Horizontal Gaze Nystagmus

The Horizontal Gaze Nystagmus test is often called “HGN.” The HGN evaluation is one of the field sobriety exercises used as part of the NHTSA Standardized Field Sobriety Exercises.

HGN is usually the first exercise performed. Keep in mind that many things can cause nystagmus, not just the consumption of alcohol.

The definition of horizontal gaze nystagmus is an involuntary jerking of the eyes as the eyes gaze toward the side. The person experiencing the nystagmus is unaware of its occurrence.

During the HGN exercise, the officer will move the stimulus during at least 14 passes in front of the subject.

During a DUI investigation, the suspect is permitted to refuse to participate in the HGN evaluation. That refusal might be mentioned at trial as an indication of “consciousness of your guilt,” but only if you were warned of some adverse consequence and still refused to participate.

Attorney for HGN Evaluations in Tampa, FL

Even if the officer does the HGN evaluation perfectly, many things can cause horizontal gaze nystagmus (HGN) other than the consumption of alcohol or the use of controlled substances.

We understand the most common mistakes that can be made by the officer when conducting the HGN evaluation or other field sobriety exercises.

From our main office in downtown Tampa, we represent clients throughout Hillsborough County. We know the problems with the way HGN is administered and evaluated by officers with the Tampa Police Department, the Hillsborough County Sheriff’s Office, and the Florida Highway Patrol.

With offices in downtown Tampa and New Port Richey, we represent clients throughout the greater Tampa Bay area including Hillsborough County, Pasco County, Pinellas County, and Polk County, FL.

Call (813) 250-0500 to discuss your case.

Admissibility of Testimony about HGN

In Florida, the prosecutor with the State Attorney’s Office is not usually allowed to elicit any testimony about the subject’s performance on horizontal gaze nystagmus.

In some cases, the court might allow some testimony about HGN if the officer has sufficient training by a medical professional to administer the exercises and evaluate the subject’s performance.

For instance, if the officer is qualified as a Drug Recognition Evaluator (DRE), the officer might be able to testify about the horizontal gaze nystagmus test procedures and the subject’s performance on HGN. Likewise, the courts are very hesitant to permit any testimony about any observations of vertical gaze nystagmus.

If the testimony is deemed admissible by the court, it is important for your criminal defense attorney to challenge this testimony in front of the jury effectively.

Your attorney must be well versed on the National Highway Traffic Safety Administration (NHTSA) guidelines for the HGN. One study recently found that the HGN test was administered incorrectly in the field over 90 percent of the time.

Instructions During the Horizontal Gaze Nystagmus Test

The officer will often give the following instructions during the HGN test:

  1. I am going to check your eyes. (Please remove your glasses).
  2. Keep your head still and follow the stimulus with your eyes only.
  3. Do not move your head.
  4. Do you understand the instructions.

Although earlier versions of the NHTSA manuals suggested that the officer should ask the subject whether they had previously suffered a head or neurological injury that might affect the HGN, the current version of the NHTSA SFST manual contains no such suggestion.

Common Mistakes When Administering HGN

Officers often make mistakes when administering the HGN including:

  1. moving the stimulus too quickly on individual passes;
  2. moving the stimulus too slowly on individual passes;
  3. holding the stimulus closer than 12 inches;
  4. holding the stimulus more than 15 inches away;
  5. not holding the stimulus for at least four seconds at maximum deviation;
  6. looping or curving the stimulus in an upward, downward, or circular motion.

If any of these mistakes were made during the administration of the HGN exercise, the test and its results are not reliable. In other words, the officer must administer HGN in accordance with NHTSA requirements and protocol.

Motions to Exclude or Suppress HGN Testimony

The courts have found that opinion testimony about HGN constitutes scientific evidence. For this reason, such testimony requires a qualified expert witness and should not be admitted as lay observations of intoxication.

In State v. Meador, 674 So.2d 826 (Fla. 4th DCA 1996), the Florida Supreme Court explained why testimony about HGN should be excluded from trial unless the traditional predicates of scientific evidence are satisfied.


Some courts have allowed for the admission of HGN testimony when the officer has sufficient experience and training in HGN testing.

When the defense attorney doesn’t make the proper objections, the courts have even allowed the officer to testify about the so-called Tharpe’s Equation, which is a purely law enforcement created “formula” used to correlate a blood alcohol level based upon the angle of onset of the observed nystagmus (BAC = 50 – angle of onset). Even in those cases, the courts have found that testimony is only allowed only in limited circumstances and only when there is a confirmatory blood, breath, or urine test.

Additional Resources

How to Challenge the Horizontal Gaze Nystagmus Test – Read an article published on the NCDD website concerning the horizontal gaze nystagmus test on how it works and how to exclude or challenge it. Learn why most States view the Horizontal Gaze Nystagmus (HGN) test as scientific evidence.

This article was last updated on Thursday, December 7, 2023.