{"id":286651,"date":"2016-07-08T18:39:47","date_gmt":"2016-07-08T23:39:47","guid":{"rendered":"https:\/\/content.findlaw-admin.com\/ability-law\/dui\/arrests\/what-is-horizontal-gaze-nystagmus-hgn.html"},"modified":"2026-04-09T16:40:56","modified_gmt":"2026-04-09T21:40:56","slug":"what-is-horizontal-gaze-nystagmus-hgn","status":"publish","type":"page","link":"https:\/\/www.findlaw.com\/dui\/arrests\/what-is-horizontal-gaze-nystagmus-hgn.html","title":{"rendered":"What Is the Horizontal Gaze Nystagmus (HGN) Test?"},"content":{"rendered":"\n
\n

What Is the Horizontal Gaze Nystagmus (HGN) Test?<\/h1>\n\n
\n
\n By Amy Vandervort-Clark, J.D.<\/a> | Legally reviewed by Melissa Bender, Esq.<\/a>\n | Last updated\n
\n \n <\/i>\n Legally Reviewed\n <\/summary>\n\n
\n

This article has been written and reviewed for legal accuracy, clarity, and style by FindLaw\u2019s team of legal writers and attorneys<\/a> and in accordance with our editorial standards<\/a>.<\/em><\/p>\n\n <\/div>\n <\/details>\n\n

\n \n <\/i>\n Fact-Checked\n <\/summary>\n\n
\n

The last updated date refers to the last time this article was reviewed by FindLaw or one of our contributing authors<\/a>. We make every effort to keep our articles updated. For information regarding a specific legal issue affecting you, please contact an attorney in your area<\/a>.<\/em><\/p>\n\n <\/div>\n <\/details>\n<\/section>\n\n\n

\n

Horizontal gaze nystagmus (HGN) is the involuntary jerking<\/span> of one\u2019s eye when it gazes to the side. Alcohol use exaggerates this jerking motion. A police officer<\/span> can use the HGN test<\/span> to observe whether a driver is more likely to be impaired. While this test doesn\u2019t directly measure blood alcohol content<\/span> (BAC<\/span>), it can still affect a DUI\/DWI<\/span> case.<\/p>\n\n <\/div>\n\n\n\n

Driving under the influence<\/span> (DUI) laws are strict to combat drugged and drunk driving<\/span>. Law enforcement<\/span> officers<\/span> receive specialized training to look for signs of impairment<\/span>.<\/p>\n

When law enforcement<\/span> suspects you of driving under the <\/span>influence<\/span> of alcohol<\/span> or drugs, they will likely ask you to perform a series of field sobriety tests (FSTs<\/span>). There are three accepted standardized field sobriety tests<\/span>: the walk-and-turn test<\/span>, the one-leg stand test<\/span>, and the horizontal gaze nystagmus test<\/span> (HGN).<\/p>\n

The HGN test<\/span> is evidence of impairment<\/span> in\u00a0DUI cases<\/span><\/a>. This article explains the science and laws behind the HGN field sobriety test. But if you were arrested for drunk or drugged driving, get legal advice from a DUI lawyer<\/a>.<\/p>\n

Horizontal Gaze Nystagmus and Impairment<\/span>: The Science<\/h2>\n

\u201cNystagmus” refers to the bouncing or involuntary jerking<\/span> of the eye. As your eye moves side to side<\/span>, this motion can be noticeable.<\/p>\n

Yet, HGN has no noticeable effect on your vision. Impaired drivers<\/span> might not realize when their eyes are showing signs of higher blood alcohol levels<\/span>.<\/p>\n

How Alcohol Affects Eye Movement<\/span><\/h3>\n

Police officers<\/span> can observe\u00a0horizontal gaze nystagmus<\/a>. This eye movement<\/span> becomes more pronounced when impaired by alcohol and some\u00a0illicit and prescription drugs<\/a>, primarily depressants.<\/p>\n

Alcohol depresses the central nervous system. It slows our reflexes and reduces coordination. As a result, it has a noticeable effect on our ability to control eye movements<\/span> smoothly and accurately.<\/p>\n

Can You Cheat the HGN Eye Exam at a Traffic Stop?<\/h3>\n

You cannot control or stop these movements from happening. When a law enforcement<\/span> officer<\/span> asks you to keep your head still and follow a pen or finger with your eyes, this triggers nystagmus or eye jerking when you are under the influence. If the police officer<\/span> sees sustained nystagmus when your eye is at maximum deviation<\/span> from the center, this is strong evidence of intoxication.<\/p>\n

Is the HGN Test<\/span> Accurate?<\/h3>\n

Taken as a whole, the three-part field sobriety test can accurately determine a blood-alcohol concentration<\/span> (BAC<\/span>) of 0.10% or higher 83% of the time, according to a study cited by the\u00a0National Highway Traffic Safety Administration<\/span><\/a>\u00a0(NHTSA<\/span>). The other two components are the one-leg stand (OLS) and walk-and-turn (WAT) tests.<\/p>\n

Multiple NHTSA<\/span>-funded studies since 1977 have shown the HGN screening to be a valid indicator of alcohol <\/span>impairment<\/span>. It\u2019s the most accurate part of the\u00a0three-part FSTs<\/span><\/a>. The precision of the test increases to 92% as an officer gains additional training and experience conducting the HGN test<\/span>.<\/p>\n

How Police Conduct the HGN Test<\/span><\/h2>\n

Officers conduct the HGN test<\/span> in a well-lit area or with a flashlight. The subject\u2019s eyes<\/span> will face away from the police cruiser\u2019s flashing lights and the lights of passing cars. These extra lights can skew results.<\/p>\n

Checking for Unrelated Factors<\/h3>\n

The officer then tells you they are going to check your eyes. They will ask whether you wear contact lenses<\/span> or have any medical condition<\/span> that would affect the results. You will need to remove any eyeglasses, as they can make it difficult for the officer to observe your eyes.<\/p>\n

The police officer<\/span> will look for equal pupil size<\/span> and equal tracking in your eyes. If your pupils are different sizes or your eyes move at different rates, this may indicate a head injury<\/span><\/a> or other serious condition. They will observe your resting nystagmus<\/span>, which could mean a medical condition<\/span>.<\/p>\n

Evidence of nystagmus does not always signify drug or alcohol <\/span>impairment<\/span>. Movement of inner ear fluid<\/span>, eye strain, neural activity, or brain damage<\/span><\/a>, among other issues, can cause abnormal nystagmus, according to the NHTSA<\/span>.<\/p>\n

Testing Eye Motion<\/h3>\n

The officer then asks you to follow an object. This is usually the tip of a pen or pen light, placed 12 to 15 inches away and slightly above eye level<\/span> with only your eyes.<\/p>\n

The officer is looking for the following three clues in each eye:<\/p>\n