation. Maximum deviation is the point at which the eye has moved fully to one side and cannot move any further.

The officer should move the stimulus from the center position to the person’s far left at a rate taking at least two seconds, hold for at least four seconds, and then move back to the center position at the same two-second rate.

It’s important to count those seconds to make sure the minimum of four seconds is met.

You also want to make sure they do not last too long because that can induce fatigue nystagmus.

Onset Angle of Nystagmus

The final phase of the HGN test is a set of passes used to determine whether the onset of nystagmus occurs prior to the eye’s movement to a 45-degree deviation.

The officer should move the stimulus very slowly at a rate that would take at least four seconds to move the stimulus to the person’s shoulder or at a rate of no more than 10 degrees per second. What matters to the officer at this point is whether or not your client’s eyes start to jerk.

If they do start to jerk, the officer is required to stop and hold the stimulus steady to confirm two things:

  • Is the jerking continuing?
  • Did the jerking occur before the 45-degree angle?

There should be no doubt that the stimulus is stopped and being held in that position.

Testing Conditions

One final thing to pay attention to during the HGN test is whether or not optokinetic nystagmus could be present. This is the jerking of the eyes caused by full-field visual motion. Situations, where there is a lot of traffic or distractions, might cause the person to lose focus on the stimulus and look beyond it.

Roadside Testing Terminology

  • Field Sobriety Test (FST): Refers to any test the officer administers on the side of the road.
  • Standardized Field Sobriety Test (SFST): Refers to three specific tests that have been researched for accuracy and follow a specific protocol to determine whether or not a person should be arrested. These tests are the HGN, the Walk-and-Turn, and the One-Leg Stand.
  • False Positive: When a test incorrectly indicates a condition is present. For example, when the field sobriety tests indicate that a person was above the specified BAC, but a breath or blood test shows that they were actually below the specified BAC.
  • Nystagmus: The medical term used to describe the involuntary jerking of the eyeballs.
  • Resting Nystagmus: The jerking of the eyes when looking straight ahead at a stimulus.
  • Lack of Smooth Pursuit: When the eyes jerk or “bounce” as they follow a smoothly moving stimulus.
  • Nystagmus at Maximum Deviation: When the eyes begin jerking after four seconds while looking all of the ways to the side.
  • Fatigue Nystagmus: Also known as endpoint nystagmus, it’s caused by holding the eye at maximum deviation for 30 seconds or longer. It has nothing to do with being tired.
  • Vertical Gaze Nystagmus (VGN): An up and down jerking of the eyes occurs when the eyes gaze upward at maximum elevation. The presence of this type of nystagmus is associated with high doses of alcohol for that individual and certain other drugs.
  • Optokinetic Nystagmus: the jerk nystagmus induced by full-field visual motion.