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Houston Criminal Defense Attorney - What test do officers rely on the most for DWI investigations?

Houston Criminal Defense Attorney - Brian Foley - Board Certified in Criminal Law.


When you get pulled over for speeding after having alcohol on your breath the police are going to do a DWI investigation. They are trained to do a number of field sobriety tests.


There are three standardized filed sobriety tests but police rely on one of these more than any other. It is called the HGN or Horizontal Gaze Nystagmus test.


Nystagmus is a condition in which the eyes make repetitive, involuntary movements. These movements may be horizontal, vertical, or rotary, and they may be fast or slow. Nystagmus can be a result of a variety of factors, including certain medical conditions, medications, and alcohol or drug use.

There are two types of nystagmus: spontaneous and induced. Spontaneous nystagmus occurs when the eyes move involuntarily even when a person is not looking at anything specific. Induced nystagmus occurs when the eyes move in response to a specific stimulus, such as moving your head from side to side.

Symptoms of nystagmus may include difficulty focusing on objects, dizziness, and blurred vision. In some cases, nystagmus may cause discomfort or difficulty with activities such as reading or driving.

Nystagmus is usually diagnosed through a comprehensive eye exam. Treatment may depend on the underlying cause of the condition and may include medications, eye patches, or vision therapy. In some cases, nystagmus may resolve on its own without treatment.


Horizontal gaze nystagmus (HGN) is a type of nystagmus in which the eyes move horizontally, or from side to side, in an involuntary, repetitive manner. HGN is often used as a test for alcohol impairment, as it is a common effect of alcohol on the body.



During this test, a police officer will ask the person being tested to follow a moving object, such as a pen or finger, with their eyes while keeping their head still. The officer is looking for specific signs of HGN, such as a lack of smooth pursuit (the ability to follow the moving object smoothly) and distinct nystagmus at maximum deviation (the point at which the eyes are moved as far to the side as possible).


As a former prosecutor I've trained police related to DWI investigations and I can tell you that the one test they believe in more than any of the others is the HGN. It is said that this is the only test that your tolerance for alcohol cannot assist you in completing. Your eyes will involuntarily jerk as you try to move them side to side. In other words, you can't fake it on the HGN.

HGN is not always caused by alcohol impairment, and other factors can also cause the eyes to move horizontally. However, HGN is a common symptom of alcohol impairment and is often used as evidence in drunk driving cases. If you have been arrested for drunk driving and have been subjected to the nystagmus test, it is important to seek the advice of an experienced criminal defense attorney to understand your rights and options.

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