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Horizontal Gaze Nystagmus

The standardized field sobriety tests that is often considered the most accurate in detecting alcohol impairment is the horizontal gaze nystagmus test. On this page, you can learn more about how a police officer administered this test and intoxication clues that a police officer will look for. Read on for more information!


In Connecticut, the first test often given is the horizontal gaze nystagmus test. Nystagmus sounds like a complicated word. But, it just means an involuntary jerking of the eyes. The person experiencing the nystagmus is not aware that the jerking is occurring. The horizontal gaze nystagmus test is based on the idea that involuntary jerking of the eyes becomes noticeable when a person is impaired by alcohol.

The theory is that the higher the blood alcohol concentration goes, the sooner the eyes will jerk as they move to the side of the person’s head. Many police officers consider the horizontal gaze nystagmus test to be the most reliable test. Before giving the horizontal gaze nystagmus test, police will look for signs of possible medical impairment. This includes pupil size, resting nystagmus, and tracking ability.

Administering the Horizontal Gaze Nystagmus Test

When administering the horizontal gaze nystagmus test, police officers usually have the person follow the motion of a small pen with the eyes only. The tip of something that contrasts with the background is used as the stimulus. When administered, the police start with the left eye. They look for three specific clues. As the eye moves from side to side, the officer is checking to see if the eye moves smoothly or whether it jerks a lot.

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The theory is based on the idea that as people become more impaired by alcohol, the eyes show a lack of smooth pursuit as they move from side to side. When the eyes move as far to one side as possible, the police officer is trying to see if the eyes jerk distinctly. This is a clue of impairment. Also, the officer is looking to see if the eyes jerk prior to a 45-degree angle. This is because onset of nystagmus prior to 45-degrees is another sign of impairment.

Taking the Horizontal Gaze Nystagmus Test

The driver taking the horizontal gaze nystagmus test has to be instructed to look straight ahead. They must know to keep the head still while following and focusing on the stimulus until told to stop by the officer. The stimulus has to be about 12 to 15 inches in front of the driver’s eyes. This is for ease of focus. It must be held at eye level, so that the eyes are wide open and looking directly at the stimulus. 

Police officers are trained to receive an acknowledgement from the driver that the stimulus is at a comfortable distance. They are supposed to document this confirmation. Eyeglasses should be removed so the police officer can make a more accurate determination of the driver’s performance. If the person cannot see the stimulus without wearing eyeglasses, the test must be allowed wearing the glasses. Also, hard contact lenses should be removed. This is to avoid any dislodging when the eyes are out at maximum deviation. It prevents damage to the eyes.

Intoxication Clues

When administering the HGN test, the police look for three clues to determine if alcohol intoxication has taken place. 

First Clue

The first clue that the police are looking for is the involuntary jerking of the eyes. This is also called the lack of smooth pursuit. Police officers are trained to look for the person’s inability to pursue a stimulus smoothly while simultaneously focusing horizontally. If the driver moves their head at any time, the score may be invalid. This happens regardless of which clue the officer is looking for. An example of smooth pursuit would be a marble rolling across a flat surface. If a driver is under the influence, the eyes will jerk. 

The police officer is trained to check the left eye first by moving the stimulus to the officer’s right. The stimulus has to be moved smoothly. This happens in order to bring the driver’s eye as far to the side as it can go. Any shaking hand movements by the officer could potentially induce nystagmus in the driver’s eyes.

This could possibly invalidate the test. Police are instructed to make two or more passes in front of the eye. This is to be absolutely certain about the presence of nystagmus. If this clue is scored as nystagmus, the driver is assessed one point. But, just because the driver has this clue in one eye doesn’t mean it will occur in the other eye.

Second Clue

The second clue the police are looking for is distinct jerkiness at maximum deviation. After the police officer has checked the first eye for smooth pursuit, the same eye has to be checked for distinct jerkiness at maximum deviation. The way the police perform this test is by moving the stimulus to the side until the eye has gone as far to the side as possible.

When the eye is that this point, no sclera (white of the eye) will be visible in the corner of the eyeball. The eyeball must remain at that position for two or three seconds. This must happen for the police officer to attempt to discern eyeball jerkiness. If the police officer can’t make this distinction from a slight nystagmus, the benefit of the doubt goes to the driver.

A common mistake that police officers make is not bringing the eyes out to the side as far as they can go or too rapidly returning the stimulus. This can incorrectly score this part of the test. During this part of the test, a certain degree of discomfort is experienced, which causes a slight twitching of the eyes when they are at maximum deviation. If the police officer returns the stimulus too quickly, it can cause a natural nystagmus which can be mistaken for one caused by alcohol intoxication.

Third Clue

The third and final clue is what is referred to as the angle of onset. This clue is the most difficult to evaluate, but the angle of onset is perhaps the best indicator of the presence of the other clues. This correlation doesn’t work conversely, however. The presence of either of the first two clues doesn’t guarantee the third clue will be present.

The driver will be told to follow the stimulus until they are looking down a 45-degree diagonal angle. To estimate this 45-degree angle, the police officer is told to place the stimulus between the driver’s ear and nose on the side being tested. This estimation of the angle is critical because scientific studies have shown that as the intoxication of a person increases, the angle will decrease.

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