Vertical Gaze Nystagmus (VGN)

Vertical Gaze Nystagmus (VGN)

Vertical Gaze Nystagmus (VGN) is a type of nystagmus that is commonly used in the field of sobriety testing. It is often added on to three standardized field sobriety tests (FSTs) that law enforcement officers use to determine if a driver is under the influence of alcohol or drugs. This article will discuss what VGN is, how it is used in sobriety testing, and its accuracy in detecting impairment.

What is Vertical Gaze Nystagmus (VGN)?

Nystagmus is an involuntary movement of the eyes that occurs naturally in everyone. Vertical gaze nystagmus, however, is a type of nystagmus where the eyes move up and down, rather than side to side. When a person is sober, their eyes should be able to move up and down smoothly without any jerking or bouncing. However, when a person is under the influence of alcohol or drugs, the nystagmus can become exaggerated, causing the eyes to jerk or bounce as they move up and down.

How is VGN used in Sobriety Testing?

During a VGN test, the officer will hold a stimulus, such as a pen or finger, in front of the driver’s eyes and ask them to follow it with their eyes as the officer moves the stimulus up and down. The officer will be looking for three clues to determine if the driver is impaired: lack of smooth pursuit, distinct and sustained nystagmus at maximum deviation, and onset of nystagmus prior to 45 degrees.

Lack of Smooth Pursuit: The officer will check to see if the driver’s eyes follow the stimulus smoothly, without any jerking or bouncing. If the driver’s eyes do not move smoothly, it could be an indication of impairment.

Distinct and Sustained Nystagmus at Maximum Deviation: The officer will move the stimulus up and down to check for the point where the eyes move as far up as they can go. At this point, if the eyes are still jerking or bouncing, it could be a sign of impairment.

Onset of Nystagmus Prior to 45 Degrees: The officer will also check to see if the nystagmus begins before the eyes reach a 45-degree angle. If the nystagmus begins before this point, it could be a sign of impairment.

Accuracy of VGN in Detecting Impairment

The accuracy of VGN in detecting impairment has been debated. Some studies have shown that VGN is an effective tool in detecting impairment in drivers, while others have shown that it is not a reliable indicator of impairment.

Understanding Vertical Gaze Nystagmus Test

Before we delve into challenging the VGN test, it is important to understand how the test works. Nystagmus is an involuntary jerking of the eye that can be caused by alcohol or drug use. The VGN test involves the officer observing the driver’s eyes as they follow a stimulus, such as a pen or finger, moving up and down. The officer will be looking for any signs of nystagmus, such as jerking or bouncing of the eyes.

Challenging the VGN Test

There are several ways to challenge the VGN test in court. Here are some of the most common:

  1. Officer’s qualifications: The officer administering the VGN test must be properly trained and certified to do so. If the officer cannot provide proof of their training and certification, the results of the test may be challenged.
  2. Medical conditions: Certain medical conditions can cause nystagmus, even in sober individuals. For example, inner ear infections or injuries, head trauma, or brain tumors can cause nystagmus. If the driver has a medical condition that could cause nystagmus, the VGN test results may be challenged.
  3. Environmental factors: The VGN test must be performed in a controlled environment. If there are distracting lights or noises, or if the ground is uneven or slippery, the results of the test may be challenged.
  4. Improper administration: The officer must follow strict guidelines when administering the VGN test. If the officer does not properly position the stimulus or move it at the correct speed, the results of the test may be challenged.
  5. Interference: If the driver wears glasses or contacts, or if they have a lazy eye or astigmatism, the VGN test may not be accurate. The driver should inform the officer of any such conditions before the test is administered.
  6. Alcohol levels: Even if nystagmus is present, it does not necessarily mean the driver is under the influence of alcohol. Nystagmus can be caused by other factors, such as fatigue or caffeine consumption.
  7. Refusal to take the test: Drivers have the right to refuse to take the VGN test, and the prosecution cannot use the refusal as evidence of guilt. However, refusing to take the test may result in the driver’s license being suspended.


Challenging the accuracy of the VGN test in court can be an effective way to fight a DUI charge. By understanding the potential flaws in the test and knowing how to challenge them, a driver can improve their chances of a favorable outcome in court. It is important to seek the advice of an experienced DUI attorney who can guide you through the legal process and help you build a strong defense.